Im building my motor going to be sleeved ls vtec. Aiming for somewhere around 400+ its my daily. I have a greddy sp2 i know its tiny but will it hurt my motor? I know i can make a lot more power easily with a 3" but can i just up the boost to reach my goal with this small exhaust. For example i made 400whp with a 3" exhaust at 15psi, can i just do like 18psi with the 60mm exhaust so it can make the same power as with the 3"? or is this bad for the motor and not make it reliable?

 

Why not just put a 3"

 

Where are you getting these numbers from and arriving at these weird conclusions? Sounds like you're asking if you should make the engine work harder to overcome exhaust restriction to make the same power. That doesn't sound too bright to me and that you're asking a question you already know the answer to. Also not too bright. Maybe you should have asked or searched for "turbo when to go to 3" exhaust". I found this.
http://www.tercelreference.com/terce...st_theory.html

"As for 2.5" vs. 3.0", the "best" turboback exhaust depends on the amount of flow, or horsepower. At 250 hp, 2.5" is fine. Going to 3" at this power level won't get you much, if anything, other than a louder exhaust note. 300 hp and you're definitely suboptimal with 2.5". For 400-450 hp, even 3" is on the small side.”
Now you should research if any of that is accurate.

 

if all you are looking for is 400 and nothing more 2.5" will make it but its not optimal

350hp+ should be 3" for maximum performance