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Thread: Catalytic converter bypass ???

  1. #1
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    Catalytic converter bypass ???

    In my state there are no emissions tests and my cat. has gone bad. I was wondering if it is possible to bypass the cat. by putting a straight pipe in instead. To me it sounds ok, but I dont know how to take care of the O2 sensors then or how they will react. So is there a way to bypass the sensors into making the computer think the cat. is working? What are the effects?

  2. #2
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    You need to take care of the post-cat sims, do some searching here to find out what to do.

    I'd put some resonators instead of the cats so the sound doesn't change much.


    Quote Originally Posted by ClintonM3 View Post
    the Audi A4 is a tweaked, cosmetically enhanced, cross platform, put-together overnight VW.
    Quote Originally Posted by joeuser528e View Post
    Staggard setups are for cars that you park at show meets. They go well with 5 screens and 3 12"s.

  3. #3
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    You'll throw a CEL for bad cat's, but it won't affect driveability. +1 on sims.

    FYI - tampering with emissions is a federal crime, regardless of local laws.

  4. #4
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    I just did it...

    I straightpipe even my resonator. Only my catback is there.



    Sound video:

    http://www.youtube.com/watch?v=Xd_FaLZ_9CU

    The throttle response is 10x better and you will gain few HPs too. No problems with O2 sensor, the sensor is before the cats.

    But this is not my daily drive car... So if you care about your planet and drive your car everyday, don't do it.
    BMW Car Club Brasil council member.

  5. #5
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    On USA spec 1996+ (ie OBD2) a second set of O2's measure after the cats.

    These do not provide information for the "tune" in the ECU, but do throw a CEL.

  6. #6
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    I never got any CEL when I replaced my cats with fflow boxes, and AFAIK, no sims are being used.

    maybe euro model is different to US
    1998 E36 Euro M3/4/6 Aegean Blue/Ox Blood
    2003 E46 M3 AW/Bl - sold
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  7. #7
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    I'm not positive either, but I think only the us model has the two extra post-cat sensors.

  8. #8
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    Has anyone else done this on an obdII? I have some bad cats and am thinking about doing this on my 328is.

  9. #9
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    oh no, don't do it, save the planet

  10. #10
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    OBD2 cars will throw a check engine light for bad catalytic converters because of the post-cat o2 sensors. The car will run fine since the ECU only uses the pre-cat o2's to adjust fuel. Active Autowerke carries o2 sensor simulators for OBD2 cars if the check engine light bugs you.

    Good catalysts are not restrictive. The gains from removing the cats are too little to justify dumping harmful gases into the air. Ditch your resonators and throw in a couple high flow cats. You will be just fine without that extra 1hp.

  11. #11
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    Quote Originally Posted by DiScoTeknix View Post
    Good catalysts are not restrictive. The gains from removing the cats are too little to justify dumping harmful gases into the air. Ditch your resonators and throw in a couple high flow cats. You will be just fine without that extra 1hp.

    + 100000000

  12. #12
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    Quote Originally Posted by DiScoTeknix View Post
    OBD2 cars will throw a check engine light for bad catalytic converters because of the post-cat o2 sensors. The car will run fine since the ECU only uses the pre-cat o2's to adjust fuel. Active Autowerke carries o2 sensor simulators for OBD2 cars if the check engine light bugs you.

    Good catalysts are not restrictive. The gains from removing the cats are too little to justify dumping harmful gases into the air. Ditch your resonators and throw in a couple high flow cats. You will be just fine without that extra 1hp.
    Agreed.

  13. #13
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    most euro cars only have the 1 lambda sensor/cat sensor
    The church is close, but the road is icy, The pub is far, but i will walk carefully

  14. #14
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    Quote Originally Posted by DiScoTeknix View Post
    Ditch your resonators and throw in a couple high flow cats. You will be just fine without that extra 1hp.
    And how much more do High Flow cats cost compared to resonators? shitloads more
    1998 E36 Euro M3/4/6 Aegean Blue/Ox Blood
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  15. #15
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    Quote Originally Posted by DiScoTeknix View Post
    OBD2 cars will throw a check engine light for bad catalytic converters because of the post-cat o2 sensors. The car will run fine since the ECU only uses the pre-cat o2's to adjust fuel. Active Autowerke carries o2 sensor simulators for OBD2 cars if the check engine light bugs you.

