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  1. #1
    Zatara's Avatar
    Zatara is offline Porsche Enthusiast
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    Why a second set of catalytic converters?

    Why did Porsche design a two cat system? Most of the after market exhaust systems for street use tout the fact the 2d cat is not necessary. Naturally this is true in terms of not affecting the O2 sensor, but since the cats are necessarily restrictive to the flow of exhaust gases, why would Porsche go to the expense of the 2d set of cats?

    Put another way, what am I doing, besides improving exhaust flow, when I buy an after market exhaust such as APE and bypass (remove) the 2nd catalytics?


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    1BlinkGone is offline Porsche Prophet
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    Re: Why a second set of catalytic converters?

    There are 2 and 3-stage cats. My guess is that Porsche is doing stage 3 in a separate unit instead of all in one canister. I am not certain of this, however. They could be two 3-way cats in series for all I know.

    FWIW, I was also pleased to find out that in CA you no longer have to go with an OEM cat replacement like we used to.
    (legislated into law somewhere around 1998 fwd, we had to install high-cost factory OEM units when they went bad )

    Now we can buy CARB-approved aftermkt units as we did before 1998. Go figure. Hope somebody here can give you some more clarity to your question.

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    RobMason's Avatar
    RobMason is offline Porsche Orator
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    Re: Why a second set of catalytic converters?

    Quote Originally Posted by Zatara View Post
    Why did Porsche design a two cat system? Most of the after market exhaust systems for street use tout the fact the 2d cat is not necessary. Naturally this is true in terms of not affecting the O2 sensor, but since the cats are necessarily restrictive to the flow of exhaust gases, why would Porsche go to the expense of the 2d set of cats?

    Put another way, what am I doing, besides improving exhaust flow, when I buy an after market exhaust such as APE and bypass (remove) the 2nd catalytics?
    The second cats help clean up the exhaust gasses when the engine is first started up and before the primary cats are heated up to operate at full efficiency. After the primary front cats are heated up, the secondary cats provide little additional benefit. That is why you get your car smog tested when it is at normal operating temperature. This is what I have read before.

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    Re: Why a second set of catalytic converters?

    Quote Originally Posted by Zatara View Post
    Why did Porsche design a two cat system? Most of the after market exhaust systems for street use tout the fact the 2d cat is not necessary. Naturally this is true in terms of not affecting the O2 sensor, but since the cats are necessarily restrictive to the flow of exhaust gases, why would Porsche go to the expense of the 2d set of cats?

    Put another way, what am I doing, besides improving exhaust flow, when I buy an after market exhaust such as APE and bypass (remove) the 2nd catalytics?
    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.

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    gobucs is offline Porsche Person
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    Re: Why a second set of catalytic converters?

    I am new to this forum so forgive me if this question is not timely- how do you feel about the carnewal modification to the cayman s exhaust?

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    Re: Why a second set of catalytic converters?

    Quote Originally Posted by gobucs View Post
    I am new to this forum so forgive me if this question is not timely- how do you feel about the carnewal modification to the cayman s exhaust?
    Look here:

    http://www.planet-9.com/cayman-boxst...t-exhaust.html

    To get back on topic: Secondary cats would only be there to reduce emissions right? Some other countries may use start up emissions in their standards.

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    cbzzoom's Avatar
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    Re: Why a second set of catalytic converters?

    Quote Originally Posted by bretster View Post
    To get back on topic: Secondary cats would only be there to reduce emissions right? Some other countries may use start up emissions in their standards.
    Correct, most of Europe follows the NEDC (New European Driving Cycle) which involves a cold start.

  8. #8
    berty987 is offline Porsche Activist
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    Re: Why a second set of catalytic converters?

    Quote Originally Posted by K-Man S View Post
    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.

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