Im using the pi x r-squared formula to get area to see whats better... a 3.5 inch turbo pipe.... or a dual 2.5 from the cats back.... From ultimate racing... it seems the dual combined has a larger area so it would be better than the single pipe right? Am i doing this correctly?

 

yep, i dunno which is better but thats the right equation

 

I've always heard that for turbo's bigger is better.

(2.5/2)^2*pi*2 = 9.82

(3.5/2)^2*pi = 9.62

Overall the 2.5" has just a bit more total x-sectional area. However, if you look at the flow profile of a fluid in a tube, it will slow down near the surface and be the fastest in the center. Your 2.5" pipe has more surface and would therefore restrict the flow more than the 3.5". Since the two are so close in x-sectional area, I believe that the greater surface of the 2.5" would restrict the flow enough to overcome its slight increase in area and therefore cause a slightly greater restriction overall.

Therefore, the 3.5" would be better.

-D'oh!