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cruise rpm: how low is too low?
#819272
10/01/10 09:04 PM
10/01/10 09:04 PM
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Joined: Jan 2003
Posts: 28,068
Irving, TX
feets
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OP
Senior Management
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Irving, TX
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I'm back on the Imperial planning thing again.  With a stock or very mild 440, what is a reasonable cruise rpm that will return the best mileage on the highway and not lug the engine down? I'm talking about moving over 5,000 lbs of car at 65 mph. The originally came with 3.23 or 2.94 gears. They would turn 2500 rpm at 65 mph but were sluggish around town. I plan on dropping a 3.73 gear out back and sliding my 518 OD in the car. That will be equivalent to a 2.54 gear. Those gears will have the engine spinning 2,000 rpm at 65 mph. Will a mild 440 have the torque at those speeds to pull that heavy of a car? Disregard my turbos. They will not make any difference at cruise. I remember when we dropped the TKO in the 440+6 RR with a 3.54. It got better mileage at 75 mph than it did at 65 mph. The engine was lugging at the lower speeds. The carbs could have used a little attention. That car weighed about 3900 lbs. When I had the 833 OD and 3.23 gears in the hot rod it would pull 75 mph at 1800 rpm and 90 mph at 2200. That was using stock heads, the 236*/.474" and 232*/.483" cam, and a 3800 lb car. In this case the engine didn't feel like it was doing any work until 2700 and over 110 mph. I know the smallish ports on a RB will help keep port velocity up compared to other engines. However, they're not going to be the same as some of the late model engines turning lower speeds in lighter cars. The 9:1 or lower compression I'm going to run won't help. I won't be able to get away with 1600 rpm at 65 mph like the LS1 cars. I fired off an email last week to Comp Cams to get suggestions for a bump stick but haven't heard anything back yet.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: cruise rpm: how low is too low?
[Re: ahy]
#819274
10/01/10 11:56 PM
10/01/10 11:56 PM
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Joined: Jan 2003
Posts: 28,068
Irving, TX
feets
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Irving, TX
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Perhaps you don't know me that well. I will have a pair of turbochargers providing a little "assistance" when necessary. When I travel to the mountains, I carry my atmosphere with me.  During cruise, the turbos are just idling in the breeze. Once there's a load on the engine they will spool up and start making horsepower. The rpm will be a little low but if I load it at 2400 they might start to spin. That's on the bottom end of where they start working. That's why I like smallish turbos for a street car. The response is nice.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: cruise rpm: how low is too low?
[Re: feets]
#819275
10/02/10 03:11 AM
10/02/10 03:11 AM
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Joined: Oct 2003
Posts: 21,345
Marysville, O-H-I-O
70Cuda383
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Too Many Posts
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Marysville, O-H-I-O
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keep it above 2000 rpm at 65 for sure.
my "mild 440" (383 stroked to 438, eddy heads, XE275HL cam, 10.5:1 compression) was originally geared with 3.21s in the rear axle, 28" tires, and the TKO-600 with a .64OD.
at 65 mph, my rpm was down around 1700 and it was downright painful, just barely enough rpm to keep running. at 55 mph, I was better downshifting into 4th.
I swapped to 4.56 gears in the rear, and can still do 75mph at just over 2500 rpm (65 is about 2400rpm), and it does great. nice and smooth, and feels like I could cruise there forever.
**Photobucket sucks**
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Re: cruise rpm: how low is too low?
[Re: feets]
#819276
10/02/10 12:40 PM
10/02/10 12:40 PM
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Joined: Jan 2005
Posts: 12,419
Kalispell Mt.
HotRodDave
I Live Here
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I Live Here
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Posts: 12,419
Kalispell Mt.
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I don't know about a turbo motor but it seems to me the lower the RPM the better the MPG as long as the cam is right meaning little to no overlap, the exhaust allows raw fuel to go through the engine at high speeds because of scavenging and at low RPM it allows exhaust to go up the intake and dilute the air fuel mix so it don't burn properly. Also running the engine slower decreases the intake manifold vaccume because of the higher throttle angle, this means the pistons fight vaccume less (think about engine brakeing how the high vaccume slows you down). Higher compression and a later closeing intake valve can increase fuel atomization by forcing the raw air and fuel back and forth across the valve seat, also this allows a much higher expansion ratio that helps the engine operate efficently and lose less heat to a large combustion chamber. I think that would be a good thing in a turbo motor as long as you keep the dynamic compression ratio low for when you are under boost.
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: cruise rpm: how low is too low?
[Re: feets]
#819278
10/02/10 01:01 PM
10/02/10 01:01 PM
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Joined: Jan 2003
Posts: 104,346
Garden Grove, CA
OzHemi
Penguin-hating Ginger
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Penguin-hating Ginger
Joined: Jan 2003
Posts: 104,346
Garden Grove, CA
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Quote:
I'm back on the Imperial planning thing again.
Maybe an engine that does not need to rev alot would be an option  
I am in the LS1/6 speed camp....cool to see the tach at 1600 rpm at 70 
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Re: cruise rpm: how low is too low?
[Re: OzHemi]
#819279
10/02/10 01:13 PM
10/02/10 01:13 PM
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Anonymous
Unregistered
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Anonymous
Unregistered
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2 things.....
1. Keep the engine in the/or middle of the cams power range and you'll get the most efficiency out the combo-'gas milage'
2. make sure you have full advance at your cruise rpm.
as for too low, my 1500 cruises at 1900 on the freeway, but thats in the cam range as well as the computer dialing the timing in full
Last edited by 1wild&crazyguy; 10/02/10 01:14 PM.
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Re: cruise rpm: how low is too low?
#819280
10/02/10 01:17 PM
10/02/10 01:17 PM
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Joined: Nov 2007
Posts: 21,831
Kirkland, Washington
Pacnorthcuda
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Kirkland, Washington
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Quote:
2 things.....
1. Keep the engine in the/or middle of the cams power range and you'll get the most efficiency out the combo-'gas milage'
Wow---cool! My cam's power band is 2000-6200. So the middle would be a 4100 rpm 'cruise'. And that will get me best mileage.
I guess I can't afford not to step on it.
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Re: cruise rpm: how low is too low?
[Re: Pacnorthcuda]
#819281
10/02/10 01:22 PM
10/02/10 01:22 PM
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Anonymous
Unregistered
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Anonymous
Unregistered
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Quote:
Quote:
2 things.....
1. Keep the engine in the/or middle of the cams power range and you'll get the most efficiency out the combo-'gas milage'
Wow---cool! My cam's power band is 2000-6200. So the middle would be a 4100 rpm 'cruise'. And that will get me best mileage.
I guess I can't afford not to step on it.
Hey joker....read it again...I said in the cams power range, or yes even in the middle..and guess what...despite the rpm you will get the best milage and the motor will run most efficiently.
It's proven by many builders all over the world....
so you can cruise 2400 and still be there....
It's called help, recognize it sometime...
I'm now reminded why I almost NEVER post here.
Last edited by 1wild&crazyguy; 10/02/10 01:24 PM.
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Re: cruise rpm: how low is too low?
[Re: stumpy]
#819284
10/02/10 01:46 PM
10/02/10 01:46 PM
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Anonymous
Unregistered
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Anonymous
Unregistered
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Quote:
1wild, whats got your panties in a wad? Your reply was way over the top. Take a chill pill and relax. Haven't you ever heard of humor? You seem to be a little sensitive.
Oh yeah...I've heard humor, but that wasn't close to humor.
BTW maybe ya'll wear panties, but me?.. I wear a rhinestone CK piece myself........ thats humor.
I hope people reading these threads can tell the diff, cause if not...they are getting screwed big time by confusing responses like his....
Last edited by 1wild&crazyguy; 10/02/10 01:46 PM.
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Re: cruise rpm: how low is too low?
[Re: feets]
#819287
10/02/10 05:46 PM
10/02/10 05:46 PM
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Joined: Jul 2005
Posts: 757
Toronto, Ont, Canada
boydsdodge
super stock
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super stock
Joined: Jul 2005
Posts: 757
Toronto, Ont, Canada
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Excuse me for jumping on your post, but I was just going to ask about the same sort of thing. May I? Building a 408 stroker and I have a Tremic 5 speed with .64 5th. I am looking for opinions on what final gear you guys might think is best. My friend said if I am cruising the car at 2000 2200 I will still be using the idle feed in the carb. I don't think so, but I am by no means mister no it all. TKO600 408 Comp hydraulic roller, http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=651&sb=2Edelbrock closed chamber heads for a 10.7:1 CR mildly cleaned up and gasket matched. LD340 intake rear tires 27" Thermoquad from Demon Sizzler. TTI steps 2.5" pipes to side exit. was thinking that 3.91 gears would be about perfect. Thanks.
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Re: cruise rpm: how low is too low?
[Re: bordin34]
#819288
10/02/10 06:08 PM
10/02/10 06:08 PM
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Joined: Dec 2005
Posts: 1,159
Cruising!
