on an 05 exhaust, is it 2 1/2'' (f150 stx) I tryed the search and it never pulled anything up.

 

The stock exhaust pipe is 2.5"--at least on my FX4.

 

so then whats the point of changing the whole pipe setup if most places just give you a 2 1/2 in set up, but thanks man.

 

stock piping on the 4.6L and 5.4L is 2.5" the pipe on the 4.2L is 2.25" and the diesels use 3"

 

Just make sure you clean up the factory Y-pipe where the two 2 1/2" pipes combine into one 2 1/2" pipe. Even the most popular after-market systems install after the Y-pipe and use 3" piping from there back. But guess what? That factory Y-pipe exits at 2 1/2" and no matter what you do after that, you still pass everything through that 2 1/2" hole first. Either go with a proper dual 2 1/2" inlet to 3" outlet Y-pipe or go dual 2 1/2" pipes into either dual mufflers or a double inlet single muffler. I chose the DI/SO setup myself.