What's the secret to retaining good gas mileage in a performance application?

[Vector]

I KNOW there are motors out there delivering 400 horsepower, and still get 20mpg on the highway. What is the secret to retaining such great driveability in a motor like that? Is it keeping the compression ratio stock? I know EFI has a lot to do with it. But I'm not really sure of the specifics. Do emissions equipment have anything to do with gas mileage?

The oxygen sensor in the exhaust probably has a lot to do with it, am I right? I want to get full-length headers, and I've heard that using those will not work very well with the o2 sensor. Would this seriously affect my gas mileage?

[HotRoddin]

Originally Posted by Vector

I KNOW there are motors out there delivering 400 horsepower, and still get 20mpg on the highway.

There is !!! ??? You're gonna have to show me some "official" specs to get me to buy that one.

There is no question however that the MPG vs HP ratio has vastly improved in the last few years, and it's all about efficiency. The combustion chambers have been designed to burn fuel more efficiently, the fuel/air charge is metered more precicely, there is less friction, everything is lighter, there is a crapper load of little things they have done that all adds up to better fuel economy for the HP produced. The car factories all have huge Engineering staffs working on that stuff all the time. I don't think you can pin it down to any one or two things, it's a combination of lots of things.

Rod

[GhettoPop]

Originally Posted by Vector

I KNOW there are motors out there delivering 400 horsepower, and still get 20mpg on the highway.

Well, i doubt any fox body is doing that, but the new dodge charger/magnum/300 srt8's w/ the 6.1 425 hp hemi might get close to 20mpg highway, but keep in mind these engines only run w/ 4 cylinders on the highway to reduce fuel consumption.

I may be off your original question though. Do you have a specific stang your looking to get better MPG with? need more info

[Vector]

Well, I don't have a stang, I've got a truck with the 302. And down the line I'm looking at having heads/cam/intake/exhaust but I was wondering if it's too much to hope for that a good combination can retain decent mileage with 'light driving'. I get about 15-17mpg city 20 highway. Is it too much to hope that I can keep it like that? With more power available and torque won't the engine have an easier time of it anyway?

Also the o2 sensor, will it hurt mileage to use long-tube headers? Thanks!

[4barrel]

If I'm not mistaken, the new 6.0 ls2 Corvette gets 28 MPG, right?

Originally Posted by 4barrel

If I'm not mistaken, the new 6.0 ls2 Corvette gets 28 MPG, right?

WHAT??!! I don't see how that's at all possible. A 6.0 liter engine getting almost 30mpg??!! I'm calling B.S.

Originally Posted by Vector

Well, I don't have a stang, I've got a truck with the 302. And down the line I'm looking at having heads/cam/intake/exhaust but I was wondering if it's too much to hope for that a good combination can retain decent mileage with 'light driving'. I get about 15-17mpg city 20 highway. Is it too much to hope that I can keep it like that? With more power available and torque won't the engine have an easier time of it anyway?

Also the o2 sensor, will it hurt mileage to use long-tube headers? Thanks!

I doubt you'll be able to retain your current 15-17mpg with aftermarket heads, cam & intake. I bet you'll be able to stay close to those #s though. Maybe somewhere between 13-16mpg with a well tuned motor & a light foot. When you're into modifying cars or trucks to go faster, and produce more power...getting good gas mileage just isn't part of the deal really. Unfortunately you can't have your cake & eat it too.
As for the oxygen sensor...the header company might be able to help you on where to tap into the header, or the exhaust tubing and thread in the O2 sensor. I'm pretty sure as long as it's within 18" from the exit of one of the headers...you're truck's (or car) computer can still adjust accordingly. I'd do some research with header companies first. Look into the reputable ones and talk to their techline...if they have one.

[lx mike]

I have a heads, cam, intake and longtube headers and i get around 20-23 mpg on the highway. that with a 5 speed and 3.73 gears. i have around 290-300 rwhp.

[4barrel]

Originally Posted by Simi Stang

WHAT??!! I don't see how that's at all possible. A 6.0 liter engine getting almost 30mpg??!! I'm calling B.S.

