hello people's: I'd go to his site for sizes but I believe it reduces down going to the turbo. So I'd say it's 3.5 and reducing to accept the 3" pipe cumming from the turbo.
Just to let you know he told me he is making a 3.5 MAFP to make the whole pc. 3.5" He said a couple of months and that was in March 2011.
IBBY