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Exhaust bypass pipe BETWEEN the cats on 03 Boxster S

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I've been wanting to do the exhaust bypass mod on my 03 Boxster S, However I'm thinking of doing it between the cats vs after both cats. I also want to add valves so I can adjust the volume and sound. I'm thinking of adding 1" pipe, before the second cat. It would also give a slight HP increase as well. Anyone have any input? Anything I should watch for? Any reason it shouldn't be there?


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1997 - 1999 (2.5L) was equipped with 2 catalytic converters and 4 O2 sensors.

2000 - 2004 (2.7L and 3.2L) were equipped with 2 warm up catalytic converters (precats as part of header assembly), 2 main catalytic converters (secondary cats) and 4 O2 sensors.

I was able to remove the secondary cats on my '04 without triggering an emissions error as the sensors are before and after the cats in the header. However, in that configuration, I do not know if it would pass emissions inspection. (I live in florida - no emissions; and the car is now a race car anyway.)

I eventually removed the other set of cats too, and had the DME reprogrammed with a PIWIS to change the emissions standard to euro 3 or less.

I'm not sure removing the cats made a big difference in HP. I don't have dyno numbers before/after. Maybe a little increase, but I did it for weight and to not have to go to the expense of replacing cats.

Take a look at Maxspeed Motorsports. (No affiliation) He has some interesting stuff for exhausts including a "CEL Fix." No idea if it works, but I have his headers and bypass pipes and am happy with them.

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