QuickDodge
super stock
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super stock
Joined: Dec 2005
Posts: 1,159
Cruising!
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A lot of the mopar M bodies (Diplomat, etc.) had 2.45 rear gears. Some had 2.21(?) gears as well. These cars had V8's with carbs, no fancy engine management systems. Many of these cars had the lean burn system, which was a primative computer controlled engine management system. But a LOT of folks claim the cars performed better when converted to a standard electronic ignition system and carb. The website www.fueleconomy.gov has fuel economy rating for cars. I just looked up the mileage for a 1985 Dodge Diplomat. The civilian diplomat was rated at 22 mpg on the old rating system. The Diplomat police car was rated at 15 mpg. Both cars had 318 engines and 3 speed transmissions. The police cars had 4 bbl carbs, 360 heads,etc. The police cars typically had 2.94 gears, while the civilian cars had the 2.21 gears. I'm not certain of the exact weights of these cars, but a 318 M-body would probably have a similar pounds per cubic inch ratio as a 440 Imperial. The engines in the M-bodies had very small cams. Some of the 318's of this era only had 120 horse power which peaked at a very low RPM. (3500 - 4000 rpm?) It might be that an Imperial could get by with a smaller engine if a person wanted to maximize fuel economy. Perhaps a 383 with a turbo? A 383 with 10 psi of boost might make similar power to a 440 with 5 psi of boost? GM built a LOT of full size cars like the Chevy Caprice with 2.76 gears and a 200R4 or 700R4 overdrive tranmission. Some of these cars were fuel injected and some had computer controlled carbs. They even built some with 2.56 gears and the overdrive transmission. With these kinds of gears a lock up converter is essential. I don't know if a standard carb could be tuned to meter fuel effectively at such low cruise speeds. Feets, if you don't mind, keep us posted if you build a 440 Imperial for economy. It would be really cool to see a 20 mpg Imperial! Edit: After re-reading the original post, it appears that Feets wants performance and economy. (Instead of the absolute best economy) In this case, certain portions of the above answer should be dis-regarded. One thing I'll add though. The 518 transmission does not have a very good first gear ratio for launching a heavy car. So a high stall, lock-up converter would probably be the best for performance and economy.
Last edited by QuickDodge; 10/02/10 06:26 PM.
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Re: cruise rpm: how low is too low?
#819291
10/03/10 02:59 AM
10/03/10 02:59 AM
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Joined: Jan 2003
Posts: 3,031
Erda, UT
67Charger
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master
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Erda, UT
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Just a perspective... My '67 gets about 4 mpg around town due to 496" with .607 lift and 259° @ .050 duration pulling through a Holley 1000HP. It will burn your eyes with raw fuel. During the Silver State we sustained 125 - 140 mph for 90 miles turning between 4600 and 5000 rpm, we got nearly 11 mpg. My torque peak is 598 lb-ft at 3800 (pretty flat from 3300 - 5900), my HP peak is 574 at 5800.
11.33 @ 118.46 on motor
10.75 @ 125.35 w/ a little spray
Now, high Speed Open Road Racing - Silver State Classic Challenge, Nevada Open Road Challenge, Big Bend Open Road Race
Rocky Mountain Race Week 2020, 2022 2.0, Sick Week 2023
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Re: cruise rpm: how low is too low?
[Re: 67Charger]
#819293
10/03/10 10:39 AM
10/03/10 10:39 AM
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Joined: Jan 2003
Posts: 28,068
Irving, TX
feets
OP
Senior Management
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OP
Senior Management
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Posts: 28,068
Irving, TX
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67 Charger and 1wild, there are a couple things you guys are missing.
Charger, your engine will not run will at low rpm with that cam. It has no chance of being efficient at cruise rpm. Instead, you have to crank that thing up to make any kind of cylinder pressure. The huge overlap makes for a very bad intake charge unless the intake port velocity is extremely high.
1wild, you're forgetting that you have to fuel the engine to hit those higher rpm. My Concorde's 3.5 V6 makes peak torque at 2800 rpm. The most efficient cruise rpm is 2200 to 2300. If I run the engine up that extra 500 rpm the additional throttle opening (and resultant fuel requirement) along with the increased drag (air and pavement) combine to lower mileage my 3-4 mpg.
The hot rod made peak torque at 4800 rpm. There's NO WAY I would ever cruise that high. It got 17 mpg running 2400 rpm.
My biggest concern with an Imperial is the weight. A car half it's weight (like a LS1 Vega or container ship) can get away with tiny rpm. This thing is going to need a little more torque to get by.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: cruise rpm: how low is too low?
[Re: Twostick]
#819295
10/03/10 12:52 PM
10/03/10 12:52 PM
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Joined: Sep 2007
Posts: 14,889
up yours
Supercuda
About to go away
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About to go away
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Posts: 14,889
up yours
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Many years ago, I swapped the 3.23 gears out of my 64 300 for 4.10's. Mind you this was a mild 413 with the street hemi grind cam in it and a three speed stick. Highway mileage jumped from 12 to 15 mpg. Very consistently too.
Being in the cam's sweet spot is the key, as mentioned. Odds are you will want to look in the small cam part of the catalog, not need for big lift or duration numbers. Don't forget today's cars are optimized in regards to friction reduction. Roller cams, no drag calipers, etc.
They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.
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Re: cruise rpm: how low is too low?
[Re: Supercuda]
#819296
10/03/10 12:58 PM
10/03/10 12:58 PM
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Joined: Jan 2003
Posts: 28,068
Irving, TX
feets
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I'll leave it stock for a little while. If I play with it, it'll get a turbo-friendly cam that doubles as a nice cruiser.
The vacuum gauge is cheap and easy. Swapping out rear gears to play with that gauge is far from cheap and easy.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: cruise rpm: how low is too low?
[Re: feets]
#819297
10/03/10 12:59 PM
10/03/10 12:59 PM
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Joined: Sep 2007
Posts: 14,889
up yours
Supercuda
About to go away
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About to go away
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Posts: 14,889
up yours
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Oh I wasn't swapping gears for mileage by a long shot, but it was a nice bennie.
They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.
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Re: cruise rpm: how low is too low?
[Re: feets]
#819298
10/03/10 02:12 PM
10/03/10 02:12 PM
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Anonymous
Unregistered
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Anonymous
Unregistered
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Quote:
67 Charger and 1wild, there are a couple things you guys are missing.
Charger, your engine will not run will at low rpm with that cam. It has no chance of being efficient at cruise rpm. Instead, you have to crank that thing up to make any kind of cylinder pressure. The huge overlap makes for a very bad intake charge unless the intake port velocity is extremely high.
1wild, you're forgetting that you have to fuel the engine to hit those higher rpm. My Concorde's 3.5 V6 makes peak torque at 2800 rpm. The most efficient cruise rpm is 2200 to 2300. If I run the engine up that extra 500 rpm the additional throttle opening (and resultant fuel requirement) along with the increased drag (air and pavement) combine to lower mileage my 3-4 mpg.
The hot rod made peak torque at 4800 rpm. There's NO WAY I would ever cruise that high. It got 17 mpg running 2400 rpm.
My biggest concern with an Imperial is the weight. A car half it's weight (like a LS1 Vega or container ship) can get away with tiny rpm. This thing is going to need a little more torque to get by.
There is NO set cruise rpm, just the popular 'as low as you can go' thought.
Stay in the effective power range of the cam and you WILL get the most efficiency.
BTW as the rpm go's up....the squeeze/dynamic pressure is less due to less time for cylinder filling.
Maybe you meant to say keeping the r's high for exhaust SCAVANGING, which pulls on the intake.
The mix going through the runner might be puddling...but if the comp is ideal/high ...it will help mix it up/burn it..
Not everyone is running around with a cam as big as his...but his situation is the PERFECT example of what I'm talking about, it's just that you guys are running teenzy weenzy cams and can't grasp this even though it would put you near 'your idea' of the ideal cruise rpm anyways...
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Re: cruise rpm: how low is too low?
#819299
10/03/10 02:28 PM
10/03/10 02:28 PM
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Joined: Jan 2005
Posts: 12,419
Kalispell Mt.
HotRodDave
I Live Here
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I Live Here
Joined: Jan 2005
Posts: 12,419
Kalispell Mt.
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You guys are a trip  If all you guys get better MPG with lower gear ratios and bigger cubes how come we don't get 50mpg when we run 540 inch strokers with huge cams and 4.56 gears? Look at some big rig trucks, some of them get 5mpg weighing 50,000 lbs and aeodynamics worse than an imperial yet compareatively they have very tiny engines and they turn very low RPM going down the road. Mabey it is just because they are diesel? OK then how come my moms huge luxury maxyvan with everything under the sun inside weighed over 6000 lbs with terrible aero but still got 18mpg? Oh yeah it had a very small engine turning very low RPM, 225 slant six with 2.94 gears and very tall tires. You only need X amount of tq to maintain your speed going down the road, if your engine is produceing any more it is wasteing fuel. Most cars need less than 50hp to keep them going 70mph, if your engine can not do that at part throttle at 1500 or so RPM something is wrong with your engine. You may indeed need more power to go up a hill but that is why we have transmissions with multiple speeds, just let it shift. BTW for cam selection I think a turbo and an MPG car need similar cams, very little to no overlap, not too big of duration and decent amount of lift. 
