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  1. #1
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    Need help! Catalytic Converter with DPs setup - suggestions?

    Hey everyone, I need some suggestions for my situation. Awhile ago I bought an aftermarket exhaust after falling in love with the sound. It was a downpipe-back exhaust, so it removed the secondary cats.

    Next, I fell in love with the idea of catless DPs for the performance gains. So once I install my new DPs I'll be completely catless which is a no-go for me as I can't feel right about just dumping all those fumes out the pipe when they could be treated.

    I need a catalytic setup that I can weld into the aftermarket exhaust that hopefully won't harm the performance gains from the catless DPs. I'm of course willing to sacrifice some performance in the name of treating the exhaust gases, and passing the sniffer test we have here in Ontario (very similar to California regulations as far as I know).

    I was thinking of getting a shop to cut the factory cats back a bit on their flanges to widen the opening to the size of the aftermarket exhaust (2.5") and weld the original secondary cats back in rather than purchasing something like the Magnaflow hi-flow cats as from what I've heard, they don't work all that well.

    Can anyone see any issues with this setup? Is there another setup I should be considering that will keep the car legal on the sniffer and not dumping really smelly fumes out the back?

    Any input would be appreciated! Thanks very much for your help!

  2. #2
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    Why not just get DP's with hi-flow cats?

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    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    Why not just get DP's with hi-flow cats?
    Well perhaps I was misled, but I was told in the long run that cats that close to the motor end up getting toasted pretty quickly, especially the hi-flow versions. I've already purchased the AR catless DPs, so I'm sorta stuck there. Anyone see any big problems with welding in the stock cats to the aftermarket exhaust after cutting the flange to increase diameter to match? Are the primary and secondary cats in the system identical other than their position in the original system?

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    Click here to enlarge Originally Posted by desean81 Click here to enlarge
    Well perhaps I was misled, but I was told in the long run that cats that close to the motor end up getting toasted pretty quickly, especially the hi-flow versions.
    Isn't the first set of stock cats right there? Having cats that close to the motor just means they heat up faster. The E46 M3 had cats in the header for this very reason.

    Click here to enlarge Originally Posted by desean81 Click here to enlarge
    I've already purchased the AR catless DPs, so I'm sorta stuck there.
    Perhaps, are they installed or still in box? AR may be willing to exchange.

    Click here to enlarge Originally Posted by desean81 Click here to enlarge
    Anyone see any big problems with welding in the stock cats to the aftermarket exhaust after cutting the flange to increase diameter to match?
    It will work but kind of a ghetto solution that will than make the exhaust tough to sell. I think your best bet is still DP's with hi-flow cats.

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    DPs went in today as I had some unexpected spare time. Exhaust smell is quite noticeable so I was hoping to do something pretty quick and get it cleaned up. I agree that the weld in is sorta ghetto, perhaps catting with some proper aftermarket cats would be a cleaner solution. I just thought to use the OEM Boysen cats as they seem to last. I suppose I'm back to the drawing board at this point.

    I had posted this question in another forum and was told that if I were to install aftermarkets / OEM in the secondary position that they may never heat up sufficiently to work. Now from your response, you seem to be leaning that way too... I'm just hoping to work something out that'll get me enough to pass the sniffer and clean up the smell and not dump raw HC's out the tailpipe! Click here to enlarge

  6. #6
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    Click here to enlarge Originally Posted by desean81 Click here to enlarge
    DPs went in today as I had some unexpected spare time. Exhaust smell is quite noticeable so I was hoping to do something pretty quick and get it cleaned up. I agree that the weld in is sorta ghetto, perhaps catting with some proper aftermarket cats would be a cleaner solution. I just thought to use the OEM Boysen cats as they seem to last. I suppose I'm back to the drawing board at this point.

    I had posted this question in another forum and was told that if I were to install aftermarkets / OEM in the secondary position that they may never heat up sufficiently to work. Now from your response, you seem to be leaning that way too... I'm just hoping to work something out that'll get me enough to pass the sniffer and clean up the smell and not dump raw HC's out the tailpipe! Click here to enlarge
    The AR downpipes are a high quality piece. You can sell them to basically anyone and order up a set with cats if you want to.

    If you don't like that solution and want to hang onto your catless DP's, seems welding in is what you will need to do although it isn't the cleanest solution.

    Really up to you, those are your options. You can also get a new rear section with integrated cats and sell your current exhaust.

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    Is it possible to use your stock midpipes with your exhaust? The factory midpipe mounted cats work well enough for me with my AR downpipes. I really don't care for that raw exhaust smell myself (unless it's leaed fuel!), and the stock stuff seems to work well at fixing that.

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