I'm getting a new exhaust on my jeep (i6 4.0L). I'm upgrading it to 2.5" w/a Carsound cat and Flowmaster muffler. The piping up TO the muffler is already 2.5" but AFTER the muffler the piping (tailpipe) goes down to 2.25", the length of this piece is a few feet long. Some people say to leave it and its fine but I'd rather up the tailpipe to 2.5" too. I'm having a little difficulty finding a prebent mandrel piece in 2.5", but i did find one in 3". I know you can go too big for piping size, but would this piece matter since its after the cat and muffler? could it still be a problem?

 

Any change in size along the piping can be an obstruction. Does the flowmaster have one or two exits? If it splits to dual exaust tips, then 2.25 should be fine.