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.

2000 at 65 MPH would be about 2300 at 75. With the right cam, that would probably give optimal fuel consumption on a fairly level road. Moderate hill climbing and acceleration may need a downshift though... kinda like my Hemi Durango that runs about 2100 at 75. In the DD Durango that's fine. I often take long trips and appreciate the (relativly) good fuel economy. Downshift to accelerate or climb a grade >4% is OK.

In a hot rod I'm not sure I'd like the same setup. Its fun having power in reserve without downshift. I guess 200-300 RPM more at cruise would make a difference... or more displacement. Have you thought about making the 440 a 493 or 496?

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.

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.

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 dont know if it helps or not but my 86 buick t type has 3.42s a 2004r with lu(3,200 stall) and 26 inch tires and at 78 it runs at 2,350 rpm.I have no problem with cruise pickup,it only takes a second for the turbo to spool up.And i would have way less torque than you do.Even with 28 inch tall tires no pickup problems either.

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

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.

Nobody else is having a problem with his post so maybe it just you who doesn't get it.

I have 2.45 gears in my 73 Charger with a 400 and in 3rd gear it easily cruises at 65-80mph and will pull up any hill easily without needing to downshift or give it much more throttle. My 400 has stock pistons, a mild cam, and a 670cfm 4bbl so I would think a 440 in a C-body would pull about as good as my 400 B-body.

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=2
Edelbrock 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.

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.

I don't think I'd want to go any lower than 2000 at 65mph. Any lower and I think you risk just lugging the engine when you hit a hill or a strong headwind. But it also depends a lot on your cam, compression and the rest of the combo. IMO if you get it between 2000-2300 at 65 you will be doing just fine.

Quote:

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My friend said if I am cruising the car at 2000 2200 I will still be using the idle feed in the carb.

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Your thermoquad will be off the idle speed circuit before that rpm.

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

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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.

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.