there seem to be a bit of conflict on how much exhaust is to much Mike K. seems to think that any thing over 2 1/4 would only hurt power.(on a stock engine) i'm getting a asp header and thinking of adding a 2 1/2 cat back system on a pretty much stock nx2k. only other mod's are a pulley set and probly a O.E.M intake to soon follow. has any one had a negative effect, because of too little back pressure. Thank's any possitve feed back would be great

I have a full 2.5" exhaust in my current (soon to be removed ) SR20 with nothing more than an intake/timing advance and it pulls really good up top. Down low i noticed a slight drop in low end torque (could be from a small exhaust leak at the secondary flange, meh.) but a big gain in mid to high end power.

I've had them both 2.5 & 2.125 on my full bolt on(-cams) SER. Dynoed both....
It's true 2.5 will ultimately generate the highest HP numbers. BUT I didn't begin to see any real different in HP until with 2.5 until at least 6500RPMs...then there was a difference in the end of about 5 extra HP up top.
Down low you will give up some torque... probably 5-8lb. until break even point of about 5500. This is presuming a good mandrel bent design in both cases. With cams added it's a different story. You'll give up a little more down low .but then the sacrifice becomes worth it because the numbers up top really jump.
You need to ask yourself where do you spend most of of your time or where do you want the most power. For me ,as this is my daily, when I finally made my choice, giving up much of anything down low was not worth it, when you figure with all bolt ons you might generate 135-140lb. of torque, if you're lucky.

If you are gonna have a ASP header you will definitely want a 2.5" exhaust.

Thanks, that kinda goes along with what i was think. i think i'll still get the exhaust but not until a few other mods.

Proper size also depends on the bends on the exhaust. Regular crush bent tubing, or mandrel bending. I ran 2.5 inch piping, but the bends were crush bent, so they shrank a little bit. I thought that it went quite well that way. I managed to make 143 WHP with that exhaust + the following..

PR cold air intake
Highport intake cam
SSAC header
Test pipe