[cmiovino]

If you're just changing the catback, it's not going to make any noticeable difference in power, even on the dyno being NA. The main power restriction is the header (and tune), which is where 90% of the gains you make come from. A few HP here and there can be picked up from changing the front pipe and overpipe.


The Invidia is a ~2.5 inch exhaust, whereas the MAP one you have is a 3". For NA applications, the 2 - 2.5 inch is generally going to make the most power because of gas velocity flow and how engines work. Bunch of science behind it, but there is an ideal diameter that accelerates the gasses out as quick as possible, increases power.


When you add a turbo, that all changes. In general, downstream of the turbo, you want a large exhaust, so 3" is going to make the most power. It's upstream of the turbo you deal more with piping sizes, spooling, gas velocity, etc.


I'd keep in mind what power level you're aiming for. If the turbo kit is going to produce something like 350+whp, then the 3" is going to be more beneficial. If you're doing a smaller kit with quicker spool or being more conservative in power, the 2.5" is likely plenty.


We're still taking small gains though. I'd personally prioritize sound and drone over 5-10whp when you're talking in the 250-300whp range of power.