Go check the window stickers at the local dealerships. The 2.73 LS1's get the same gas mileage as 3.23 LS1's. The 3.73 Lightnings get the same gas mileage as the 3.55 Lightnings.

Engineermike, are you serious? First off, I give the manufacturers window sticker all the credibility it deserves - which in my book is almost none. Just because they used the same data for both vehicles doesnt make it accurate. If I'm not mistaken, I think they use a "base line" fuel economy number for all vehicles of the same model. They certainly don't do mileage tests on every possible vehicle option combination agreed? Logic, empirical evidence & physics all indicate that for 2 vehicles with identical specs except final drive ratios, will get different fuel mileage readings when driven at the same speed over the same route in the same conditions. The engines are simply turning at different sppeds....
I would love to hear your explanation of why final drive ratios would NOT affect mileage....

I agree with all the other ascertions though...reducing drag coeficient, parasitic losses, etc, but I'm going to support the idea that mileage can & will be improved with a taller final drive ratio.

 

tanmanski, you bring up an interesting point.

An engine has the lowest BSFC near peak torque at WOT. By definition, an engine is most efficient at its lowest BSFC. What I don't know is what happens to BSFC at part throttle. My guess is that it increases since engines are most efficient at full throttle (a la diesel, which doesn't even have a throttle blade).

A Lightning makes peak torque at 3500 rpm and, thus, is most efficient at 3500. Perhaps the 3.73 gears put the engine closer to 3500 rpm at cruise.

Obviously, there are limits to this. Mileage would suffer if you gear too far in either direction. Imagine having a 1:1 rear axle. Cruise rpm might be around 400 rpm in OD, which couldn't be good for mileage. Same goes in the other direction. I don't think a 4.56 gear would help mileage any.

The manufacturer's, by the way, have different gas mileage ratings for each engine and transmission combination, for a total of 4 different gas mileage ratings for the F-body line. If rear end gears had a large effect, with only 2 possible rear end ratios, it seems reasonable that they change the window sticker.

Mike

 

More air in and out also = more fuel added to make more power.


More isnt good for mileage, the only increase you will see is from less parasitic loss (resistance to flow)

 

gears would be a big help 2.73's will help even if u have 3.23's, for a 3gpm it might be wort h the few hundered

 

Damn, this thread got super technical.
I wondered in looking for a simple yes or no. sheash.

 

Yes and no.

Fuel mileage and how to get it isn't simple at all. If it were just a matter of a CAI or higher flow muffler, or some sheet metal blades in the intake (wonder why that hasn't come up here?) to give 1, 2 or 3+ mpg on a car, we wold see them on all OEM vehicles. Remember they have to achieve certain corporate average mpg figures or pay stiff fines.

FWIW, it takes a certain amount of rwhp to maintain a given speed in a given vehicle. Burning the air and fuel produces some indicated hp in the engine. Subtract friction, pumping losses, accessory losses, etc. and you get flywheel hp. Now subtract driveline losses to get rwhp. (Obviously you could start at rwhp and go forward also.) To maintain a speed, only that amount of power is produced. Any more and the vehicle accelerates, while any less it slows down. If it take air in easier, it wil be able to operate with less throttle opening, but if the efficiency doesn't change, the same amount of fuel will be burned.

So, anything that decreases the losses helps fuel economy. Friction and pumping losses are pretty much proportional to engine size. Smaller is better. That's why Displacement on Demand works: closing the valves on half the cylinders just about halves the pumping losses. Each closed cylinder effectively has a "piston return spring" in the air cushion developed.

Simple questions rarely have simple answers.

 

OldSStroker, about your point on extra pumping losses with more displacement. . . The extra piston size or cylinders forces the engine to compress more air on the compression stroke, but you get this energy back during the power stroke. I never thought about it from this standpoint, but, as Gale states, the piston does have to do work on the intake stroke since it's working against vacuum. Larger engine = more work on the intake stroke = less gas mileage.

One other big advantage of smaller engines on gas mileage: The higher the cylinder pressure during combustion, the more power you get out of the same amount of fuel. This is made obvious by the fact that higher compression means more power even though the air and fuel flows are the same. BUT. . . at cruise, you are throttling incoming air down to -10 psi, so when you compress it, cylinder pressure is still very low. However, a smaller engine must operate with more cylinder pressure to maintain the same power, thus cruise speed, as a larger engine. Sooo. . . more cylinder pressure means higher efficiency and less fuel used.

Diesel engines, with no throttle valve, have high compression ratios and full atmospheric pressure in the intake at all times. So, cylinder pressure is higher and efficiency goes up.

Mike

 

Good points, Mike.

You don't get back 100% of course, so smaller is usually better for cruise. How much? GM says about 8% better on an Displacement On Demand (DOD) LS truck engine in EPA testing with only 4 of 8 cylinders working. Up to 25% better under some conditions. However, you still have all the bearing and cylinder wall friction of the 6 L engine even if it's only firng on 3 L of displacement. Of course when you demand more go, it's available.

On the down side for little engines, when the driver signals for a given performance (acceleration) level, the smaller engine may need to rev higher, use more torque converter multiplication and lower gears to achieve the same performance level as the lazy larger engine turning very slowly. That's more friction, pumping, and accessory loss, so unless the little engine is MUCH more efficient (BSFC), it may not get better real world economy in the higher load conditions. If you've driven in the Rockies much with underpowered cars you find them in lower gears or at WOT a lot more than higher power/weight vehicles.

OEM's keep developing the flow, swirl, combustion process, etc. so that they can use 87 octane and still maintain the power/torque the got from 92 octane. Sometimes they are able to increase CR at the same time. This helps mileage, as well as fuel cost/mile of course.

...and the beat goes on.

 

. . . as can be seen in an S2000 versus ZO6 comparison. Gas mileage is almost identical even though one has only about 40% the displacement. The S2000 engine makes alot of hp/liter, but is apparently pretty inefficient at cruise. Maybe it turns 4500 rpm at normal cruise speeds.

Mike