How do "Performance Mods" effect a car's economy???

[Mario_Marques]

Hi! i don't know what kind of car you have, but in some cases performance mods(when done right) are good for mpg, i'm one successfully case of that and i've done other good works in other cars from my clients, my car stock was about 6,5L/100 now it is 5,1L/100 and it's way faster and safe and i dont need to care too much about driving to get that.
One thing about performance, the idea of tuning is to set you vehicle to get the most performance from the fuel you inject(of course that if you inject more fuel, it will be faster but also will decrease your mpg, air only don't create horsepower, you need fuel to burn heheeheh), if you get the best, when you're not pushing too hard your fuel economy will be good.
you can also tune weight, aero, suspension(if your car has good handling you dont need to be so hard on the brakes).
gearbox and clutch should work fine, wheel alignment, good flow on intake, etc, etc.
If your car is a tdi feel free to contact me, i'm from europe but i can provide you good information, i work a lot on this engines(my english is not so good as my mechanic skills )

[Backtobasics]

I think the answer lies in whom the question is from. An ecomodder, or an average driver?
For an average driver, who uses more throttle, then the common performance modifications (Intake, exhaust, and computer programming)will probably yield better MPG.
Computer upgrades are usually units that reflash the computer, and they frequently add additional timing.
Intake modifications lower air intake restrictions, and when coupled with someone who uses the throttle can have lower pumping loses.
Exhaust modifications also reduce pumping loses, which also show as exhaust restrictions.

[Backtobasics]

I am a firm believer in exhaust efficiency. I have seen may stock exhaust manifolds, and they scream restriction. I can't begin to see how a moderate tubular header, monolith / honeycomb style converter, and low restriction muffler can NOT benefit MPG. While it may move TQ a bit, from a sheer restriction point of outgoing exhaust, the above items have to be more efficient, offering lower restriction, and there for requiring less power to PUSH the exhaust out of the cylinder.

[some_other_dave]

I agree with you about the exhaust--if the new exhaust is optimized for low-RPM flow. I would be pleasantly surprised if you could find any tuned header setup that was actually optimized for 2,000 RPM, which is where most of our engines are the most efficient.

Most or all headers are tuned to improve flow well up in the RPM range, and more often than not they sacrifice low RPM flow to do that. So most headers will tend to worsen your MPG.

There are exceptions, particularly when the stock system is really hideous. For example, a friend of mine has a classic V8 Mustang. His fuel economy went from astonishingly awful to merely horrible when he swapped the stock exhaust for a set of headers.

-soD

[stovie]

you can also get a economy cam, i can get one for my jeep at 198 dollars and it says it will increase FE by 2-3 mpg and move the torque band from 1500-3000 to idle- 2100 witch would be great for my jeep sense i don't go over 2000 rpm except when i need to speed up quickly and i hit only 2100 rpm's at max(i also think that if your ecodriving that 2-3 mpg gain could hit 4-6 or more)

I was thinking that smoothing your intake could help alot to because you don't need the tubulence in the intake if you have MPFI or direct injection so then a rough intake only hinders flow(i also believe that this kind of performance stuff is best on manual vehicles then automatics because you can control your gearing better with a manual to optimize your new power band for FE)

[gone-ot]

...headers typically tend to make the engine run slightly "leaner" because they more thoroughly remove the spent gases (sorta like interia-cleaning EGR) from the cylinders--usually! But NOT always across the engines' RPM band, because at 'some' speeds, the headers won't be "tuned" and reversion can occur.

...if the catalytic-converter & O2-sensor(s) are still there, behind the headers, then the ECU will compensate...making more HP at some RPMs and then making better MPG at others.

...if you consistently drive where MPG improves, you'll get better MPG; but if you drive at RPMs where HP improves, your MPG will suffer.