When I first got the combo together and running I emailed Ed Curtis asking about the 2.5" vs 3". I originally told him i'd be running 3" in his cam sheet but due to money I kept my old 2.5" H pipe. He told me not to worry right now as long as I have plans for the 3". He said i'll be leaving a few hp on the table up top (which is what I care about) So yeah, 3" is what my combo needs.