My 2 cents, and I'm not a Turbo expert, but I know that on alot of smaller 4 cylinder motors such as my Turbo Mini Cooper, we went with a 3" exhaust because of the amount of boost we were running on the front end, which is 18psi.
I think all the Turbo aps for the 370z from Greddy, AAM, GTM, etc. are in the 6 to 9psi range, because of the hi-compression pistons in our motor. I know a lot of the experts out there recommend running 3" in anything that is pushing over 300whp. how much more power you could get out of the 3" is to be seen, IMO I think you could probably get to 500whp with a 2.5 exhaust that has good flow. There is a lot more to flow then just the diameter of the tubing, cats, resonators, mufflers, tubing bends all play a role. Bottom line is if you are going to be for sure boosting the car later on, you should probably talk with whoever is going to be supplying the kit and building it, most of them are designing the exhaust around that application.
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