Most aftermarket headers include downpipes that I have seen, and they also say it bolts up to the OE exhaust flange. My question is this: If I get (for example) an OBX header & DP, and have a custom 2.5" exhaust made all the way back from the DP (test pipe in place of Cat) the flange where the OE Cat. bolts to the OE is what diameter? My current line of thinking is that if the flange where DP meets test pipe (2.2" or so?) that even if I have full 2.5 the rest of the way back, there is still a bottleneck/restriction at that flange.

What have those of you with full performance exhausts done? Make a custom DP? Or is the flange where DP meets Cat/testpipe not an issue?

Shoot me some ideas, thanks DSMers.

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2012 2500HD LML

so far, i got a "hi flow" cat(straight through) and n1 catback. I'm going to get a header/downpipe soon. The way that I figure, is that the hi flow cat bolted right up to the stock downpipe, and the aftermarket header will bolt up to the stock cat, so therefore, the aftermarket header should bolt up to the hi flow cat without any problem, that's just my way of thinking, I may be wrong.

1996 Eagle Talon ESi

intake, apexi N1 exhaust, hi flow cat
17" hyperblack exel tz-7's
next: AFX UDP and 60 mm throttle body

true I assumed that as well, however I was wondering if anyone considered that an issue in the exhaust system and if anyone has done anything to modify that point in specific.

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2012 2500HD LML

I have the greddy header, and custom 2 1/2 pipin back to my muffler.
No Cat
No Testpipe

we used the flange off the stock exhaust and cleaned it up so that it would fit evenly.

The greddy header will mount straight up to a stock set up.

Hope That helped
-Gabe-

My greddy header hooks up with the 2.5" greddy downpipe, which bolts to a 2.5" high flow cat, which mates up with a 2.5" greddy evo. If your downpipe was only 2.25" and you're adding a 2.5" catback, you really shouldn't notice that much of a difference. If anything, you'll notice a loss in bottom end, I did. But who really cares about anything under 3000rpms anyways? Topend is alot more fun anyways.

I have a 4-1 header.
Stock cat (soon to be replaced by a test pipe)
Custom 2.5" exhaust pipe with a glasspack resonator.
Muffler with removable silencer.

Most aftermarket headers, test pipes, and exhausts come with at least a 2.5" flange. Have fun !

John and I were thinking that, with my Pacesetter, we might cut off the weird compression joint or whatever where the DP meets up with the flange for the cat and replace it with a flex section to the rest of the system. Hopefully we'll be doing this all soon.