I'm going to be installing an 06 rsx s muffler on my car and even though I realize this will kill some power on a built motor,Im wondering what size exhaust pipng would be best from the cat-back for my b18 which I have already listed a build sheet for,basically 180whp 140 wtq.Would 2.5 be sufficient?Or should I ditch the type s muffler and go 3"?This will be a street driven car thats why Im going for a quiet-ish car.Also, can a set-up like this be tuned to pass a test only smog?

well piping can vary on a motor. I am assuming your are staying N/A, so 2.5" should be fine, but if you ever plan on boosting 3" would be recommended. As far as emmissions just have a stock cat, or another cat somewhere. Emmissions can vary depending on where u go, some people willj ust hand them out and other will reject your for the smallest things.

2.5" will support you fine NA