why do 3400's get better mpg than 3100's?

[no_doz]

is this only due to the heads & intake-or gearing? im pretty well baffled because i just looked on fueleconomy.gov & the 3400 equipped fwds (other than the van) got better mpg highway & as good if not better in town than the 3100 equipped cars-for small & large port 3100's-is this simply tranny gearing or ?

[robertisaar]

probably gearing... almost all small port 3100s got 3.33 gears, the large port got 3.08(?), the 3400s might have gone into the 2.xx range.

[asylummotorsports]

Perhaps because they are simply more efficient.

My 3500 that runs low 14's, yet it gets 25+ mpg at 3500 RPM (70 mph) on the hwy.

[robertisaar]

Perhaps because they are simply more efficient.

My 3500 that runs low 14's, yet it gets 25+ mpg at 3500 RPM (70 mph) on the hwy.

3500 =/= 3400...

the laws of thermodynamics state that a vehicle with a larger bore SHOULD get worse fuel economy with all else being equal due to a larger bore not completeing as much of the combustion process before it gets exhausted.

obviously: not everything is the same, and since we know the large port intakes, exhaust, cams and heads are the same between 3100 and 3400, it narrows down what is different to somewhere other than the engine. there MIGHT be a compression increase, but i'm not positive on it.

i know the latest 3400 montes had tall gears, but not sure on the exact number...

[IsaacHayes]

My car is 2.97 FDR....

The tune perhaps. Lean burn, lots of timing..

[robertisaar]

My car is 2.97 FDR....

The tune perhaps. Lean burn, lots of timing..

EPA would NEVER allow lean burn... timing: a larger bore requires more timing, so yes, there would be more than a 3100, but if its proportinate to the increase in bore, i don't know.

by most: i meant the heavier cars with 3100s. unfortunately L bodies died out after 1996. i'll look up the a-body numbers in a sec...

EDIT: here's tranny info in the ZIP. 2.86 in 2000-2003 on 3400 W-bodies...

[no_doz]

any exact numbers on what the large port 3100's got?

also what ratio did the supercharged GTP boat anchormobile's get?

[robertisaar]

in the interest of fairness, i'll only include w-body 3100s...

2000-2004 3100 W-body - 3.05

1997-2004 W-body L67 - 2.93 except 2004 GTP with TAPShift, that got a 3.29

[asylummotorsports]

Maybe my tune is just better than most.

On two seperate 700 mile trips (95% HWY) I recorded 25+ (US)mpg at 3500 RPM

[robertisaar]

entirely possible. a correctly tuned(meaning non emissions restricted) vehicle will always pull better fuel economy than the GM tune...

[SofaKingWeToddDid]

if the engine makes more power, it takes less of its percentage of overall power to idle or cruise the car... right?

[no_doz]

thats what i figure-meaning you can run lower rpms, & with a better tune less mpg-what gear ratios are the 5 & 6 speed fwd trans?

[robertisaar]

if the engine makes more power, it takes less of its percentage of overall power to idle or cruise the car... right?

when you start talking about that type of thing, you're diving into dynamic compression. if you open the throttle farther, the (dynamic) compression goes up, meaning better efficiency.

running taller gears takes advantage of this, since you'll have to put your foot farther down to keep the same speed at a lower RPM with the same motor.

"what gear ratios are the 5 & 6 speed fwd trans?"

there is a 5 speed? i know only of the 6.

here is info on it:

http://en.wikipedia.org/wiki/GM-Ford_6-speed_automatic_transmission

[no_doz]

the fwd 5 speeds gm mated to 3.1's & LQ1's

[robertisaar]

the fwd 5 speeds gm mated to 3.1's & LQ1's

i thought you meant autos...

so F40, 284 and 282?

F40 and a few others:

http://www.fastfieros.com/NEW_manual_transmission_ratios.htm#top

284 and a LOT more, including 282

http://bstuff.com/index.php?showtopic=45488&mode=threaded&pid=561583