What is the diff in a 2.5in and a 3.0in cat back

 

A half inch?

 

lol i no that but i mean performance wise

 

You won't get a definate answer like 5.24rwhp, 7.93rwtq. Besides, they don't make 2.5" cat-backs (well maybe for imports but only up to the muffler). Almost all cat-backs for our cars are 3" and from there they get bigger like the Mufflex cat-back. So making a decision should be easy: 3.0" cat-back

 

how bout about 69% more volume?

or simpler cross sectional area

A=(pie)r^2

do the math...

 

figured that it whould probly make more hp

 

most have 3" I-pipers, all that I seen, just some have 2.5 and others 3" exhaust outlets. I'd definetly think 3" tailpipes would flow more, but not sure how much it'd help.