since a good flowing exhaust will be less restrictive on the engine, does it improve the fuel efficiency of your car? Before/After MPG readings will be greatly appreciated

 

i would think no.. (you will have a loss of MPG) it will eliminate the back pressure resulting in the car running a little richer at times due to the loss of low end power from no back pressure on a stock motor.

 

I can still get 28-29 mpg with 75% highway driving with my 92 Si with high flow cat and Thermal exhaust.

I really didn't notice a difference in gas mileage after I put my exhaust on. Let's just say I know it didn't go down.

 

Backpressure is never a good thing, and wont improve anything. The volumetric efficiency of the exhaust system is most likely tuned in the mid-high rpm range, if you want to improve gas efficiency and dont mind sacrificing top end a bit, go 1/4 inch smaller piping to give that cruising highway rpm of around 3000 the best flow.

Now if your going for top end fuel efficiency then you want a good 4-2-1 header.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by TraKtioN &raquo;</TD></TR><TR><TD CLASS="quote">Backpressure is never a good thing, and wont improve anything. The volumetric efficiency of the exhaust system is most likely tuned in the mid-high rpm range, if you want to improve gas efficiency and dont mind sacrificing top end a bit, go 1/4 inch smaller piping to give that cruising highway rpm of around 3000 the best flow.

Now if your going for top end fuel efficiency then you want a good 4-2-1 header.</TD></TR></TABLE>

im confused here you say that backpressure isnt needed but then you say get smaller piping which increases backpressure? so what should i do to get more power get a bigger exhaust or a smaller one? please explain

IMO fiddling with the exhaust wont do much for fuel economy i doubt you would even notice it...so might as well leave it alone