Exhaust system mods are made for 2 principal reasons: to improve the sound and to increase power. For the following discussion I'd like to set aside sound improvements, and focus exclusively on increasing engine power. I apologize that the following is a bit long-winded, but it's a long logic chain...
There are many aftermarket systems that replace the factory silencer (or "muffler" or "catback"). Several have been carefully tested, and the results posted here show that as much as ~20 HP can be obtained with these systems. The systems achieve these gains by reducing the flow restriction (or "backpressure") from the silencer. The OEM system restricts the flow through 2 mechanisms: the "main catalysts" that are integral to the system, and the rather tortured path the exhaust gases must take to exit the system (here are some good pictures of the internal design of the OEM system
Droning aftermarket exhaust). The aftermarket silencers eliminate the main catalysts, and usually use a "straight-through" flow path that reduces the restriction to the flow.
The other important component of the exhaust system is the exhaust manifold. The OEM system features a small secondary catalyst that is integral to the exhaust manifold. This smaller catalyst was added to reduce the time required for the catalyst to warm up to operating temperature, and thus to become fully effective. By making the secondary catalyst smaller and much closer to the engine, the time required for it to heat up is reduced. This catalyst has oxygen sensors both upstream and downstream of the catalyst, and the ECU (Engine Control Unit) computer monitors the performance of these catalysts using data from the sensors.
The secondary catalyst provides a significant restriction to the flow. In my opinion, the restriction provided by the secondary catalyst is larger than the restriction provided by the main catalyst, since it is so much smaller in cross-sectional area. The main catalysts are larger in diameter so as to reduce their restriction to the flow, but there is no room for such a large-diameter catalyst on the exhaust manifold. The smaller catalyst will rob more power since it is more restrictive.
The need to mount a catalyst very close to the engine makes it impossible to have long, equal-length primary tubes from the exhaust ports to the collector. Exhaust manifolds that feature more or less equal length primary tubes that are long enough to properly resonate near the engine's power peak are commonly called "headers", and headers have been shown time and again to be the most effective design for increasing engine power. But you can't have headers AND have the catalyst close to the engine. The OEM manifold doesn't even try to have equal-length primary tubes, although some attention was given to this in the 986 Boxster S (see
06 987 Boxster S "high-performance" headers).
A final observation regarding the secondary catalysts mounted on the exhaust manifolds is that these catalysts apparently have a very harsh operating environment. Some aftermarket header systems that include secondary catalysts have had serious problems with catalyst failures in service.
So, from a performance point of view, the ideal exhaust system would have headers with long equal-length primary tubes, no catalysts at all, and a straight-through silencer. This might be appropriate for racing, but for street use it would be too loud, you would get CEL's (Check Engine Lights) from the oxygen sensors, and the complete omission of catalysts would be irresponsible. If you change your ECU software you could eliminate the CEL's, but the other objections remain.
If we take that ideal performance system, and then try to eliminate the issues for operation on the street, what is the best compromise? Well, we need to have a catalyst somewhere in the system. The catalyst provides a cleaner exhaust, and also reduces the noise level to more neighbor-friendly levels.
The usual approach is to eliminate the main, relatively low-restriction catalyst (in the OEM silencer), and leave the more-restrictive, more fragile, secondary catalyst in the OEM exhaust manifold. Why not reverse this? Why not use well-designed headers with no catalysts (several vendors offer these), hook these up to the existing silencer's main catalysts, and then modify or replace the too-restrictive OEM silencer with a more straight-through design?
Such a system would be less restrictive than the usual approach of using OEM headers with a low-restriction catback, and would not suffer the durability problems that "race" catalysts mounted to the exhaust manifolds have sometimes exhibited. As a final step the oxygen sensors could be relocated to the main catalysts, or the ECU software could be modified to ignore their data.
I think a system like this could have excellent performance (+25 HP??), good durability, and should be far less expensive than the "complete systems" that provide excellent performance, but cost $5000 or more.
