OK, so I have always heard that adding a custom exhaust system to your car will give you better fuel economy because there is less back pressure. However, before my system came in, I was getting around 22-24 mpg. With my system installed, and my driving as slow as (if not slower than) before, i get about 19 mpg tops. I was wondering what anyone elses exhaust has done to their fuel economy? And if the systems have helped, which one do you have?

 

anyone?

 

i don't know who told you that but they were wrong. every time i have put an exhaust on any car the milage always goes down. sorry friend

 

how much though? and which have you put on?

 

i'm a very spirited driver and i get anywhere from 15.5 to 19 on a good tank. but i have gotten up to 23 strait freeway road trip. i have the greddy evo 2 now and haven't had a chance to check the milage since i put it on.

 

why does a higher flowing exhaust hurt fuel economy then? it seems like it would help...

 

alright i answered my own question. i got the fast intentions and it boosted my fuel economy by 2-3

 

I noticed a huge hit in mpg when I had my Borla exhaust on for about a week. I saw about a 25% drop with no change in driving style. The hell with that!!!

 

its gonna be alot worse..

i cant get enough of the sound so im always on the gas...

 

yea i noticed a 2-3 mpg drop when i put my XO2 on. but when i put the fast intentions on, it increased like 2 mpg on average.

 

Exhaust on a modern OBD2 car should not matter since the A/F ratio is fixed by O2 and ecu. Things close to exhaust ports [cats] set most of engine backpressure.

Measure back pressure in front of 1st cat with and without any exhaust system period you will be surprise at how little it changes.

At idle,cruise and low-middle rpms there is almost no back pressure.
Back pressure comes in at WOT and torque peak and above rpm where no one considers mpg anyway.

 

I agree with what you're saying for the most part, but high flow exhausts (specifically true dual) do impact the exhaust velocity hence the reason why the low end power is impacted when adding these setups. The impact in exhaust is most likely the primary cause for the impacted gas mileage because the flow in and out of the head is impacted. In the end, the MAF is richening the A/F, at least at part-throttle. I think with a little tuning though, you can restore most of that lost MPG. I took off my TD because I didn't like the volume and vibration.

 

Automotive Engineers avoid these forums like the plague because of all the old wives tails and false info the members promote as truth.
When they try to correct misconceptions in Physics they usually fail because of lack of scientific education of members.

On these tiny 3.5 liter engines there isn't much exhaust flow to begin with.
The hot exhaust expands out as the exhaust valve is opened not because the piston pushes it out on upstroke. Just like people believe the piston sucks in the intake charge rather than the Earth's atmospheric pressure [14.7 psi superchargerlike] filling the low pressure void.

 

aftermarket exhaust= faster driving = less mpg. However unless your car kicked up the air/fuel ratio as far as more fuel, then it could have. However, our cars tend to run lean with exhaust and air intake mods.

 

In all the cars that I have put a "free flow" exhaust, the mileage went down. Why, you ask? Because I could never keep my foot out of it.