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: cruise rpm: how low is too low?
[Re: feets]
#819301
10/03/10 04:47 PM
10/03/10 04:47 PM
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Joined: Jan 2003
Posts: 3,031
Erda, UT
67Charger
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master
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Posts: 3,031
Erda, UT
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Hey feets I was not condoning a 5000 rpm cruise, I was making a general point about working within the efficiency range of a given cam. I'm sure I could have done much better than 11 running around at 70 instead of 140, but that wasn't the point. I was simply giving an example of what happens when you get past the heavy overlap and let the cam do what it was designed to do. Secondly, as my post stated it was simply "For perspective..." not a suggestion to imitate my setup for best mpg. ...but I'm sure you figured that out.
11.33 @ 118.46 on motor
10.75 @ 125.35 w/ a little spray
Now, high Speed Open Road Racing - Silver State Classic Challenge, Nevada Open Road Challenge, Big Bend Open Road Race
Rocky Mountain Race Week 2020, 2022 2.0, Sick Week 2023
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Re: cruise rpm: how low is too low?
[Re: 67Charger]
#819302
10/03/10 05:34 PM
10/03/10 05:34 PM
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Joined: Jan 2003
Posts: 28,068
Irving, TX
feets
OP
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Irving, TX
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No problem with your post.
1wild is suggesting that buzzing around at a bazillion rpm is going to do wonders for mileage.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: cruise rpm: how low is too low?
[Re: Twostick]
#819303
10/03/10 08:47 PM
10/03/10 08:47 PM
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Joined: Jan 2003
Posts: 8,180
Detroit, MI
CokeBottleKid
master
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master
Joined: Jan 2003
Posts: 8,180
Detroit, MI
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Quote:
A vacuum gauge will be your best friend here. I had a '79 F100 with an inline 300 many moons ago with a 600 holley and headers that got 10 mpg period no matter what. 2.75 gear with a 3 on the tree and it cruised at 65 with 10" of vac. Switched the gears to 3.50's and no other changes it got 16-18 mpg and cruised with 18" of vac at 65 and was spinning 1000 rpm faster.
If you are staying N/A with this and putting in that 518 trans I would just advance the stock cam 4 deg and put in a set of 3.55's maybe 3.73's. That will lower the peak torque rpm and it should help get all that mass moving around town. It should do better than most pick-up trucks for mileage on the highway because the aero should be better and the weight is similar.
Kevin
Your results are a function of poor carb/distributor tuning vacuum is your enemy for efficiency.
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Re: cruise rpm: how low is too low?
[Re: CokeBottleKid]
#819304
10/03/10 09:21 PM
10/03/10 09:21 PM
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Joined: Jan 2004
Posts: 27,347
Today? Who Knows?
1_WILD_RT
Management Trainee
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Management Trainee
Joined: Jan 2004
Posts: 27,347
Today? Who Knows?
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Quote:
vacuum is your enemy for efficiency.
 Care to explain what this means??? For many years vacuum gauges were mounted in cars & labeled as an "Economy Meter" & you seem to feel vacuum is bad??? Please explain...
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Re: cruise rpm: how low is too low?
[Re: 1_WILD_RT]
#819306
10/04/10 12:37 AM
10/04/10 12:37 AM
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Joined: Jan 2003
Posts: 8,180
Detroit, MI
CokeBottleKid
master
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master
Joined: Jan 2003
Posts: 8,180
Detroit, MI
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Everytime someone asks about fuel economy it never fails this is brought up every time. And every time people refuse to acknowledge simple thermodynamic/fluids laws. Fact the area of a pressure/volume graph = the work used to move the fluid. This is what we call pumping losses... The more pressure (vacuum in this case) and the more volume (displacement) the more work required. Also the lower the RPM the less dynamic mechanical friction you have. So it's pretty much opposite of what everyones been saying, if you want greatest gas mileage you want to 'lug' your engine at a low RPM (low friction and flow rate) with the throttle blades open as far as you can (low vacuum). This is exactly why all the new motors use active cylinder shut-down in combination with drive by wire. As far as why the old vacuum gauges listed 'economy' in the high vacuum range? Simple, high vacuum also means you're not accelerating hard .
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Re: cruise rpm: how low is too low?
[Re: 1_WILD_RT]
#819308
10/04/10 01:09 AM
10/04/10 01:09 AM
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Joined: Jan 2003
Posts: 8,180
Detroit, MI
CokeBottleKid
master
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master
Joined: Jan 2003
Posts: 8,180
Detroit, MI
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I didn't say you have to use MDS and drive-by-wire to realize the benefits of low RPM and vacuum. Hence 98-02 LS1 6 speed camaros, drive by foot, no MDS but low cruising speed. All you need is FI and a good tune (which he has 1 at least ) .
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Re: cruise rpm: how low is too low?
[Re: HotRodDave]
#819309
10/04/10 07:34 PM
10/04/10 07:34 PM
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Anonymous
Unregistered
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Anonymous
Unregistered
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bwaaaaaaa haahahahahahah
[directed at hotroddave]
your kidding me right?
This is about getting the most mpg/efficiency with WHAT YOU HAVE, not your moms mini van...
you are really stretching it to come out right about this in some way.
Really though... quit the bs already man.
I'm trying to help, not take your lime light away..
Last edited by 1wild&crazyguy; 10/04/10 08:02 PM.
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Re: cruise rpm: how low is too low?
[Re: 67Charger]
#819310
10/04/10 07:51 PM
10/04/10 07:51 PM
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Anonymous
Unregistered
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Anonymous
Unregistered
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Who said to cruise 4800 rpm?
oh, you did..or should I say you were trying to credit me for your statement.
ha ha ha...come on people...
prove ME wrong.
and ya better re read what I am saying and less of what you are wrongly reading in.
say... what rpm does your mech advance total out at?
and has anyone considered the fact that a lot of guys with hotter setups don't run the vac canister to make up for your stated below average full advance rpm's?
don't know about everyone else here, but I run 26*initial/34* total no vac can and all in by 2200 rpm, my ported, x headed, .528 solid cam'd 340 w/750 dp single plane intake 4 spd 3.73 gears gets 15-16 all day long and I cruise around 3-3400 rpm.
I ran vac can before but beyond 4* over the total.. it would cack when I stabbed it to the floor because the timing does not fall off faster than my foot.
good luck, I'm through wasting my time with people who confuse and misunderstand what I say.
have fun down shifting....'oh but wait...you then have accelerate to keep speed' LOL
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Re: cruise rpm: how low is too low?
#819311
10/04/10 07:52 PM
10/04/10 07:52 PM
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Anonymous
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Anonymous
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btw...your mom mini van gets better mph, gears aside, BECAUSE THE CAM ONLY OPERATES IDLE TO 3800RPM!!!!!!!!!!!!!
DUHHHHHHHHHHHHH
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Re: cruise rpm: how low is too low?
[Re: feets]
#819312
10/04/10 07:56 PM
10/04/10 07:56 PM
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Anonymous
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Anonymous
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Quote:
No problem with your post.
1wild is suggesting that buzzing around at a bazillion rpm is going to do wonders for mileage.
no, you are simply stuck in the box your build and won't recognize other's examples.
Feets, you are kind of being a dk about this.
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Re: cruise rpm: how low is too low?
#819313
10/04/10 08:02 PM
10/04/10 08:02 PM
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Joined: Jan 2003
Posts: 28,068
Irving, TX
feets
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1wild, are you really a 16 year old or do you just play one on the net?
I gave you my example. The hot rod's torque curve really lit up around 3000 rpm and peaked at 4900 AT WIDE OPEN THROTTLE.
I got my best mileage cruising between 2300 and 2400 AT PART THROTTLE.
What you seem to forget is that the higher speeds induce higher drag. Stick your hand out the window at 30 mph and try it again at 80 mph. I bet you'll feel a difference. The car feels the difference too.
Now either play nice or go away.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: cruise rpm: how low is too low?
[Re: feets]
#819314
10/04/10 08:10 PM
10/04/10 08:10 PM
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Anonymous
Unregistered
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Anonymous
Unregistered
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Quote:
1wild, are you really a 16 year old or do you just play one on the net?
I gave you my example. The hot rod's torque curve really lit up around 3000 rpm and peaked at 4900 AT WIDE OPEN THROTTLE.
I got my best mileage cruising between 2300 and 2400 AT PART THROTTLE.
What you seem to forget is that the higher speeds induce higher drag. Stick your hand out the window at 30 mph and try it again at 80 mph. I bet you'll feel a difference. The car feels the difference too.
Now either play nice or go away.
You need to recognize what 'stay in the cam range' means.
