I have an 87 Trans Am, 305 TPI stock, I just ordered from Summit, a dynamax hi flow cat and a magnaflow cross flow muffler, im having a shop I deal with custom bend the pipes. now for my question, I went with 2.5 inch all the way, is that good enough considering my car is stock and not facing any real crazy mods in the future, ( ford injectors and fastchip ) or should I have went with 3 inch, I read the post about back pressure but will it matter if I am running all stock pretty much except a KN air fliter, should I see a decent power increase? So far I have followed the mods on the suggested mod page for tpi cars KN filters, synthetic oil platinum plugs so far so good, any help or insight will be appreciated,

I would think 2.5" would be sufficient if your not running 5000rpm+ all the time. I'm no expert on this so a more experienced exhaust guy should know. I went with the 3" in my sig, last cat-back ill ever buy even if i up the cubes motor wise, im also dealing with a guy on hooker 2055 headers.