Torque effect on mpg.
Thread Starter
Burning Brakes
Joined: Sep 2006
Posts: 967
Likes: 0
Torque effect on mpg.
I've never been one to really scoff at what I spend in gas for a car like a corvette but I started thinking this morning that when I do some work to the engine and improve the torque and horse power I may end up making the mpg better.
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My theory is that if the engine makes better torque at lower rpms then the car will be moved with less effort. This means that to cruise at the same speed the engine is working easier and at a lower rpm.
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Wouldn't this also mean that at that lower rpm its burning less fuel therefor getting more mileage per gallon?
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Am I way off here?
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My theory is that if the engine makes better torque at lower rpms then the car will be moved with less effort. This means that to cruise at the same speed the engine is working easier and at a lower rpm.
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Wouldn't this also mean that at that lower rpm its burning less fuel therefor getting more mileage per gallon?
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Am I way off here?
Melting Slicks
Joined: Mar 2004
Posts: 3,472
Likes: 18
From: Lake Ozark, MO
From what little I know... the principle sounds...sound. I see it all the time with modified 4 wheel drive trucks where the guys install the 36/44 inch tires that change the gearing so much that the engine operates outside it "torque zone" and get lousy fuel mileage without a gearing change.
Le Mans Master
Joined: Jan 2007
Posts: 5,485
Likes: 7
From: Peekskill NY
I'm not so sure more HP & torgue will give you better milage based on your theory. RPM's will still be the same at specific speeds because of the rear gear ratio and transmission. Those two components have the largest impact on gas milage in my opinion.
My vette has an M20 wide ratio trans & 3.08 gears in the rear. At 60 it's at about 2,300 rpm. My friends 70 with a M21 & 3.36 gears revs at 3,000 rpm @ 60 mph. I'll get better milage.
Glenn
My vette has an M20 wide ratio trans & 3.08 gears in the rear. At 60 it's at about 2,300 rpm. My friends 70 with a M21 & 3.36 gears revs at 3,000 rpm @ 60 mph. I'll get better milage.
Glenn
Race Director
Joined: Nov 1999
Posts: 10,643
Likes: 8
From: Foxfield CO 1970 Convertible
If you can raise the torque enough to run in a higher gear (or better yet, get an overdrive transmission) then you will see an increase in mpg. Of course, if you enjoy the torque so much that you end up hot-rodding around everywhere and flooring it at each light, your mileage will decrease... 
Instructor
Joined: Mar 2006
Posts: 124
Likes: 1
From: Bowmanville Ontario
I remember reading in a government fuel efficiency brochure that an engine is running at it's peak efficiency when it's running at it's peak torque. So for maximum fuel economy I'd an build the engine to peak it's torque at the speed/RPM that you normally drive at and make as much torque as you can at the RPM's lower than that.
Your car won't be fast but it'll be quick! ( And your horsepower numbers won't be impressive to the idiots who only look at that number)
Your RPM's won't change, but your throttle position will.
Your car won't be fast but it'll be quick! ( And your horsepower numbers won't be impressive to the idiots who only look at that number)
Your RPM's won't change, but your throttle position will.
Instructor
Joined: Jul 2007
Posts: 245
Likes: 2
From: Hope Indiana
With all due respect, your logic is "off". You have to think about this in terms of efficiency, or bsfc. If you do something to improve performance that makes the engine more efficient at pumping air, reduce friction, better fuel atomization, etc then you'll get both a hp boost and a mpg decrease. If you're simply cramming more air and consequently fuel into the cylinders then you're not really making the engine more efficient.
If you prefer to think in terms of torque, think about this.... Really in all actuality you're rarely driving your car "on the torque curve" anyway. For a given gear ratio, vehicle weight, and aerodynamics, unless you change your driving style you're not affecting the torque you are actually using out of your engine. But, if you reduce the parasitic torque on the engine (again pumping work, friction, etc) you're making your engine behave more efficiently and therefore using less fuel to make the same amount of torque at the rear wheels.
It's all boils down to efficiency... plain and simple.
If you prefer to think in terms of torque, think about this.... Really in all actuality you're rarely driving your car "on the torque curve" anyway. For a given gear ratio, vehicle weight, and aerodynamics, unless you change your driving style you're not affecting the torque you are actually using out of your engine. But, if you reduce the parasitic torque on the engine (again pumping work, friction, etc) you're making your engine behave more efficiently and therefore using less fuel to make the same amount of torque at the rear wheels.
