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Old 11-08-2008, 04:54 PM   #1
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Exhaust Setup Questions

Hello, I have a 1998 Porsche Boxster with a 2.5i engine. Currently I have the stock muffler setup with the two cats in the mid-pipe section. It has the two 02 sensors before and after both cats. I am going to buy these headers
And I have a Fabspeed cat back system which includes the midpipe and muffler. The Fabspeed system removes the only 2 catalytic converters the car has and the system doesn't have any 02 sensor bungs. The headers have 02 sensor bungs. So the question is what should I do with the 02 sensors? Do I unplug all 4 of them or do I route the two pre-cat sensors into the headers and leave the 2 post-cat sensors unplugged? How do you think this would affect the mixture, and what are some of the affects or running the car without catalytic converters, I am expecting it to be loud and get a CEL but are there any harmful affects? Thanks so much for any help!!!

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Old 11-08-2008, 05:08 PM   #2
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My personal opinion (I know, you didn't ask) is that you're causing yourself more problems than you will ever gain.

Look into secondary cat bypass pipes, but I'm not really sure of the early Box layout.
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Old 11-08-2008, 10:21 PM   #3
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If you go this route, you will obviously be cat-less. If you want to continue down this path, here is some advice.

Plug the first set of O2s into the headers. Looking at those headers, they have two bungs per header. You'll want to use the bungs AFTER the collector so that you are reading exhaust from all 3 cylinders, not just 1 per bank. The second set of O2s you would simply remove from the system (just unplug them from the socket and ideally put something in the socket on the car side to keep the connectors from getting contaminated from the weather). Alternatively, you could try welding in a set of bungs to the exhaust midpipes and inserting the secondary O2s back into the system with a spark plug de-fouler, which may trick the DME into being happy, or maybe it won't. My guess is it won't.

The DME computer uses input from the first set of O2s to control the mixture. The second set are only used to monitor catalyst efficiency (or in your case, the lack thereof). They have no effect on the mixture or anything to do with engine performance.

Whether you remove the secondary O2s entirely or run them behind a de-fouler, you do not have cats now, and the North American DME program does monitor cat efficiency per OBDII regulations. So, you will likely throw a CEL (check engine light). This will not effect the performance of the car in any way, but if the car throws another code for something actually being wrong, you won't know it. Also, depending upon what state you're in, this may cause issues related to passing inspection.

If you have access to a PST2 or PIWIS (the Porsche computers used to program various things in the car and read out codes), you can re-flash the DME to the RoW (European) program, which ignores the secondary O2s (and also has some other differences as well- does not require the clutch to be down to start, will not run the secondary air pump, and a few other things). This way the car will run happily and will not throw a CEL with your setup.

You will pick up some performance at the expense of social responsibility with this setup. If you do decide to continue this project, I would strongly recommend trying to find someone with a PST2 to re-flash your DME.
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Old 11-09-2008, 07:52 AM   #4
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Thanks so much, this is sooo helpful!!!

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