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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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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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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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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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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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Irving, TX
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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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Posts: 8,180
Detroit, MI
CokeBottleKid
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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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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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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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