You ran with it looking for a flaw in a 'generalized statement'
WHEN DID I TELL 'YOU' TO CRUISE AT A GAZILLION RPM?
I DIDN'T, SO YOU NEED TO PULL YOUR 'FEETS' OUT OF YOUR MOUTH. 
playing??? is that what you do on here?? 
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Re: cruise rpm: how low is too low?
[Re: feets]
#819318
10/04/10 08:47 PM
10/04/10 08:47 PM
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Joined: Jan 2003
Posts: 6,591
Canton, Ohio
Sport440
master
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master
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Posts: 6,591
Canton, Ohio
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How low is to low?? I think the engine will let you know. It will depend on the cam , comp and cubes and weight. If I remember correctly my 5.4/330 CI motor F150 cruises at about 1800 to 2000 RPM with 3.31 gearing at 65 MPH Its fine on the flat and mild grades before it needs to down shift. A 440 CI motor set up similer should be able to cruise at even a lower RPM. But, Like I said, the engine as set up will let you know. As far as motor Efficiency goes. They are most efficient per CI at there max torque RPM. The Problem with that. Is that these cars are Grossly overpowered. At Max torque you could be making 500# plus easy on a near stock 440. Burning All kinds of fuel in its efficent max torque rpm. Again the problem, We need very little torque compared to what these engine can produce to say, Cruise at 65 MPH So with a resticted throttle the engine itself may be in a less efficient per CI, it is in a Far more efficient state in reguards to fuel consumption. A restricted throttle reduces volumetric efficiency per CI, making it run like a Smaller motor. Technically if the fuel to air ratio stays inline, and it does. The motor isnt in efficient at all. It just runs like a fuel efficient smaller motor. mike
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Re: cruise rpm: how low is too low?
[Re: Sport440]
#819319
10/05/10 12:35 AM
10/05/10 12:35 AM
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Joined: Mar 2008
Posts: 390
kentucky
superbyrd
enthusiast
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enthusiast
Joined: Mar 2008
Posts: 390
kentucky
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i for years ran a 73 charger with this combo.....
318,wieand action plus,out of the box holley 600,mopar orange box and distributor,36 degrees total timing,manifolds,2 1/4" duals with h-pipe,40 series flows,full length exhaust,833 aluminum overdrive 4-speed,performance clutch,8 3/4" rear with 2.94 gears and sure-grip. 215/70/15's on 15X7 cragars. drove this car 3 times to columbus to the nats,(1000 mile round trip) got 23-25mpg running 75-80 mph. tached 1800 rpm at 75 mph. car pulled hills just fine. was it a powerhouse throttling up at 70mph in overdrive?? no. thats what i had a 4-speed for.if someone is trying to figure out how to keep a sizeable amount of power,while still in a higher gear,and low cruise rpm,i say,stop being lazy and downshift already. i'm not sure what "lugging the engine" at low rpms would be entailing,i mean,fire trucks,cop cars,and wreckers do it all day long sitting still.pulling alot of amps,pumps,etc.just my 2 cents......
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Re: cruise rpm: how low is too low?
#819320
10/06/10 04:45 PM
10/06/10 04:45 PM
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Joined: Oct 2010
Posts: 46
Can.
Hellrico505
member
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member
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Posts: 46
Can.
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Hi there, Long time reader-first time posting. I'll give my .02. I spent tremoudous amoung of hours on dyno and test track doing SAE test and stuff. What I suggest, do a highway test. Find yourself a portable tank that can be weighted(5gallon is good and a good weightscale) At cruise speed, switch tank and time record for 1/2 hour ride. make the ratio for pound of fuel/hrs. This will provide you with all energy loss while running at cruise speed. Do it a couple of time, to see if it repeat. Volume is innacurate, weight is much better. If you have acces to a rolling dyno that measure fuel rate, O2. You could install vacuum gauge, exhaust gas recorder or other. Spin the dyno at 65 MPH at a steady RPM. Measure fuel rate without load, that will give fuel rate needed for all the vehicule resistance component(tire, axle, trans, engine). Keep it steady at 65 mph and then start to load until it lug (or your assumption of lug, should be at kickdown point). Make sure you measure increment of fuel rate vs hp during that session(as futur reference). Example: without load: 5pounds/hrs +10HP : 5.1 pounds/hrs +20HP: 5.2 pounds/hrs, etc. Deduct no-load-dyno-fuel-rate from highway-fuel-rate. This give you aerodynamic resistance fuel rate and front tire rolling resistance. Now, since you are still attach to the dyno(caus you did the highway test before going on the dyno), take the highway-fuel-rate, load the dyno to that fuel-rate, keep the fuel rate steady(or dyno load) and reduce dyno rpm until it lug. Prepare to be suprise how deep it will go. If you go that route (deeper RPM) it will downshift more frequently than before. Other point to consider. if per example, you find that it could do confortably 65mph at 1500RPM, the engine may lug if you go 75mph at 1700RPM, 85mph at 2000rpm. Energy demands is sqareroot of speed. As far as future number increment test. If you ever do a modification on the car(change tire, synthetic lube, different rearend, manual trans, overdrive, procharger, etc) you will be able to compare the improvement on your next dyno visit. If you don't have acces to dyno, there are other way(hitch and BIG camper), but poorly accurate. Even dyno result can be questionnable(don't ask). I did that countless time with class 8 truck. Fuel(gas, nitro, diesel, E85, Hybrid) is energy, regardless of what it does.  Rej
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Re: cruise rpm: how low is too low?
[Re: Hellrico505]
#819321
10/06/10 05:28 PM
10/06/10 05:28 PM
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Joined: Jan 2003
Posts: 28,068
Irving, TX
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That's a great way to do some testing but I'm not going to buy several different ring and pinion sets to do that over and over.
I was trying to get a general idea.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: cruise rpm: how low is too low?
[Re: OzHemi]
#819325
10/06/10 08:03 PM
10/06/10 08:03 PM
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Joined: Jan 2003
Posts: 28,068
Irving, TX
feets
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That's part of the building process. Step one of the mechanical bits I believe.
1) get a car
2) put stuff on it
3) cuss the car
4) kick the car
5) throw stuff at the car while hopping around on your good foot
6) fix stuff you broke while upset with the car
7) dream of driving the car some day
8) cuss car
9) fill car with gasoline
10) strike match
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: cruise rpm: how low is too low?
[Re: 1_WILD_RT]
#819326
10/06/10 11:43 PM
10/06/10 11:43 PM
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Joined: Sep 2003
Posts: 1,688
Marlboro, NY, USA
Rick_Ehrenberg
top fuel
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top fuel
Joined: Sep 2003
Posts: 1,688
Marlboro, NY, USA
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Quote:
[is your enemy for efficiency.
<snip>
Care to explain what this means??? For many years vacuum gauges were mounted in cars & labeled as an "Economy Meter" & you seem to feel vacuum is bad??? Please explain...
One of the 3 main reasons that diesels get better MPG is the lack of a throttle = almost no vacuum, lots less pumping losses.
(The other 2 are higher CR and more BTUs per gallon of fuel).
Back to the Imp: Will this be carb's or SMPI? (I hope it is the latter).
Rick
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Re: cruise rpm: how low is too low?
[Re: Rick_Ehrenberg]
#819327
10/07/10 01:00 AM
10/07/10 01:00 AM
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Anonymous
Unregistered
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Anonymous
Unregistered
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20.1-1 comp, a power band of idle to 4000rpm, 20 somthing speed tranny with 2 speed rear...
and the cruise rpm is...... like I said, in the cam range.
It IS still apart of the total equation of optimal gas milage.
it all add's up... and anyone denying that needs to start over.
Rick...will even you deny this?
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Re: cruise rpm: how low is too low?
#819328
10/07/10 04:47 AM
10/07/10 04:47 AM
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Joined: Apr 2010
Posts: 10,570
Sunny South Florida
Golden-Arm
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Quote:
btw...your mom mini van gets better mph, gears aside, BECAUSE THE CAM ONLY OPERATES IDLE TO 3800RPM!!!!!!!!!!!!!
DUHHHHHHHHHHHHH
so, the engine shuts off, at 3801 rpm? seems to me, the cam operates at any speed, the engine is operating at. have i missed something here? if the cam doesnt operate over any specific rpm, the engine would shut off. perhaps you meant something else, but were too busy yelling?
"When Tyranny Becomes Law, Rebellion Becomes Duty"
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Re: cruise rpm: how low is too low?
[Re: Golden-Arm]
#819329
10/07/10 06:13 AM
10/07/10 06:13 AM
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Joined: Oct 2003
Posts: 21,345
Marysville, O-H-I-O
70Cuda383
Too Many Posts
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Too Many Posts
Joined: Oct 2003
Posts: 21,345
Marysville, O-H-I-O
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Quote:
Quote:
btw...your mom mini van gets better mph, gears aside, BECAUSE THE CAM ONLY OPERATES IDLE TO 3800RPM!!!!!!!!!!!!!
