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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: 28,067
Irving, TX
feets
OP
Senior Management
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OP
Senior Management
Joined: Jan 2003
Posts: 28,067
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
Joined: Sep 2007
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: 28,067
Irving, TX
feets
OP
Senior Management
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OP
Senior Management
Joined: Jan 2003
Posts: 28,067
Irving, TX
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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
Joined: Sep 2007
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
master
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master
Joined: Jan 2003
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: 28,067
Irving, TX
feets
OP
Senior Management
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OP
Senior Management
Joined: Jan 2003
Posts: 28,067
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
Unregistered
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Anonymous
Unregistered
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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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