In the April 2005 issue of 5.0 Mustang and Super Fords, Steve Turner wrote in his article, "The new Corvette gets 28 mpg."

But don't ever take someones word for anything. Always go see for yourself.
http://www.chevrolet.com/corvette/sp...s/#fueleconomy
Fuel Economy Engine EPA Estimated MPG city/hwy.
6.0L LS2 V8
4-speed automatic transmission 18/26
6-speed manual transmission 18/28

[Hethj7]

I was making a little less power than Mike (about 265 at the wheels) and got 24 mpg or even a little better on the highway. However, it drastically dropped off once I was off the highway, to the tune of 13 mpg or so (if I was strictly in the city).

[HotRoddin]

Originally Posted by 4barrel

http://www.chevrolet.com/corvette/sp...s/#fueleconomy
Fuel Economy Engine EPA Estimated MPG city/hwy.
6.0L LS2 V8
4-speed automatic transmission 18/26
6-speed manual transmission 18/28

Falling from an airplane maybe lol ... or in some Chevy Engineers dreams

Seriously, I might buy that if it runs on 4 cyls on the open road like GhettoPop said the Magnum did, otherwise Chevy must have tested it on solid rubber tires, stripped to bare fiberglass with a 40 MPH tailwind.
Remember also that all the veh manufactures are in hot water right now for vastly overstating fuel economy.

Rod

[GhettoPop]

that vette 28 mpg from the highway seems overtsated as HotRoddin mentioned. have you ever bought a new car that got the manufacturer's stated highway MPG? no one i know has.............the vette is good for low 20's MPG on highway, case closed..........who cares anyway, its an f'ing chevy!

back to real vehichles equipted w/ 302's, Vector, w/ heads/cam/intake your milage will go down, but i think the power added will be more than worth the trade off! how many miles do you really make? unless you do TONS of driving, i would not worry about it. if you do make tons of milage, buy a 4 cyl daily driver and make that 302 a real monster!!!!! and BTW, the long tube headers might increase your MPG, even if just slightly.

[Vector]

Originally Posted by GhettoPop

that vette 28 mpg from the highway seems overtsated as HotRoddin mentioned. have you ever bought a new car that got the manufacturer's stated highway MPG? no one i know has.............the vette is good for low 20's MPG on highway, case closed..........who cares anyway, its an f'ing chevy!

back to real vehichles equipted w/ 302's, Vector, w/ heads/cam/intake your milage will go down, but i think the power added will be more than worth the trade off! how many miles do you really make? unless you do TONS of driving, i would not worry about it. if you do make tons of milage, buy a 4 cyl daily driver and make that 302 a real monster!!!!! and BTW, the long tube headers might increase your MPG, even if just slightly.

Hey, yeah I really do want to make this thing a monster . I only drive to and from work every day, so I don't put a ton of miles on it really. I asked about the long-tubes because this book tells me the oxygen sensor being so much farther down-stream in the exhaust system, it does not have as great an accuracy for measuring unburnt hydrocarbons. Thus it can't adjust the air/fuel ratio as efficiently, thus perhaps it would impact gas mileage. I don't know.

[4barrel]

I haven't test drove a new 'Vette. I just know what the envoice on the window says at the dealerships. I think it does use the 4-6-8 cylinder cut out on the highway.

[Gearhead999]

Keep your foot out of it !!!!

Will improve mileage.

[Unit 5302]

If you want a camshaft, stick with a low duration/high lift scenario. Match your components. Course, you'd be better off going with a turbocharger/centrifugal supercharger setup that is not building a lot of boost below 2500rpm.

Keep your gears in the 3.73's or taller neighborhood. Have the engine dyno tuned to make sure the entire setup is working together properly.

The biggest part of fuel economy was mentioned above. Efficiency. If your engine is running well, you'll get a lot better fuel economy.

Weight is your enemy as well, but lets be honest, there is a limited amount of weight you can pull out of a 4 passenger daily driver without starting to sacrifice your driving experience. The 5.0 engine rocks the house on fuel economy when it's running right. Toss the 4.6L into a lake. I was probably in the 260hp range with my 87GT, and 27-28mpg on the highway was standard.