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Thread: Replacing stock exhaust with headers and a hi-flow cats questions

  1. #1
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    Replacing stock exhaust with headers and a hi-flow cats questions

    I know. Another header thread. I've searched many, many threads and I've found no conclusive answers anywhere to some of my questions concerning replacing the stock exhaust manifolds and adding hi-flow cats downstream. My state/city only runs an OBD readiness check, no visual or sniffer.

    My intentions are to install my AA stage 1 supercharger on my 330i 6spd but before I go forward with that project, I want to replace the stock exhaust with headers and downstream cats. I plan on having AA install their canned tune and code out the post-cat O2's so those won't be an issue but I'd still like to retain the cats to reduce or eliminate the raw exhaust smell that's usually associated with a cat delete. So here goes...

    - I'd prefer to avoid any installation issues, cutting, rewelding flanges, etc. so eBay headers aren't going to be a consideration.
    - I really like the Supersprint headers and I'm not opposed to spending money on a quality part, but at half the cost I'm considering BimmerBrakes Gen 3 or AA. The only issue I can see with the BB headers is working around the tubes on the 4-6 to get them bolted up. It looks less than ideal for working space. I wish the original V1's were still available. The Magnaflow headers have changed design that don't appear to differ all that much from the OEM design so I'm not considering those. The BB headers aren't nearly as affordable as they were 5 years ago either.

    I've also considered these from Schmiedman but the equal-length tubes look even more difficult than the BB headers when it comes to install the rear and the outlet flanges are specific to this kit and will only mate to their section 1, so other headers/exhaust will not be compatible down the road without modification should I decide to make any changes.
    https://www.schmiedmann.com/en/produ...roduct=SCM5254

    Additionally, this midpipe has to be lengthened to clear the 6 spd, so I might as well just cut and weld cats in place of the first resonator on my stock exhaust.

    - Metal high-flow cats. Since I won't need the rear O2's, my thought was to remove the first resonator and install cats in its place.
    - Two cats, single in, single out?
    - Single cat with dual in, dual out?
    - Should I also add a crossover after the cat?
    Magnaflow has pretty much any configuration, single or dual to fit any installation, but I've read that some of them fail quickly or develop a rattle.

    As it turns out, Schmiedman also has some metal 400 cells, although I haven't been able to find any reviews or comments. https://www.schmiedmann.com/en/produ...t=SCM5254KATUR

    In short, I'd like some feedback, positive or negative, suggestions, comments, issues with my plan...What I really need is some long-term, real world experiences. Wins, fails, etc. Any insight is greatly appreciated. Thanks.

  2. #2
    This is one of the things holding me back from installing my headers as well. I have the BB Gen3 which I will get ceramic coated eventually. Got them right before this price hike.

    The cat has to be hot to work so the closer you can get it/them to the collectors, the better.

    Personally, I think I'm going to run single cat, dual in, single out, 3" piping. Muffler and resonator choices TBD.

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  3. #3
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    Quote Originally Posted by sparky300 View Post
    - Metal high-flow cats. Since I won't need the rear O2's, my thought was to remove the first resonator and install cats in its place.
    Not recommended, because;

    Quote Originally Posted by DeathTrap View Post
    The cat has to be hot to work so the closer you can get it/them to the collectors, the better.
    If the cat is way down by the first resonator, its only function is going to be to restrict the exhaust... It'll never get hot enough to do its job. That's why you always see the cats immediately after the manifold / collector.
    -Chris


  4. #4
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    I have the BB Gen3 which I will get ceramic coated eventually
    Whatever header I end up with will also be ceramic coated before the install

    The cat has to be hot to work so the closer you can get it/them to the collectors, the better
    Correct, limited placement for sure. Behind the trans after the offset is about the only location.

    Personally, I think I'm going to run single cat, dual in, single out, 3" piping. Muffler and resonator choices TBD
    Any timeframe on your install? Any ideas which cat you're considering? I'd like to keep the factory pipes all the way out the back, that's why I'm considering the Schmiedmann 400 cpi cats but I'm still open to options

    Not recommended, because; If the cat is way down by the first resonator, its only function is going to be to restrict the exhaust... It'll never get hot enough to do its job. That's why you always see the cats immediately after the manifold / collector.
    Is this your experience or conjecture? From my understanding, BMW put the cats where they are to pass cold-start emissions and to have a single type of exhaust for 50-state emissions rather than a 49-state exhaust and a specific CA-only exhaust. If I recall, the euro exhausts are set up this way, albeit with primaries and secondaries. 400 cpi cats will not restrict the exhaust flow to any significant degree.

    The cats will still light off further downstream, it'll just take a bit longer. Regardless, headers and 400 cpi downstream cats are better than headers and a cat delete and it should eliminate the raw exhaust smell. The benefit of reducing underhood temps is a bonus.

    This guy did it the way I imagine it should be done. He used SS stepped headers and 400 cpi cats. Granted it's on a Z4, but it's similar to what I have in mind, doing it the right way and using v-band clamps.

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    Last edited by sparky300; 06-28-2019 at 10:15 PM.

  5. #5
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    Man I drove myself crazy with this too and finally settled on modifying S54 headers/cats to fit our cars. I've had the parts sitting in my garage for a year and still haven't gotten around to getting the M54 flanges welded on.

    I kept looking at the Schmiedmann kit but came to the same conclusion about the cats being too far downstream with that and most custom setups.

  6. #6
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    It appears the Schmiedmann cats are set up virtually identical to the Supersprints with the only difference being the collector flanges are ball & socket type versus OEM flat flange and gasket and mate to the Schmiedmann headers. My guess is that either system will light off and work just fine, it'll just take a few minutes longer than the factory system.

    If my TiG skills were better I'd be tempted to try exactly what you're doing. I'm the long run it's far simpler.

    At this point I'm leanng toward AA headers and the Schmiedmann cats installed in the first resonator location of the factory midpipe.

    Graphing the log files of my post-cat 02's shows the voltage continuously bouncing from .1 to .9 and back again so my stock cats are most likely toast anyway.
    Last edited by sparky300; 06-29-2019 at 09:51 PM.

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