Mine is: Langdon headers to 2-1/2" pipe to 2-1/2" Y to single 2-1/2" pipe to a 2.5"/2.5" muffler and out 2.5" It flows very well and I'm not having any driveability issues with it. I'm also running an offy intake and 390 CFM 4 barrel.
I wouldn't go over that though. The single 2.5" works well. If you want dual go 2" or less.
