Well i was just wondering at what point a 3" exaust is really needed for proper air flow? I just got a new borla muffler, and the dude put it on, and he had to use some 2.5" section of pipe all bent and curved to make it fit right, but i have 3" before the muffler, but 2.5" after the cat, and 2.5" resonator. It seems to pull better than the magnaflow that i had on there, but with the way this new muffler is mounted, i have to think that all this piping is flowing like a 2.5" exaust. I hate the way the muffler looks by how its mounted, and the tip sticks out too far, i'm just not happy with it i guess. So i am thinking of getting the Pacesetter cat back, witch is a true mandrel bent 2.5" cat back, and i will try and use the borla muffler thats on there now. Is 2.5" piping all i would need for running 8 psi? I only plan on runing no more than 8 psi until the motor is rebuilt, so do you guys think that 2.5" will be plenty for 8 psi? I *want* to get the Hahn 2.5" o2 housing, and downpipe in the near future for a true 2.5" turbo back set up. Is this enough? What do you guys think?

Well its generally accepted that 2.5" is good for up to 11sec cars so my guess it wouldn't be needed to at least past 15psi (although I'm sure it wont hurt whatever your running). I wouldn't worry about it until crossing the 300hp mark

2.5" will be just fine. They say 2.5" is good up to ~500hp....not that 3" would hurt though :)

All I know is i had that nasty short hrc downpipe plumbed into stock piping to a gutted cat, and then a non-turbo greddy sp catback to finish it off when I bought the car... I really didnt think of checking the exhaust and I would make over 20psi runs, hehe, probably wasnt the smartest thing to do, but ohh well. Anyways, I had a 3'' downpipe custom made, 3'' flex section, and eliminated the cat and o2 sensors bungs, other than one incase i wanna hookup the o2 for the DFI. I had a senseless force clutch at the time and it was slipping, so I was only running 10psi and i felt a BIG difference...plus the clutch was slipping 10x worst, so i know i was making more power, so the next day they dropped off the greddy exhaust, and put a 4'' straight back.. damn, once again, great difference, turbo spooled so much faster after the exhaust was reworked from the previous owner being a moron. and I sold the greddy unit and paid for the downpipe and exhaust with the $$ i got :-)

i agree with brad...i didn't have a 2.5 exhausso i can't give you a difference of the two, but it gave me huge gains with a 3" exhaust, but i have mix piping 'cause of the star downpipe. the star down pipe is 2.75" then it goes to a no cat 3" exhaust then to a 2.75 resonator, and the rest is 3". unfortunately it's press bended, but hey it still flows waaaay better than stock. and it sounds good too, not loud like a v8 truck or anything, but bassy. but it sounds awesome when WOT. and your revving sounds mean as hell too. when i raced my friend with a turbo 16g civic. in the 1/4mile i beat him by 2 1/2 car length with the stock piping, with the 3" ,i raced him again and i killed him by like 3.5 to 4 car lenghts, and with a good launch. after that is what i realize it's a good idea for wider exhaust flow when boosting. try it trust me you'll thank us later.

WOW Brad. I really like the DP you got on there. Looks much nicer than the HRC issue one.

Heh, tell me about it. 175 bucks including flex section. custom is most always better, or in my eyes atleast...