I plan on having custom exhaust piping made for my 99 protege ES 1.8L as a cat-back set-up. I have noticed that most people seem to have listed 2.25" as their size of piping. Is this the biggest I can go? I haven't measured, but how big is that compared to the stock piping? Can it be any bigger? Or is bigger bad? What?
THANKS
-James

 

Larger piping reduces exhaust velocity, and thus exhaust scavaging, which hurts low-end and mid-range torque. You give up a small amount of top-end, but not enough to offset the loss of low-end, especially for autocrossing or street driving.

There is no reason to go bigger than 2" on a street driven car (unless you are planning to go turbo near-term), although for drag racers, the extra flow will help a little. Your butt-dyno may tell you that a 2.25" makes the car more powerful, but that is likely due to greater noise.

Of course, YMMV.

 

Butt-dyno. You sleigh me.

 

Anything larger than 2.25" is overkill on an N/A motor that only revs to 6800 rpm. Larger pipe will just make more noise and kill torque; in order to see more power from 2.5" or larger pipe, the engine would need to rev a lot higher than the ECU will let it.

BTW I assume your car does not have a turbo. If it does, you want BIG pipe- 2.5" at least, 3"+ preferably.