Hey so I realize this is probably a bit of a newb question, but i'd rather ask then mess anything up on my car. I'm thinking about getting a midpipe for my 8, probably the racing beat dual rez or the Agency Power. Both of these are 3" pipes, and I am currently running a Greddy SE exhaust on my car, which has 2.5" piping. Could this potentially cause any issues for me? like built up back pressure from the midpipe to the catback? I'd appreciate any help or advice on the manner. Thanks.

Your not going to have any issues i use to have greddy exhaust which was 2.5 inch with a 3inch and never had any problems at all. But all in all u try try the search button next time

well if you look at it mathematically, the 2 2.5" pipes in the axle back will have an increased volume over the 3" piping. Now, velocities may change, but I didn't go to far into fluid dynamics when I did physics in college so I'm not sure if they would go up or down or mostly stay the same. I would assume that they would stay mostly the same since the twin 2.5" pipes will have a combined volume slightly greater then the 3" pipe, but that would only factor in if you were able to flow said volume out of the engine, the stock header would be the point of restriction or choke point in that setup.

There is a thread surrounding this topic somewhere..... 3in or larger, even catless.