    Good catalysts are not restrictive. The gains from removing the cats are too little to justify dumping harmful gases into the air. Ditch your resonators and throw in a couple high flow cats. You will be just fine without that extra 1hp.
    if you ditch the resonators, your car will sound like a civic. ditch the cats, you will gain some power, use O2 simulators, and the world will naturally come to a fiery end as soon as you crank up your car.

  16. #16
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    Quote Originally Posted by kabal View Post
    And how much more do High Flow cats cost compared to resonators? shitloads more
    Unfortunately high flow cats cost quite a bit more. Magnaflows are in the $50-150 range.
    Quote Originally Posted by FatJohn325 View Post
    if you ditch the resonators, your car will sound like a civic. ditch the cats, you will gain some power, use O2 simulators, and the world will naturally come to a fiery end as soon as you crank up your car.
    How would a 2.5litre+ i6 sound like a 1.5 litre i4? Sure you will get some rasp in your exhaust note but it won't sound like a civic.

  17. #17
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    Quote Originally Posted by DiScoTeknix View Post
    How would a 2.5litre+ i6 sound like a 1.5 litre i4? Sure you will get some rasp in your exhaust note but it won't sound like a civic.
    well my friend has an S52 E36 and he removed the resonators and it sounds like a civic with a bad exhaust leak. miserable.

  18. #18
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    Just dish out $1000 for a AC Schnitzer catback like I did...........PSYCHE!!!!
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  19. #19
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    Quote Originally Posted by FatJohn325 View Post
    if you ditch the resonators, your car will sound like a civic. ditch the cats, you will gain some power, use O2 simulators, and the world will naturally come to a fiery end as soon as you crank up your car.

    So obviously you have no experience in removing cats? Maybe if you had a 4cyl it would "sound" like a civic (oh wait, that is also a 4cyl).

    I can tell you now, that my m52 in no way at all sounds like a civic.

    Oh yes, global warming is bullshit, new age hippie bullshit
    1998 E36 Euro M3/4/6 Aegean Blue/Ox Blood
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  20. #20
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    This isn't 1975, the cats aren't as restrictive anymore as the tales your daddy told you...

    I'd had a car with hollowed cats... and would never bother myself...
    -Boondoggie

  21. #21
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    Quote Originally Posted by kabal View Post
    I never got any CEL when I replaced my cats with fflow boxes, and AFAIK, no sims are being used.

    maybe euro model is different to US
    Did you ever get a CEL in your car?


    Quote Originally Posted by ClintonM3 View Post
    the Audi A4 is a tweaked, cosmetically enhanced, cross platform, put-together overnight VW.
    Quote Originally Posted by joeuser528e View Post
    Staggard setups are for cars that you park at show meets. They go well with 5 screens and 3 12"s.

  22. #22
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    none in 3 years... besides the occational taillight failure, and check washer fluid level
    1998 E36 Euro M3/4/6 Aegean Blue/Ox Blood
    2003 E46 M3 AW/Bl - sold
    1997 E36 328i AW/DG - RIP

  23. #23
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    Quote Originally Posted by kabal View Post
    So obviously you have no experience in removing cats? Maybe if you had a 4cyl it would "sound" like a civic (oh wait, that is also a 4cyl).

    I can tell you now, that my m52 in no way at all sounds like a civic.

    Oh yes, global warming is bullshit, new age hippie bullshit
    wow ive never seen someone misread something as badly as you. i agree with ditching the cats, and i also agree that global warming is hippy bs. removing resonators = civic. i have no cats on my e36, but the resonators are still in place.

  24. #24
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    This topic is covered at least once a week yet nobody listens. Removing your catalytic converters will yield little to no benefit what so ever on a stock motor. If you have an OBD1 car get an OBD2 mid pipe and call it a day.

    BTW global warming is hippie bullshit? The earth's sea level has been rising at almost 3 times its rate since the early 1900's. I'd say that's a pretty good indication the climate getting a bit hotter.

  25. #25
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    OK here is the fix!! I got a 97 328is with a Full AA exhaust headers back and i have not Cats and im running regular o2 sensors with NO CEL. What you need to do is got to autozone and pick up 2 spark plug anti foulers. they run i think like $5.50 for the pair. Because the thread sizes of the spark plugs and the O2 sensors are the same they screw right in to the fouler, thus effectively raising the sniffer part of your O2 sensor out of the main exhaust stream. Have your CEL reset if its on and it should remain off for good. My buddy has it done on his M3 as well and we have not got any in 8 months CELs. Just make sure thats you put the O2 sensors in their correct plugs and do not mix them around or else there will be a CEL later on down the road. Its soo easy and saves you a lot of money that you would spend on the simulators!!! Good luck

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