It's all boils down to efficiency... plain and simple.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Efficiency is a function of getting the SAME amount of torque from an engine with LESS fuel. Having an engine which has the ability for more torque or more torque at lower rpm's doesn't necessarily mean it is more efficient....just more POTENT. There are more powerful engines which have been tuned well and are efficient at low power levels...but most of them are likely an accidental ocurrence. People who want more power usually want high power at full throttle and at higher rpm zones. Folks who want efficiency set the engine/carb/dist. up much differently.
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
GD70 and Z-man have it right. The slower an engine turns the better the mileage. The biggest contributor to better mileage in the last 30 years is use of overdrive transmissions. Second would be fuel injection and third a computer that acts as a dozen screwdrivers and wrenches making adjustments as the engine goes through it's cycles.
A slower turning engine (and drive-train) doesn't have to fight frictional forces which aren't linear. Similar to wind resistance.
So if an engine with a peak torque at 3000 RPM is geared to run 60 MPH at 2000 RPM it will get better mileage than the same engine geared to run 60 MPH at 3000 RPM.
Here's something I've always wanted to study. The relationship of engine noise level vs. torque and HP.
A slower turning engine (and drive-train) doesn't have to fight frictional forces which aren't linear. Similar to wind resistance.
So if an engine with a peak torque at 3000 RPM is geared to run 60 MPH at 2000 RPM it will get better mileage than the same engine geared to run 60 MPH at 3000 RPM.
Here's something I've always wanted to study. The relationship of engine noise level vs. torque and HP.
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Instructor
Joined: Jul 2007
Posts: 245
Likes: 2
From: Hope Indiana
But you're right, if the poster is only looking for better mpg, then slowing down that engine gives you the biggest bang for the buck.
Drifting
Joined: Apr 2007
Posts: 1,892
Likes: 3
From: Springfield Illinois
I would say your kind of thinking on the right track, but its off a hair. My comparison my 73 L82 DD and my 75 C20 454. The 454 has seen as much as 15 MPG with a 4.11.. The overall ratio with the tire comes to about the same as a Corvette with 3.70's as far as RPM's go. The best I've ever gotten out of my L82 is 14 mpg with 3.55's. The cons about the 454, the truck is as flat as a wall, sets at a height of most 4x4's, and is one of the heaviest beast I've ever tried to push. The pro's about the vette - less weight, def. more areodynamic, higher gearing, and less cubes to fill. Therefore, the 454 will catch more air, push something heaver, at higher RPMs, has more cubes to fill, and still gets bette gas mileage than the vette. Eventhough it is 1 mile, just think of the possiblities once it is in the vette.
Where I think your off is that you can't add more torque and HP to your current engine to get better MPG... You would have to get a pretty lopy cam that will have a lot of valve overlap (good for performance - terrible for gas mileage).
Remember the old saying "No Replacement for Displacement"
Where I think your off is that you can't add more torque and HP to your current engine to get better MPG... You would have to get a pretty lopy cam that will have a lot of valve overlap (good for performance - terrible for gas mileage).
Remember the old saying "No Replacement for Displacement"
Drifting
Joined: Jun 2005
Posts: 1,950
Likes: 498
From: Chino Hills CA
It's not all that complicated. Stock puny engines generally have very mild cams with low lift and very little overlap, small intake and exhaust runners and small carbs. This makes a high velocity charge at low rpms which is very efficient. Large ports, intake and exhaust, carbs, cams and valves, do not have high velocities at low rpm which are not very efficient at low rpm, that's the price you pay for horsepower. Build something in the middle and you'll get back something in the middle. Carbs cannot compete with fuel injection from an efficiency point of view, however, quadrajets have very small primaries, and for a carb, are pretty efficient at low load and rpm. my 2 cents.
Le Mans Master
Joined: May 2006
Posts: 7,708
Likes: 266
From: indy indiana
2 more cents worth and just throwing this into the mix.
One of the Ford Pickup magazines did a test years ago. Twas a 300,6cyl truck,pulling a race car on a trailer. Where ever it was they tested, there was a slight incline. They tested time and MPG,each way. With no other change except for putting in a 460 V-8, they repeated the test. Of course the 460 was faster but the surpriseing thing was the 460 got better gas milage even going up hill compared to the 6 banger going down hill.( remenber,it a SLIGHT oncline.) so, more horsepower/tourque, less you have to put your foot into it.
One of the Ford Pickup magazines did a test years ago. Twas a 300,6cyl truck,pulling a race car on a trailer. Where ever it was they tested, there was a slight incline. They tested time and MPG,each way. With no other change except for putting in a 460 V-8, they repeated the test. Of course the 460 was faster but the surpriseing thing was the 460 got better gas milage even going up hill compared to the 6 banger going down hill.( remenber,it a SLIGHT oncline.) so, more horsepower/tourque, less you have to put your foot into it.
