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to si and mikedsilva
first: How did I know that Bill V would provide superb insights? Maybe history? (Just giving Bill a hard time, for I've found his insights super helpful for my 1993 964 over the years.)
i cannot answer the back pressure question (but i bet i know who can and will!).
As Bill V and others have accurately pointed out, 964 exhaust (starting with the cat) is really just three pieces (plus some connecting parts of course).
FWIW, I've run my 964 with all possible combinations of exhaust parts from the cat and down. My car came with "cat delete" and "G pipe" (=secondary muffler bypass). I've since purchased the "cup pipe" (from Sid at Vertex. made by Schnell, IIRC), as well as a great OEM secondary muffler.
OK, so i now had essentially an exhaust sandbox in which to play.
Running the primary muffler bypass (="cup pipe") and the secondary muffler bypass (="g-pipe" or "swan pipe") at the same time resulted in headaches within 3 minutes for me. way too loud, outside and in, at any rpm range. and my BUTT DYNO indicated that there was only performance degradation in both HP and torque.
(Perhaps that's an indicator of need for back pressure, Bill V?)
more to come shortly
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88 Venetian Blue Coupe "B"--gone, but not forgotten
93 964 C2 Coupe "BNC" ("PNC" on some days!)--PSS9s, Big Reds, Strut Brace, Recaro SRDs, Schroth 5-pts, DAS bar
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