As Cheuk stated, the lag is in the turbo, not in the exhaust. With a turbo a 5" exhaust pipe would have no more turbo lag than a 2.5". Also, with turbo's sound is not much of an issue, as the exhaust pules get broken up quite a bit by the turbo itself. That is why most "turbo" mufflers are a straight through design. Turbo exhaust is inherently quieter than N/A.

I would reccomend the 3", even for low boost. That way you do not have to ever worrying about replacing it later, and you KNOW your exhaust system is not causing you any problems.

 

Another thing is if i dont install a cat would it trigger a CEL?
I might just install a high flow cat and have no worries.Without a cat might give you an extra 3 HP if lucky.!!!!

 

No cat will trigger a CEL on the stock ECU. You would need to modify the second 02 sensor to return lower voltage to fool the ECU into thinking that you have a cat.

A good high flow cat should not hurt you HP wise. Just ONE car without a cat creates as much emissions as 100 cars with cats.

 

Thanks Random!!
Im sound gonna purchase the car sound Hi flow cat.Im going with the 2.5inch inlet Cat,and doesnt even cost much.I can get it for $170US or less which is about $110US.