Post cat O2 sensor and custom down pipe back exhaust - Mercedes Benz SLK Forum

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#1 Old 04-28-2015
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Post cat O2 sensor and custom down pipe back exhaust

Hello everyone,

For the past couple weeks, I've been working hand in hand with an exhaust fabrication shop creating a 2.5" downpipe-back exhaust for my '99 SLK230. The thing that has me a little frustrated is the fact that this car is a California Emissions compliant car so it has the weird, restrictive cat setup and a post-cat O2 sensor.

I heard of some people on here just swapping these cats for one higher flowing cat but I'm still debating on whether I should go with a cat-less design or buy a performance/california/OEM/aftermarket cat. The thing that has me stumped is the fact that half of people say that the O2 sensor controls fuel, while the other half say it does nothing but monitor.

If the O2 sensor controls fuel, then I'm going to be forced to buy a ridiculously expensive cat that I don't necessarily want. If it doesn't matter then I'll put a non-california cat on and not worry about the O2 sensor freaking out.

So, what's the word? Does this O2 sensor control the ECU in any way? Has anyone tried running a non-california cat in this car? Has anyone run an O2 simulator with any success?

Also, where I live, there's no emissions testing, so I could run open headers if I want, .

Thanks guys!
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#2 Old 04-29-2015
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currently, on my car, i have a fully custom exhaust after my header. it's 2.5" diameter, full titanium, with a high-flow cat, no muffler or resonator. within the next month or so, i will be adding in a performance header, which will be heat wrapped. if your state doesn't have emissions, i would just say screw it and run no cat, or just a high flow cat. keep in mind that running no cat, is federal offense, having to do with the EPA. if you're worried about your ECU, you can always tune it to run with only one o2 sensor, the primary one. in my previous car, that's what i did. i had a full race exhaust with no cats, no muffler, or resonator. the ECU was flashed to run using only primary o2 sensor, the secondary one was removed all together.
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#3 Old 04-29-2015
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Thank you so much for the very knowledgable reply!

That sounds identical to the setup I'm having run with the exception of the titanium. How do you like the sound? You wouldn't happen to have a video or anything of how the car sounds would you?
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#4 Old 04-30-2015
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i dont have a video unfortunately. i love the sound as it is right now, it's loud but not rice-y since it's only a 4 cylinder. my problem i had with my previous car, since it was also a 4 cylinder, and the whole system was stainless steel, it was very rice-y.
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#5 Old 05-01-2015
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I'm glad that it sounds good! If you have the time, I would be incredibly appreciative of a video, but I can understand if you're busy. I got a Magnaflow high flow cat, a Magnaflow 2.5" straight through 11" Muffler and the down pipe-back piping is yet to be done. Hopefully I can get a supersprint header and down pipe soon and add that into the mix. I'll post progress pics and results if anyone is interested!
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#6 Old 05-10-2015
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I think I answered my question as I was typing. Flowmaster's are the junk mufflers, right. Magnaflows are decent but Borla are the best. My question is: Any actual performance gain? I have an SLK32 AMG and there is not much to be gained even with removing 4 cats and the resonator.
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#7 Old 05-11-2015
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Well, I would be able to tell you how good the Magnaflows are today, as my full exhaust was scheduled to be put on today, but seeing as I hit someone else's rear bumper this past Sunday and tore the front of my SLK to pieces, this project is literally now on my back burner...... Oh well. HOWEVER, you are correct, the Flowmasters will sound bad because they are baffled mufflers. You'll get a very farty noise from the pipes. According to common practice opening up your exhaust will cause you to *lose* less power because you don't have backpressure working against the piston head while it tries to push the gasses out. By this logic the most free flowing muffler will give you the best performance and a straight through muffler design, such as Magnaflow or Borla will allow your engine to breath a lot better. Whether or not you'll be able to feel it, I don't know. A lot more so on the 32 AMG than a 230, though. I'll guarantee you that. .

Man, I'm so sick about wrecking my car. Hopefully they see enough value in it to not total the thing. I don't think there's that much damage, but it looks really, really bad.... Grrrrrr...
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#8 Old 05-11-2015
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Sorry to hear that. Maybe a sign to upgrade to that R171 SLK 350


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#9 Old 05-12-2015
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I agree with Jeff. Look on the brightest side. An upgrade is always nice. If it's totaled and has some value buy it back and part it out. Good Luck
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