DUHHHHHHHHHHHHH
so, the engine shuts off, at 3801 rpm? seems to me, the cam operates at any speed, the engine is operating at. have i missed something here? if the cam doesnt operate over any specific rpm, the engine would shut off. perhaps you meant something else, but were too busy yelling?
come on guys. knock it off. you're arguing semantics now! anyone with a 3rd grade education knows what he meant! let's avoid picking on each other, and debate the actual topic on hand instead of nit picking using proper english. this is an internet forum, not a college masters paper
**Photobucket sucks**
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Re: cruise rpm: how low is too low?
[Re: feets]
#819330
10/07/10 12:12 PM
10/07/10 12:12 PM
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Joined: Jan 2003
Posts: 16,123
Grand Haven, MI
patrick
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I Live Here
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Quote:
I'm back on the Imperial planning thing again.
With a stock or very mild 440, what is a reasonable cruise rpm that will return the best mileage on the highway and not lug the engine down?
I'm talking about moving over 5,000 lbs of car at 65 mph. The originally came with 3.23 or 2.94 gears. They would turn 2500 rpm at 65 mph but were sluggish around town.
I plan on dropping a 3.73 gear out back and sliding my 518 OD in the car. That will be equivalent to a 2.54 gear.
Those gears will have the engine spinning 2,000 rpm at 65 mph. Will a mild 440 have the torque at those speeds to pull that heavy of a car?
Disregard my turbos. They will not make any difference at cruise.
I remember when we dropped the TKO in the 440+6 RR with a 3.54. It got better mileage at 75 mph than it did at 65 mph. The engine was lugging at the lower speeds. The carbs could have used a little attention. That car weighed about 3900 lbs.
When I had the 833 OD and 3.23 gears in the hot rod it would pull 75 mph at 1800 rpm and 90 mph at 2200. That was using stock heads, the 236*/.474" and 232*/.483" cam, and a 3800 lb car. In this case the engine didn't feel like it was doing any work until 2700 and over 110 mph.
I know the smallish ports on a RB will help keep port velocity up compared to other engines. However, they're not going to be the same as some of the late model engines turning lower speeds in lighter cars. The 9:1 or lower compression I'm going to run won't help.
I won't be able to get away with 1600 rpm at 65 mph like the LS1 cars.
I fired off an email last week to Comp Cams to get suggestions for a bump stick but haven't heard anything back yet.
FWIW, my '96 ram was 4700 lbs w/o me. it had an NVG3500 (.73 OD, IIRC) 318 magnum, cat back, open element air filter, and MP computer. rear gears were 3.21 with 31" tall tires. 75mph was 2000 RPM. rode down the highway fine and pulled fine from 60MPH in 5th.
I think the key is wide LSA and moderate duration to keep overlap and potential cam surging down....
another comparison, a friend has a '96 impala SS with 3.73's or 4.10's (can't remember which) and a T-56, it's 4200-4300lbs w/o driver, the engine is a 396CID LT1, with a comp hydraulic roller in the 230@.050 range, and it goes down the freeway at 70 mph in 6th just fine...lower than that he does get a little bit of surge in 6th...
1976 Spinnaker White Plymouth Duster, /6 A833OD
1986 Silver/Twilight Blue Chrysler 5th Ave HotRod **SOLD!***
2011 Toxic Orange Dodge Charger R/T
2017 Grand Cherokee Overland
2014 Jeep Cherokee Latitude (holy crap, my daughter is driving)
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Re: cruise rpm: how low is too low?
#819331
10/07/10 12:27 PM
10/07/10 12:27 PM
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Joined: Jan 2003
Posts: 16,123
Grand Haven, MI
patrick
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I Live Here
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Posts: 16,123
Grand Haven, MI
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Quote:
This actually makes no sense because the cams "power range" is only at WOT, no one cruises at WOT unless you have a slant six in a dump truck. All new cars cruise at a much lower RPM than the peak TQ because they get the best MPG at a lower RPM. People just say this beause they want an excuse to drive fast, I however don't need any excuse, I just drive fast because it's fun
You are mistaken, it's called torque...ya know that thing that comes on when you get into the power band of the cam...
While it's true that lower rpms typically net you more mpg, it's also staying in the torque curve/power curve/band or whatever you decide to call today..
As in ...if your mill has a cam advertised 3000-6500, you are better off reving inside of 3000 than 2000 rpm.
Ever have to down shift and stomp on it to get up that hill?
you wouldn't be mashin the pedal if you were in the usable power range to begin with.
ahh you'll figure it someday. 
uhh, again, the torque/HP peaks you usually see are based on WOT....run a dyno sweep of the motor at 1/2 throttle and see if the HP and tq peak RPMS change....
1976 Spinnaker White Plymouth Duster, /6 A833OD
1986 Silver/Twilight Blue Chrysler 5th Ave HotRod **SOLD!***
2011 Toxic Orange Dodge Charger R/T
2017 Grand Cherokee Overland
2014 Jeep Cherokee Latitude (holy crap, my daughter is driving)
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Re: cruise rpm: how low is too low?
[Re: patrick]
#819332
10/07/10 12:48 PM
10/07/10 12:48 PM
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Joined: Jan 2003
Posts: 16,123
Grand Haven, MI
patrick
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I Live Here
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Grand Haven, MI
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another point of reference, my 5th ave I have a mag headed 318, 9:1 comp, with a 259/259 adv, 208/208@.050, on 112LSA cam, A500, 27" tires and 3.55's....65mph is ~2000 RPM. my commute is 2 lane, with a somewhat hilly portion, doing 55 mph (~1800 RPM), I've never had it lug/surge or downshift.
same thing with my old motor, which was a stock long block 360 with a comp XE262. the old motor would surge a bit at 45mph in OD, the new one doesn't...
if it were my car feets, even if it is heavy, I'd go 3.55
s-3.73's, since you'll probably be running 28-30" tall tires, your 518, and a higher stall lockup (2800-3200RPM) to get the weight moving from a standstill.
for a cam, I'd probably run bullet's HR276/378 lobe on the intake, probably a 114 LSA, installed at 108, not sure on the exhaust, if you want more or less duration on the exhaust with a turbo car...I'd personally build the engine with an idle-5500 RPM powerband...
1976 Spinnaker White Plymouth Duster, /6 A833OD
1986 Silver/Twilight Blue Chrysler 5th Ave HotRod **SOLD!***
2011 Toxic Orange Dodge Charger R/T
2017 Grand Cherokee Overland
2014 Jeep Cherokee Latitude (holy crap, my daughter is driving)
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Re: cruise rpm: how low is too low?
[Re: patrick]
#819333
10/07/10 01:26 PM
10/07/10 01:26 PM
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Anonymous
Unregistered
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Anonymous
Unregistered
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uhh, again, the torque/HP peaks you usually see are based on WOT....run a dyno sweep of the motor at 1/2 throttle and see if the HP and tq peak RPMS change....
Yeah and you'd still be in better shape than you would below the operating advertised range of the cam.
Your new vs old motor is a good example of what I'm saying, again.
However, I do agree with the advice you give in the above post about cam range idle to 5500, thats where he needs to be if he's putting mph over power/rpm.
I would say with a cam like that one mentioned, a cruise rpm of 2200 rpm would be nice.
To all those who understood what I said, thank you.
To all those who didn't understand but then gave examples proving true what I said, thanks.
And to all those who decided they would chime in about nothing other than my delivery of what I said, thanks but don't worry about me.
ps, When I do the caps lock deal..it's more of a LOL while responding. 
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Re: cruise rpm: how low is too low?
#819336
10/07/10 04:12 PM
10/07/10 04:12 PM
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Joined: Jan 2003
Posts: 28,068
Irving, TX
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OP
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Quote:
To all those who understood what I said, thank you.
To all those who didn't understand but then gave examples proving true what I said, thanks.
What about my example proving you wrong? My cam was designed to operate between 2500 and 5500 rpm. I should know. I worked with Comp Cams to design it.
The car got MUCH better mileage at 2300 to 2400 rpm than anywhere above that. The higher you revved the worse the mileage got.
It's called DRAG.
The engine may be more efficient at WOT but that changes at part throttle. My computer says I was using 12% throttle opening to hold 2400 rpm.
Unlike some of these guys, my hot rod was instrumented nicely.
Now, unless you can come up with evidence that I am wrong in my example or that I was using alien technology to invalidate the laws of physics, please leave my thread. You have made yourself unwelcome in this topic through your behavior and mockery of others.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: cruise rpm: how low is too low?
[Re: feets]
#819337
10/07/10 04:33 PM
10/07/10 04:33 PM
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Anonymous
Unregistered
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Anonymous
Unregistered
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Quote:
Quote:
To all those who understood what I said, thank you.
To all those who didn't understand but then gave examples proving true what I said, thanks.
What about my example proving you wrong? My cam was designed to operate between 2500 and 5500 rpm. I should know. I worked with Comp Cams to design it.
The car got MUCH better mileage at 2300 to 2400 rpm than anywhere above that. The higher you revved the worse the mileage got.
