cat removal

[Blue-S]

Originally Posted by ChrisZang

Hi,

this question came up over a discussion with some of my buddies the other day and I was hoping that some members here could settle the dispute.

When removing one sets of cats, which set will most likely bring me into LESS trouble when smogging the car? (or in other words which set is "more green").

My hypothesis is that the set of cats closer to the muffler is doing the "heavy lifting" and the one closer to the headers is doing less of the work.

But my buddies disagreed.

So who knows the answer?

If the Boxster is like other vehicles that use a light-off catalyst in or near the exhaust manifold and a main catalyst further downstream, then your belief is correct. The light-off cats in the manifolds are probably just oxidation (one-way) cats and the main cats are probably three-way cats. If that is true, then the cats in the manifolds do nothing to reduce NOx emissions.

Although the car will most likely pass the California smog check with the main cats deleted, the emissions will be higher than you would have seen with all cats onboard and functioning. The car would probably produce lower emissions test values with the light-off (header) cats deleted and the main cats functioning than vice-versa, but doing so successfully requires that the downstream O2 sensors be relocated aft of the secondary cats...which is a hassle. If you just let the downstream O2 sensors hang in the open air, you will eventually set a DTC because the O2 sensor voltage never varies.

I sure hope that it will pass smog with the secondary cats deleted, because I will soon be installing a good used pair of bypass pipes!