Secondary Cats Needed?

[aladams01]

I've read quite a few posts on other boards (Cayman Club, etc.) describing the moderate gains achieved with an aftermarket exhaust system. I'm not looking to debate the power/torque gains, it seems a dyno test is the only way to tell, what I am interested in learning is whether the secondary cats are actually needed? It seems every aftermarket exhaust manufacturer removes them and claims they can pass emmisions without them. Why then would Porsche use them? Does anyone have any knowledge regarding this? I apologize for the question if this is a repost.

aladams01

[renzop]

There are a few different issues here.

The car manufacturers are required to use an OBDII compatible computer that monitors various engine sensors and reports if they have problems (the dreaded Check Engine Light). The manufacturers are also required to pass emissions both on new cars and that the emissions system will continue to work without maintenance for 100K miles.

Most manufacturers have two catalytic converters for each cylinder bank. One is very close to the exhaust manifold (even sometimes inside it ) so that it heats up very quickly on start up. Another is further downstream to catch anything the frst misses. The computer monitors the effectiveness of the cats via oxygen sensors attached to the exhaust system. These are in a feedback loop with the mixture and spark controls to keep the engine running properly.

Porsche has positioned oxygen sensors before and after the first cat. There are no sensors after the second cat. Since problems downstream of the second sensor cannot effect it, the aftermarket has realized that the second cat can be removed and the OBDII system will not display any check engine light warnings.

This does not mean that removing the second cat will keep the air as clean as leaving it on. Also since the second cat is hidden inside the second muffler many states inspections might not realize that the emissions system has been compromised.

I look at it this way, would Porsche have put it there for no reason? Would they have put a second cat there if the car would have passed emissions with one? Would Porsche have purposely reduced the performance of their sports cars and made them sound worse? No, of course not.

On the other hand, would removing the 2nd cat cause problems with the motor? No, once we are that far downstream there is nothing the exhaust can do short of plugging it up that could impact the motor.

So this comes down to three questions:

1) Will your car pass an emissions test without the 2nd cat?

2) Will removing the 2nd cat void your warranty?

3) Do you think that possible extra emissions are acceptable, environmentally speaking?

Which leads to a single, final question:

Are all these doubts and extra costs worth the small performance gains and sound improvements?

[berty987]

I think porsche made the 987 to comply with EU standard 4 emissions. The new cars need to comply with even tighter emissions of level 5. Personally I would seek out a performance exhaust with 100 or 200 cell performance cats and if possible retain the dual set up in both sides. If your car does not comply with the stated emissions you may be foreced to refit your old exhaust or buy a new one form Porsche !! From what i've read so far on forums and tech pages its the primary cats that offer the biggest gains to being converted to performance items. The secondary cats and silencers offer little gain from being replaced with "performance " items. The boxster S already utilises a gas flowed exhaust manifold to improve performance , I'm not sure this can be much improved on, but are generally an integral part of the primary cats , hence replaced as a single item.