It's called DRAG.
The engine may be more efficient at WOT but that changes at part throttle. My computer says I was using 12% throttle opening to hold 2400 rpm.
Unlike some of these guys, my hot rod was instrumented nicely.
Now, unless you can come up with evidence that I am wrong in my example or that I was using alien technology to invalidate the laws of physics, please leave my thread. You have made yourself unwelcome in this topic through your behavior and mockery of others.
Yeah...and where was it degreed at???? 
Last edited by 70Cuda383; 10/08/10 02:52 AM.
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Re: cruise rpm: how low is too low?
#819338
10/07/10 04:50 PM
10/07/10 04:50 PM
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Joined: Jan 2003
Posts: 32,946
Grand Prairie,Texas
stumpy
I Win
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I Win
Joined: Jan 2003
Posts: 32,946
Grand Prairie,Texas
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My 408 stroker is running a Mopar 508 cam which has a recomended operating range of 3000-6000 rpm and I get my best gas milage at 2300 2500. Anything above that and my milage drops rapidly.
(I said to keep it to the argument and knock off the personal attacks)
Last edited by 70Cuda383; 10/08/10 02:50 AM.
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Re: cruise rpm: how low is too low?
#819339
10/07/10 05:28 PM
10/07/10 05:28 PM
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Joined: Jan 2003
Posts: 28,068
Irving, TX
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Last edited by 70Cuda383; 10/08/10 03:00 AM.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: cruise rpm: how low is too low? *DELETED*
[Re: stumpy]
#819340
10/07/10 07:50 PM
10/07/10 07:50 PM
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Anonymous
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Anonymous
Unregistered
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Re: cruise rpm: how low is too low?
[Re: stumpy]
#819342
10/07/10 10:12 PM
10/07/10 10:12 PM
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Joined: Mar 2008
Posts: 390
kentucky
superbyrd
enthusiast
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enthusiast
Joined: Mar 2008
Posts: 390
kentucky
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Last edited by 70Cuda383; 10/08/10 02:54 AM.
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Re: cruise rpm: how low is too low?
[Re: stumpy]
#819343
10/07/10 10:14 PM
10/07/10 10:14 PM
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Anonymous
Unregistered
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Anonymous
Unregistered
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Last edited by 70Cuda383; 10/08/10 02:56 AM.
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Re: cruise rpm: how low is too low? *DELETED*
[Re: superbyrd]
#819345
10/07/10 10:19 PM
10/07/10 10:19 PM
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Anonymous
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Anonymous
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Re: cruise rpm: how low is too low?
[Re: stumpy]
#819346
10/07/10 10:21 PM
10/07/10 10:21 PM
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Anonymous
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Anonymous
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Last edited by 70Cuda383; 10/08/10 02:57 AM.
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Re: cruise rpm: how low is too low?
#819347
10/07/10 10:22 PM
10/07/10 10:22 PM
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Joined: Dec 2008
Posts: 1,200
UK
602heavy
pro stock
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pro stock
Joined: Dec 2008
Posts: 1,200
UK
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Last edited by 70Cuda383; 10/08/10 02:58 AM.
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Re: cruise rpm: how low is too low?
#819348
10/07/10 10:22 PM
10/07/10 10:22 PM
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Joined: Mar 2008
Posts: 390
kentucky
superbyrd
enthusiast
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enthusiast
Joined: Mar 2008
Posts: 390
kentucky
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Last edited by 70Cuda383; 10/08/10 02:58 AM.
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Re: cruise rpm: how low is too low?
[Re: 602heavy]
#819349
10/07/10 10:29 PM
10/07/10 10:29 PM
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Anonymous
Unregistered
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Anonymous
Unregistered
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Last edited by 70Cuda383; 10/08/10 02:59 AM.
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Re: cruise rpm: how low is too low?
#819351
10/07/10 11:03 PM
10/07/10 11:03 PM
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Joined: Jan 2005
Posts: 12,419
Kalispell Mt.
HotRodDave
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I Live Here
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Kalispell Mt.
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Before I did the big super duper MPG 318 build I had the bone stock 318 2bbl in my 68 cuda, it came with a 2.76 gear and cruised well below the 2400 RPM TQ peak it came from the factory with, now I had an otherwise identicle rear end with 2.94 gears and thought I would swap it in for a little more acceleration and to get it up higher RPM at cruise because like you I though it would get the best MPG cruising at the TQ peak, that is when I found out I was very wrong, cruising was right at 2400 RPM however MPG went down drastically, next rear end swap was to a 8.75 with 2.76 gears and MPG went right back up to where it was with the 2.76 7.25 rear even though RPM was once again well below the TQ peak. After that I built a 904 with LU and brought RPM down more and MPG went up another 1mpg.
After that I built a similar motor but with a few changes, the compression was increased, I added a 4bbl carb and headers, so the peak TQ RPM should have gone up significantly (I never dynoed it) and it definately felt like it did but guess what? MPG went up a bunch more. I got a best of 29mpg lugging the snot out of that motor well below the peak TQ. I really wish there had been higher gears available for the 8.75, I am sure I would have gotten over 30 MPG with that combo. That motor was not working hard at all to pull the car at that high of a gear, even at 70 mph (2200 RPM) it was still pulling 17 inches vaccume so there was way more power there than was nessacary.
Cruising at your peak TQ only gives you the best MPG if you have to run WOT to maintain your cruise speed at that RPM.
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: cruise rpm: how low is too low?
[Re: HotRodDave]
#819352
10/07/10 11:15 PM
10/07/10 11:15 PM
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Anonymous
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Quote:
Before I did the big super duper MPG 318 build I had the bone stock 318 2bbl in my 68 cuda, it came with a 2.76 gear and cruised well below the 2400 RPM TQ peak it came from the factory with, now I had an otherwise identicle rear end with 2.94 gears and thought I would swap it in for a little more acceleration and to get it up higher RPM at cruise because like you I though it would get the best MPG cruising at the TQ peak, that is when I found out I was very wrong, cruising was right at 2400 RPM however MPG went down drastically, next rear end swap was to a 8.75 with 2.76 gears and MPG went right back up to where it was with the 2.76 7.25 rear even though RPM was once again well below the TQ peak. After that I built a 904 with LU and brought RPM down more and MPG went up another 1mpg.
After that I built a similar motor but with a few changes, the compression was increased, I added a 4bbl carb and headers, so the peak TQ RPM should have gone up significantly (I never dynoed it) and it definately felt like it did but guess what? MPG went up a bunch more. I got a best of 29mpg lugging the snot out of that motor well below the peak TQ. I really wish there had been higher gears available for the 8.75, I am sure I would have gotten over 30 MPG with that combo. That motor was not working hard at all to pull the car at that high of a gear, even at 70 mph (2200 RPM) it was still pulling 17 inches vaccume so there was way more power there than was nessacary.
Cruising at your peak TQ only gives you the best MPG if you have to run WOT to maintain your cruise speed at that RPM.
Finally seriousness...
To bounce off of that...I would think the 8 3/4 would have more drag due to the larger internal parts surface contact.
I know there are 2.93 gears in the 7 1/4 but never had any 2.94's in one.
I never said to run in the peak torque, I simply stated that if you stay in the advertised or figured advertised [based upon cam position] operating range..that you would see the better of mpg.
btw, that is some really good mpg dave, good job.
as for gears for the 8 3/4, 2.45 was the highest I've seen in an old imperial.
I'm sure 'us that know' realize It all add's up thats for sure, it's not just one thing...it's the combo or total package.
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Re: cruise rpm: how low is too low?
#819353
10/07/10 11:16 PM
10/07/10 11:16 PM
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Anonymous
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and....that there are many ways to skin a cat in this game.
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Re: cruise rpm: how low is too low?
[Re: HotRodDave]
#819354
10/07/10 11:39 PM
10/07/10 11:39 PM
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Irving, TX
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The real point behind this wasn't finding the absolute best mileage. It was to find the best RPM to push 5200+ lbs of car down the road. The aerodynamics of a vending machine will not make the job any easier.
I want to cruise at a low enough rpm to reduce engine noise and save fuel while still having the torque to go up minor inclines at 60 mph. Running too low of an engine speed with a container vessel like this will bog down the engine. At what point will that happen with a mild cam with specs similar to stock stuff?
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: cruise rpm: how low is too low?
[Re: ahy]
#819356
10/08/10 12:43 AM
10/08/10 12:43 AM
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A turbo cam is very similar to a stock cam. The overlap is minimized to prevent blowing raw fuel into the exhaust. A street style turbo cam will make for a fine efficient cruiser. I'm a wee bit familiar with thermodynamics. I had a 4 gear in the hot rod and pulled it out. I didn't care for the manual with the turbos. Also, a luxury car should shift for itself. As for making it truly one off, I'd do something far more drastic than that. However, I've got to build the cruiser before I really go off the deep end with one.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: cruise rpm: how low is too low?
