the size of the exhaust depends on the size of the exhaust side of the turbo, most people run a 3" with just about every turbo, i know the gt3076 is actually a 2.5 inch outlet, i think the gt35r is a 3", im not sure about any of the bigger turbos though
the biggest of T3 exhaust housings is 3" i think and even T4's and GT series turbos end at 4" exhaust but the GT42RS might be 5" outlet, but anything will do, 3.5" will work but im going to try and do something else with my setup, i still have to talk to someone about it and see if it will work, for a big turbo the bigger is often the better, for 400-500 HP you could use up to 4" but your going to have problems with it moving around and with clearance problems when it crosses over and around the rear suspension with an exhaust bigger then 3", and you shouldnt have to worry about about problems, you will notice less lag i would say if you went from a 3" O2 housing into a 3.5" downpipe and catback exhaust, i hope that information helps
I have a 3" 02 with my turbo kit and I am just exploring the possibility of maybe going with a 3.5-4" dp/exhaust. Another possibility is to do a new 3.5" 02 section and mate that up to either a 3.5"-4" exhaust. Just something to think about doing in the future?
Anyone care to guess what kind of hp+ going from a 3" to a 3.5/4" dp/exhaust? Say for a 500whp car?
im not sure about 500 HP but i heard of a case of a 700 HP supra gaining something like 50-60 WHP just be changing from a 3" downpipe with 4" catback to a 4" downpipe mating to the 4" catback so im pretty sure it will at least be a hefty gain over a 3" at 500whp on our cars, im sure it wouldnt be very heard to get someone to do a custom 3.5/4" O2 housing into a turboback side exit, at the side though i would recommend maybe making a transfer from the 4" pipe into two 2 1/2" pipes for the tip to help out on ground clearance, the only thing is i would check with any of the vendors on here that make exhaust to see if it can clear over and around the rear suspension in the back but those bends will hurt performance, for a big exhaust like that to get the most out of it i recommend a side exit