http://autos.msn.com/research/vip/Spec_Glance.aspx?year=1995&make=Eagle&model=Talon&trimid=-1
the 2g was very close for gas millage but differant engines were used.

http://autos.msn.com/research/vip/Spec_Glance.aspx?year=2007&make=Chevrolet&model=Cobalt&trimid=-1
the supercharged colbalt get worse milage than the na colbalt.

first off, to eliminate the chassis and driveline from the process, you need to look at the fuel consumption for a given engine output level with the engine on a stand.

Next, there's no magic involved -- making a certain amount of HP requires a certain amount of air, which also requires a certain amount of fuel. Of course, any engine makes power that you cannot measure at the crank.. this is power used to turn the alternator, or to warm the engine, or to pump the engines oil, etc.

It is my opinion that given two engines making identical peak power at the same RPM, with similar internal losses and accessory loads, the NA engine will use less fuel.

Of course, where are you going to find 2 identical engines that make the same amount of HP in both NA and turbo versions? If one of them is a 3 liter V6 vs a 2 liter I4 turbo, the internal differences will be intrinsically different due to different # of bearings, balance shafts, # of valves, etc.

I think as a practical matter, a smaller, lower displacement turbo engine can make similar peak power when its boosting, but be lighter and consume less fuel when its not boosting. The A4 2.0T gets better milages than the A4 3.2 V6.

Under boost i think an NA engine does better. Turbo combustion is less efficient than NA combustion i think, and most turbos run rich under boost. running rich == using more fuel than strictly necessary.

In around town driving, my A4 2.0T gets better mileage than my 2.8L NA V6 passat, which gets MUCH better mileage than my 3.5L I6 NA M5. The A4 is 200hp, the VW is 190hp, and the M5 is ~250hp. The A4 is actually the heaviest of the vehicles and has the most drivetrain losses. It's also got the most efficient engine design with the most modern technology.

KA-T_240;181426 wrote:
i actually have no personal expeince. never drove on the highway with my car. I have delivered them but dont fill the tank up. i got just under 27 with a auto, which in 240s when stock gets better gasmileage. but it was -20 so thats a big side affect right there

I also think I would have better gas mileage with a bigger turbo, because it would take more rpm to spool. on the highway at 80mph, I am at 3200-3300rpm in 5th gear. which when I am full throttle at the rpm at am in the upper teens in boost. so if it hit a bump oops 10psi, lol

Type much?

It all depends on your tune. It doesnt matter whether your turbo or N/A when you aren't in boost. most turbo people just have their cars tuned to be lean at low RPMS. Lean obviously means better gas mileage. i guess air swirling thought a turbo/Intercooler/ic piping might make a tiny bit of difference on gas mileage, but I highly doubt it.

Volumetric Efficiency

I also think I would have better gas mileage with a bigger turbo, because it would take more rpm to spool. on the highway at 80mph, I am at 3200-3300rpm in 5th gear. which when I am full throttle at the rpm at am in the upper teens in boost. so if it hit a bump oops 10psi, lol

if so, IIRC, a turbo vehicle should get better gasmileage while cruising on the highway. I am happy, i got 29mpg with the 240 driving 80 on way back from souix falls with no cruise control.