I am going to put together true duals (over the axle) this summer and plan to run 2 1/2 out the back. Would it be better to go down to 2 1/2 inch right after the collectors (before the x-pipe) or right after the x-pipe, or will this make any difference at all? Sorry if this is a stupid question, just didn't know if there was any difference in performance, sound, etc.

 

It will hurt performance to downsize the piping. I don't think it will matter where you do it though, either way it will be a restriction.

 

After. There has been a couple local cars that did a 3" X dumped and then did 2.5" under axel pipes. No performance loss either. No matter what it'll still flow better than any 3" catback.

 

on a bolt on car...dual 2.5" exhaust won't be a real restriction...

relative to dual 3" i could be...but you won't lose hp from it i don't think.

i would save the money and weight and ground clearance by going 2.5" right off the collectors