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So many shops have NEVER seen a 928, we often get cut some slack because they aren't sure about what they should be seeing. Full length headers on my 91 Mustang would not make it past the visual inspection more than maybe 10% of the time at BEST, but I don't know of any 928's that have failed due to headers, and I have seen more than a few.
Cat back is not smog related, just the normal do you have a muffler type inspection by Highway Patrol etc. so RMB is usually no issue as long as it isn't insanely loud.
Getting the sound "right" means a lot to me, more than plus or minus a few HP, so I am still fiddling a bit with my exhaust. I don't think outside of a track car the benefits of headers are worth the chance of failing an inspection or the increased heat in the engine compartment.
X or Y merge pipe and high flow modern cats have real torque and hp gains, so both are part of any planned exhaust for myself. Use OBD II style cats with a Carb EO# and inspection should go fine. Mufflers are a matter of personal taste, I like a single two or three chamber Magnaflow at the rear.
Thermal barrier coating is very appealing to me, so I am planning on factory 85/86 exhaust manifolds with JetHot extreme coating. I'm not sure on doing the merge (head pipes) yet mostly due to cost and chance I may redo the design again. Reducing under foot temps though sounds worth the money.
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