View Single Post
Old 01-03-2012, 01:54 PM   #3
geaux_tigerss's Avatar
Drives: 2018 Summit White 2SS M6 Redline
Join Date: Nov 2011
Location: Orange, TX
Posts: 593
Exhaust size doesn't matter to header selection. You do need to be sure of the cat back size so that you get appropriately sized hf cats or connecting pipes. I have seen both 2.5" and 3" cat backs.

That said, take a piece of string and wrap it around your cat back piping in a straight run (not on an elbow). Do not stretch the string, but wrap it around the pipe and mark the string where it meets itself. Measure the length of marked string. This will give you the circumference of the piping. Then it's a simple d=C/pi to figure diameter (7.85" for 2.5" and 9.42" for 3"). I'm not sure if the 2.5" and 3" diameters refer to inside diameter or outside, so the measurement may not be exact due to wall thickness. I would guess that less than 9.42" circumference is 2.5" and greater is 3".

However, make sure to check the sleeve clamp where the exhaust meets the stock manifolds. Many 3" catbacks neck down to 2.5" to be compatible with the stock manifolds.
2018 2SS Summit White M6, NPP, Redline
geaux_tigerss is offline   Reply With Quote