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The stock 964 exhaust system consists of the following:
Cylinder 4-5-6 heat exchanger/header, which flows forward and then leftward to join the other side
Cylinder 1-2-3 heat exchanger/header, which flows forward and then leftward after the flow merges with the other side
Catalytic converter, which flows rearward, has an O2 sensor bung on its inlet side, and bolts to the heat exchanger assembly with a gasket and four bolts (this is the part that results in heat on the left side of the car such as steam after a drive in the rain)
Primary muffler, which sits inside the rear bumper and flows rightward, connecting to the catalytic converter with a spherical kind of fitting
Secondary muffler, which sits inside the right corner of the car, reverses the flow of exhaust gases coming forward from the primary muffler and then going rearward through the tailpipe, connecting to both of those with the same kind of fitting as the primary muffler and cat connect with (this is responsible for steam on the right side of the car after driving in the rain)
Tailpipe, which is supported only by its fitting to the secondary muffler
Heat cross-over pipe, which connects to the rear clean air side of both of the heat exchangers and has an inlet that is fed by the big electric HVAC fan on the left top side of the engine, which itself gets air from underneath the engine shroud that gets pumped in by the engine-driven cooling fan
Flexible ducts, one on the front of each heat exchanger, that direct hot, clean air from the heat exchangers up to the HVAC system
Common modifications:
1. Primary muffler bypass / "cup pipe": my car has this from a previous owner and sounds great, very livable but you know when the throttle is open
2. Secondary muffler bypass / "G pipe": this replaces the secondary muffler and is named for its general shape, just reversing the flow of exhaust gases without also muffling them, supposedly also sounds good
3. Cat bypass / "test pipe": this replaces the catalytic converter and should have an O2 sensor bung so your ECU does not get confused, which is not a problem because it is designed for the O2 sensor to read before the catalytic converter and thus the bypass has no effect on it
4. Replace the primary muffler and secondary muffler with a single, high-performance muffler that sits in the location of the old primary muffler: Fabspeed is one of the many offering this solution
5. Replace everything from the heat exchangers on with new headers: Fabspeed also offers this and it supposedly has a big impact on performance because the two cylinder banks do not have a merged exhaust flow as the stock system has
Nobody agrees on the actual benefits of any particular modification.
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