Originally Posted by K-Man S
Porsche implements the cats necessary to meet sound and pollution restrictions and keep their fleet emissions low so as to avoid any penalties. Cats, depending upon design, may not be all that restrictive, although I've never tested the Porsche ones, the dyno difference between headers with cats and headers without typically show no improvement in power for catless headers and in some instance have shown less power than those with cats. The secondary cats and muffler design, however, do appear to be restrictive in that they dull the HP of the car by a good 15hp or so.
If you remove them, then yes you would technically be illegal and your car should be for off-road use only. Some states will check and might care, other states will not.
Some have also theorized that Porsche intentionally gave the Cayman a restrictive exhaust to hold it back from the 911, while this may be true, I think there is also something involved with the resonance factor coming from the engine and trying to provide a resonance-free system that meets european noise restrictions.
I don't strictly agree with this statement , real world testing on the benefits of de catting standard manifolds (headers) would suggest there is a gain from removing the primary cats and leaving the secondaries in place. Bigger gains may be achieved from removing or changing the type of cat in the secondary or fitting high efficency cats in the manifold and deleting the secondary items: 987 Gen 1 - All Models Manifold / Header De Cat - Articles
The official porsche literature I've seen relating to the inclusion of cats near the cylinder head on the 987 model , suggests it was done to comply with EU class 4 emissions and beyond. Hence the secondary air pump and other changes to the inlet and exhaust system , water pump etc on the 987 from the previous 986. The cats in the manifolds(headers) are placed so they heat up and work effectively in the quickest time possible , the secondary cats will reach full efficiency some time afterwards as they take longer to heat up. I suspect both cats in the 987 are 3 way type as the emissions are compliant with either the primary or secondary units removed , providing the engine and exhaust/catalyst is up to temperature. Obviously if you remove both cats you will fail emissions so its important to balance the gains against emissions and either fit sport cats or only remove one set of originals.
Regarding the design of the gen 1 cayman headers , these are definitely not optimised as comparison to the "sport" headers fitted to the 987 3.2 shows a different design theory and slightly smaller tube diameter. The 3.2S has both smaller valve size , less lift and smaller TB , compared to a cayman 3.4 (so it flows less gas), so it doesn't make sense that the cayman would have a smaller exhaust diameter or an unequal branch length manifold. The design of the cayman 3.4 exhaust header is identical to that of the early M96 engine 2.7 boxster 987 and later M97 2.7 unit in the cayman/boxster 987 (2006 onwards).
The gen 2 headers use a more equal length branch design and larger cat diameter , I can only summise this reduces back pressure and optimises torque performance as a result.
In terms of resonance , the 3.2 equal branch length manifold is a quieter manifold compared to the unequal branch design of the Cayman . A de catted 3.2 manifold is a lot closer to standard in terms of dB level compared to the equivalent cayman item. The size of the cat cartridge is almost identical between 3.2 and 3.4 manifold cats , so its not a case of resonance chamber effect.