Fabspeed bypass pipes

[Blue-S]

Originally Posted by ChrisZang

Hmmmm,

so I have two options:
a) replace 2nd ("main") cats with bypass pipes
b) replace pre-cats + headers with "hi-flow" headers w/o cats and relocate O2 sensor behind main cat

In your opinion which scenario has a higher chance to pass smog in CA?
Please note that I already went with option a

That's a good question. I bought my car in Jan 2009, and as such the prior owner had it smogged at a "corner garage" in Carlsbad...so I haven't seen a Boxster in the process of being smogged. I think that passing smog would depend on two factors:

1. The experience and work ethic of the smog inspector: If the inspector really knows Boxsters, then they will notice that your 2000+ Boxster only has the pre-cats but not the main cats. The VECI label inside the rear trunk mentions only that the car has 3-way catalysts, but it does not list how many. I have asked several techs and they tell me that the smog computer also does not list how many cats are supposed to be on the car. The main cats are much easier to see (vs the pre-cats) when bending down and looking under the rear of the car while it is parked. However, the pre-cats can be clearly seen when looking at the bottom of the engine, peeking in front of the rear wheels. Since many late-model cars have close-coupled cats (built into the exhaust manifold), I imagine that the bypass pipes would not look suspicious on a 2000+ Boxster as long as the pipes look stock. That presumes an inspector that doesn't know Boxsters well...

2. The composition of the pre-cats: I do not know for sure, but I suspect that the pre-cats are an oxidation "light-off" type of catalyst. If that is true, then there is a risk that your car might not pass the sniffer test - possibly failing due to excessive NOx. This could happen because (I think) the main cats are the 3-way type that oxidizes (burns) HC to produce CO2 & H2O, combines CO with O2 to make CO2 and reduces NOx to N2 and O2. I suspect that the engine management system (DME) controls HC and CO pretty well, but that good NOx control at light load would rely heavily on a properly functioning 3-way cat. For more detail on 3-way catalyst operation, check this out:

http://en.wikipedia.org/wiki/Catalytic_converter

Please let us know what happens when you go in for a smog check!