[Re: feets]
#819357
10/08/10 01:06 AM
10/08/10 01:06 AM
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Posts: 6,591
Canton, Ohio
Sport440
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Canton, Ohio
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Quote:
The real point behind this wasn't finding the absolute best mileage. It was to find the best RPM to push 5200+ lbs of car down the road. The aerodynamics of a vending machine will not make the job any easier.
I want to cruise at a low enough rpm to reduce engine noise and save fuel while still having the torque to go up minor inclines at 60 mph. Running too low of an engine speed with a container vessel like this will bog down the engine. At what point will that happen with a mild cam with specs similar to stock stuff?
Does your car really way 5200#
I cant see it weighing over 4200# . Reguardless there are other issues to consider.
With the right stock like cam and the right comp, 1200 RPM may be plenty/just right.
Suppose it is.
At 60 MPH can a stock 727 final gearing 1 to 1 and a stock or after market low rear axel gear get you there?
What axel gear with a tall 30" tire would you need if you dont have a overdrive tranny gear.
1.8 axel gear is what you would need for 1200 RPM with a 30" tire.
Another issue is the trannys shift down points. Its not governed to stay in 3rd at 1200 rpms., or even at 2000 rpm. so that will have to be addressed.
Conclusion, If my measly 331 CI engine can cruise my 4500# Brick truck down the road at 60 mph at 1800 rpm.
A similer built 440 should be able to do the same at a lessor say 1200 rpm. But as stated above there is more to it then that.
Without a overdrive and stock 727 tranny. Heres some numbers for a low 2.73 gear at 60 mph.
60 x 2.73 x 336 /30 = 1835 rpm, add in 5% vert slip and your looking at 2000 minimum obtainable RPM anyways. mike
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Re: cruise rpm: how low is too low?
[Re: Rick_Ehrenberg]
#819358
10/08/10 01:24 AM
10/08/10 01:24 AM
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Joined: Jan 2003
Posts: 8,180
Detroit, MI
CokeBottleKid
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Quote:
Quote:
[is your enemy for efficiency.
<snip>
Care to explain what this means??? For many years vacuum gauges were mounted in cars & labeled as an "Economy Meter" & you seem to feel vacuum is bad??? Please explain...
One of the 3 main reasons that diesels get better MPG is the lack of a throttle = almost no vacuum, lots less pumping losses.
(The other 2 are higher CR and more BTUs per gallon of fuel).
Back to the Imp: Will this be carb's or SMPI? (I hope it is the latter).
Rick
Just about one of the only completely correct posts in this thread, thanks Rick.
Feets you have your answer, the lower the better.
If you were building a high strung N/A motor with 12:1 and a long duration (like 265@.050 and greater) tight LSA cam, I'd say ya maybe you don't want to go below 1800 or so (depends on cubes and cam) but considering it's a turbo build with a really mild cam, go as low as you want.
I'm sure you'll hit your personal gear ratio limit on acceleration before it's too low for the motor.
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Re: cruise rpm: how low is too low?
[Re: feets]
#819360
10/08/10 09:16 AM
10/08/10 09:16 AM
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Joined: Jan 2003
Posts: 16,123
Grand Haven, MI
patrick
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Quote:
The real point behind this wasn't finding the absolute best mileage. It was to find the best RPM to push 5200+ lbs of car down the road. The aerodynamics of a vending machine will not make the job any easier.
I want to cruise at a low enough rpm to reduce engine noise and save fuel while still having the torque to go up minor inclines at 60 mph. Running too low of an engine speed with a container vessel like this will bog down the engine. At what point will that happen with a mild cam with specs similar to stock stuff?
I'd gear it like my old ram, which turned ~2000 RPM at 75, and I didn't have to downshift going up and down typical highway hills doing 60mph in my 4700 lb truck with only a stock internals 5.2 magnum....
which would be ~3.55's, ~30" tires with your 518....that gives an OD cruise RPM of 1650 at 60 mph, 2050 for 75 mph.
3.73's would increase those RPM numbers by ~100, 3.23's decrease them by about 100-150 rpm....
3.91's with 30" tires still give you a 60mph cruise of 1800, and 75mph of ~2300, and may help getting the mail moving from a stop better....
you could also use a relay or something to trigger lockup/unlock based on your MAP sensor, or use a vaccuum switch to do the same thing, too...
for a heavier street car, I like the combo of a loose/high stall lockup converter and tall gears....the high stall gets the motor up into the meat of the powerband quickly even with the tall gears, adn the tall gears make for a nice freeway cruiser.
what might be fun would be to try adapt an LX A580 and maybe some 2.94's...the 580 has a deeeep first, and a not so tall OD...
1976 Spinnaker White Plymouth Duster, /6 A833OD
1986 Silver/Twilight Blue Chrysler 5th Ave HotRod **SOLD!***
2011 Toxic Orange Dodge Charger R/T
2017 Grand Cherokee Overland
2014 Jeep Cherokee Latitude (holy crap, my daughter is driving)
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Re: cruise rpm: how low is too low?
[Re: Sport440]
#819361
10/08/10 09:42 AM
10/08/10 09:42 AM
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Yes, the fuselage Imperials really do weigh in excess of 5,000 lbs. Toss in 25 gallons of fuel and a 225 lb driver and it's well over 5,200 lbs.
I guess either the 3.73 or 3.91 gears will work. It will probably depend upon which I can find a speedo gear to match as well as the cost of the gears.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: cruise rpm: how low is too low?
[Re: Twostick]
#819362
10/08/10 10:17 AM
10/08/10 10:17 AM
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Joined: Jan 2003
Posts: 8,180
Detroit, MI
CokeBottleKid
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Quote:
Just because he agrees with you don't make him right. The BIGGEST reason a diesel is more fuel efficient is because it runs dead lean ALL the time and can do that because it is compression ignited. Pumping loss is absolutely a consideration but most of the loss is from the compression stroke not the intake. A Jake Brake on a diesel is an apt demonstration.
Kevin
You're debating with yourself here, no one is arguing what is the single greatest characteristic which makes a diesel efficient. We were discussing the effects of pumping losses on efficiency, welcome to the discussion....
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Re: cruise rpm: how low is too low?
[Re: CokeBottleKid]
#819363
10/08/10 12:00 PM
10/08/10 12:00 PM
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Joined: Jan 2005
Posts: 12,419
Kalispell Mt.
HotRodDave
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FWIW my Durango with a 5.9 had no problem maintinaing it's speed up most hills with out down shifting. The aero was not much worse and it weighed about 5000lbs. It cruised around 2000 at 70mph.
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: cruise rpm: how low is too low?
[Re: feets]
#819364
10/08/10 01:13 PM
10/08/10 01:13 PM
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Joined: Jan 2003
Posts: 16,123
Grand Haven, MI
patrick
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Quote:
Yes, the fuselage Imperials really do weigh in excess of 5,000 lbs. Toss in 25 gallons of fuel and a 225 lb driver and it's well over 5,200 lbs.
I guess either the 3.73 or 3.91 gears will work. It will probably depend upon which I can find a speedo gear to match as well as the cost of the gears.
what sized tires are you planning on running? same tires that are on the hotrod now, which are what, ~ 27" tall? if so, I'd probably lean more towards a 3.23 or 3.55 based on my experience with my A500/255/50R17/3.55 equipped 5th ave.
1976 Spinnaker White Plymouth Duster, /6 A833OD
1986 Silver/Twilight Blue Chrysler 5th Ave HotRod **SOLD!***
2011 Toxic Orange Dodge Charger R/T
2017 Grand Cherokee Overland
2014 Jeep Cherokee Latitude (holy crap, my daughter is driving)
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Re: cruise rpm: how low is too low?
[Re: patrick]
#819365
10/08/10 05:44 PM
10/08/10 05:44 PM
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Irving, TX
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I'd like to run the same 28" tires I have now.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: cruise rpm: how low is too low?
[Re: feets]
#819366
10/08/10 06:09 PM
10/08/10 06:09 PM
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Grand Haven, MI
patrick
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I'd probably lean more towards 3.55's if it were me.
1976 Spinnaker White Plymouth Duster, /6 A833OD
1986 Silver/Twilight Blue Chrysler 5th Ave HotRod **SOLD!***
2011 Toxic Orange Dodge Charger R/T
2017 Grand Cherokee Overland
2014 Jeep Cherokee Latitude (holy crap, my daughter is driving)
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Re: cruise rpm: how low is too low?
[Re: patrick]
#819367
10/08/10 06:15 PM
10/08/10 06:15 PM
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Remember that I'm trying to balance the gears between dragging it's boated carcass through stop and go traffic and highway use.
You know how it goes. I want it to do everything despite being the size of a container ship.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: cruise rpm: how low is too low?
[Re: challengermike]
#819369
10/08/10 11:17 PM
10/08/10 11:17 PM
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I know about the low numbered gears and turbos. A friend was running 10.30s in his turbo 440 Dart with a 2.94 gear.
Your Buick weighed 3500 lbs. Hook a bass boat to it and try again.
