3.5" or 4" dp
Postmaster
Joined: Feb 2002
Posts: 2,773
Likes: 0
From: Bonney Lake, WA
Though it wouldn't seem like it, a 4" pipe will flow 30% more air than a 3.5" pipe. I'm not sure what that equates to in lower EGT's, but it's logical that it would be substantial when under load.
Posting Guru
Joined: Feb 2004
Posts: 2,248
Likes: 1
From: Elverson, PA
The 4" downpipe doesn't really make you lose torque - it just moves it up the rpm a bit. If you're going to be running lower HP settings, a 3.5 downpipe would be fine - but if you're looking for bigger power a 4" dp is the way to go. This holds true for the Superduties - not the 1st gen. PSD's. I think the biggest dp you can fit on the earlier trucks is a 3.5 - and that takes some massaging of the firewall.
