so ive got a 98 1.8tq that is all stock right now but getting a chip in the near future. im getting a custom exhaust fabed up this week and i am wondering if i run 2.5" from the flex-pipe to a mid-car mounted muffler and then 3" the rest of the way if i will will get enough back pressure as to not hinder the performance.

 

The turbine wheel causes more than enough backpressure for the engine. You want no restrictions past the turbo. The best would be no exhaust, but you need some for the o2's. If you plan on sticking with a stock turbo, 2.5" will be fine, if you ever plan on going bigger, 3" would be more ideal

 

Our car's don't have overboosting problems so you never need to worry about back pressure. My old RX-7 though if I wanted to go 3" exhaust I needed to make sure I ported the waste gate too or it would over boost.