I don't want to spool the turbos between street lights. I want the car to get around okay on it's own before I invoke the positive manifold pressure.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: cruise rpm: how low is too low?
[Re: feets]
#819370
10/09/10 12:37 AM
10/09/10 12:37 AM
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Posts: 1,062
Amherst,NY
challengermike
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Most of those the old cars stock had 3.23s or lower and they wernt pigs to drive around town,and on the freeway you would of liked another gear.I completely get what your saying,but i think we are all looking into this to deep.I think either way you go with gears there are pros and cons.Im just thinking the dodge rams are around 5,000 pounds and are fine driving around with A518s and 3.55s with little 318s with little torque and they drive great in and out of town.Towing with a half ton ram with 4,000 pound load isnt even that bad.My other point was the converter will make up for the lack of steeper gears.As long as the converter isnt loose as a goose it wont hurt you with mileage.
Last edited by challengermike; 10/09/10 12:38 AM.
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Re: cruise rpm: how low is too low?
[Re: challengermike]
#819371
10/09/10 01:23 PM
10/09/10 01:23 PM
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Joined: Jan 2005
Posts: 12,419
Kalispell Mt.
HotRodDave
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My cuda weighs 3200 with me and a full tank of gas and with the 318 2bbl and 2,76 gears it was great in town and had a cubic inch to weight of 10.062 lbs per cube. Assumeing 1# tq per inch and adding the gear ratio into the equation I have 3.65 lbs TQ per inch at the axle at cruise speed.
Your barge has about 11.8 lbs per cubic inch. Assumeing both motors make the same TQ per inch then a 3.23 gear will give you about the same lbs TQ per pound of vehicle weight.
My cuda moved with authority through trafic and had plenty of guts on the freeway to pull almost any hill without downshifting. Personally I would have liked an even higher gear because of my time on the open roads but it was not a bad compromise with that gear ratio.
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: cruise rpm: how low is too low?
[Re: Twostick]
#819374
10/09/10 05:01 PM
10/09/10 05:01 PM
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Kalispell Mt.
HotRodDave
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Quote:
The vast majority of pumping loss is caused by compression followed by friction. To even consider vacuum resisting the piston going down during intake as a significant loss factor just defies logic and common sense IMHO. I'm not saying its not part of the loss, just not a significant part.
Kevin
Ever down shift into low and see how fast you car slows down? That is vaccume right there and that should give you an idea how much work the engine is doing to overcome that vaccume. Remember that a vaccume gauge plugged in the bottom of the carb is gonna show a lot less vaccume than than the top of the piston sees.
Any piston driven engine is very inefficent because of the great loss due to vaccume, compression, and friction. Only a small part of the energy the combustion process produces ends up at the end of the crank and an even smaller amount at the wheels. You can not do a lot about those 3 things, you can run a vaccume pump to increase vaccume under the piston to minimize the effects of vaccume above the piston but a vaccume pump still takes enery to operate, you can run all kinds of friction reduceing tricks like thin rings, narrow bearings, lightweight synthetic oil... but there is still gonna be a lot of friction loss, and to combat compression, well there ain't a lot you can do because reduceing the compression reduces the efficency of the tightly packed fuel burning and reduces the expansion ratio, a high expansion ratio can achived by increasing the compression ratio and by opening the ex valve later, you can reduce the dynamic compression ratio by closeing the intake valve later and this will reduce the compression loss and keep the octane requirment lower but you lose some energy pumping the mix back up the intake tract, you also lose TQ but some of that is gained back by the higher compression/expansion ratios. There are just too many losses of energy in a piston engine to make it very efficent and vaccume is definately high on the list.
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: cruise rpm: how low is too low?
[Re: stumpy]
#819376
10/10/10 01:27 AM
10/10/10 01:27 AM
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Joined: Jan 2005
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Kalispell Mt.
HotRodDave
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If the throttle is closed, or mostly closed like it is at cruise then there is nothing or very little to compress, therefor it is working against the vaccume. A gasoline engine has vaccume brakeing and diesel engines have compression brakeing.
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: cruise rpm: how low is too low?
[Re: stumpy]
#819380
10/10/10 02:05 PM
10/10/10 02:05 PM
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Joined: Jan 2005
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Kalispell Mt.
HotRodDave
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How can you have compression if there is virtually nothing going into the cylinders? Ever do a compression test and see how much differance there is with the throttle open VS closed? And that is only mabey 50 RPM not 2000 or so.
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: cruise rpm: how low is too low?
[Re: stumpy]
#819382
10/10/10 02:33 PM
10/10/10 02:33 PM
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Kalispell Mt.
HotRodDave
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Again I say, hook up a vaccume gauge, go for a drive then report back what your vaccume was at cruise. My 318 held 17 inches at 70 mph cruise. A piston area of 10.65716 pulling 17 inches of vaccume is 181.1717 inches of vaccume on each cylinder X 8 almost 1500 inches vaccume being pulled every other revolution of the crank, that is far from nothing. Again I am not saying the fight against compression is insignificant but the vaccume loss is huge also. Compressing air that is under 17plus inches vaccume into an area 1/10 the size it was is not hard when you have 14 PSI (atmospheric preasure)or so pushing the bottom of the piston up. Someone a little more mathmatically gifted than me can convert it to LBS for you so you can see how much it really takes to pull those pistons down, you WILL be suprised. I know when someone showed me I was totally shocked 
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: cruise rpm: how low is too low?
[Re: stumpy]
#819384
10/20/10 02:31 AM
10/20/10 02:31 AM
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Joined: Jan 2005
Posts: 12,419
Kalispell Mt.
HotRodDave
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Even if the compressed mix did not light off that preassure that resisted the pistons upward movement will still push it down on the power stroke, this is why a Diesel engine will not slow you down much going down a hill unless you have a jake brake, a diesel has very high compression and NO SIGNIFICANT VACCUME in the intake like a gasoline engine. I came up with some experiments for you to do stumpy. Get in your car, prefferably manual trans, go to the top of a big hill and let your car start coasting down the hill, engine off with the clutch in and the trans in 2nd or 3rd then once you get rolling real good let the clutch out, you will feel the engine rev up and the car will begin slowing drastically, now with the engine still off stomp the gas so the vaccume goes away and full compression can now be achieved, you will notice the car will not be brakeing near as hard as it did with the throttle closed and very little compression but high vaccume. That should prove beyond a shadow of doubt to you that vaccume has a much larger loss of energy than compression. Hope this helps
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: cruise rpm: how low is too low?
[Re: feets]
#819385
10/20/10 05:28 AM
10/20/10 05:28 AM
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Joined: Aug 2005
Posts: 403
30 miles west of EuroDisney
fbernard
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I regularly drive between 1600 and 2000 RPM now. Used to be hard to maintain anything under 1800 in 5th gear before (I have a tremec with .64 5th gear and 3.55 gears), but now I have a properly curved distributor, lots of initial advance [24°], and absolutely no knocking in the driveline down to almost idle (I can coast to 1200 and slowly ramp up again). Before this distributor, I could not maintain 90 km/h (56mph, the posted limit on backroads here) in 5th, I had to stay in 4th.
Of course, my car is much lighter, around 3000 pounds.
I drive to and from racetracks, usually between 2000 and 2500 RPM, at which point I'm slightly above the highway speed limit. I get 14 to 16 mpg.
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Re: cruise rpm: how low is too low?
[Re: 360view]
#819388
10/20/10 08:16 AM
10/20/10 08:16 AM
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Joined: Jan 2006
Posts: 8,162
USA
360view
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USA
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You can get HP and MPG estimations for the Imperial at different speeds on the road from here: http://www.bgsoflex.com/mpg.htmlI would guess an Imperial would have a Cd of approximately 0.50 and a frontal area of approximately 25 square feet. Taylor is using lbs of fuel divided by horsepower-hours in his graph, getting a best of about 0.5 lbs/hp-hr in the little circle in the upper left hand side. Bowling and Grippo want to use a slightly different BSFC in the units of gallons of fuel divided by horsepower-hours. Most gasoline blends are between 6.5 and 7.5 lbs per gallon density, so maybe use 7 lbs/gallon as a guess unless you want to weigh your local gasoline.
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Re: cruise rpm: how low is too low?
[Re: 360view]
#819389
10/21/10 09:12 AM
10/21/10 09:12 AM
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Joined: Jan 2006
Posts: 8,162
USA
360view
Moparts resident spammer
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Moparts resident spammer
Joined: Jan 2006
Posts: 8,162
USA
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Possible Correction: a Sept 1981 article by Jim Dunne in Popular Science magazine titled 'Low Drag Cars' lists the Chrysler Imperial of that year as having a Aerodynamic Cd of 0.41 pretty good... considering that the Porsche 924 of that year was 0.34 and a Corvette 0.45 The Eldorado of 1981 is listed as a whopping 0.55 shape at: http://www.oldcarmemories.com/content/view/51/114/sample quote "Interesting to note the aerodynamics on the 1981 Imperial was so good for its era that some NASCAR teams used the Imperial as race cars."
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