Quote Originally Posted by Vteckidd View Post
What size are the primaries on the DC USDM? they should be close to 1 7/8th almost all B series headers nowadays are close to that or 1.75in etc bloh blahb blah

You would be surprised how much a 2.5in collector makes. Chop off the last 8in of your USDM header and weld in a 2.5in pipe like my Narrow header and youll see good gains.
I think you're right on the size, although I haven't measured for sure. I already did the 2.5" collector mod, but I can see how it could be improved... especially if I'm going B20 bottom end.