I wouldn't add a Y if I were you. Go with a 2 in 1 out Magnaflow if you're just doing a muffler swap. Shailey over on the other forum showed good gains with this setup if I recall...Fetzer said:I've been reading up on here and seen alot of you guys have done the magnaflow 12256 muffler swap. That muffler is 1 in and 1 out, so I'm assuming your putting in a " Y " pipe before the muffler? I just looked under my truck and the muffler on my 07 is 2 in and 1 out. ????