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Thread: Wide Band O2 Sensor and Cat Convertor Question

  1. #1
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    Wide Band O2 Sensor and Cat Convertor Question

    Afternoon Guys its been a while.

    I have a general question about wideband O2 Sensor Location. I have a 95 Chevy K2500, 5.7, 7427 ECM, 4L60E. I am slow gathering parts for doing a eng swap to a fresh eng with better heads, cam etc. I will be installing an Wide Band O2 sensor when the swap is complete to help tune. I plan to use the JBA 2831 Y pipe which comes with (1) O2 sensor bung on it. I also plan on installing a Cat convertor Must likely a Mange Flow. I have seen Convectors with a O2 Bung on them.

    Here's the questions 1) does it matter where the Wide band is located? 2) does it matter if it is reading before cat or after? 3) do Cats have a few direction?

    Here is what I was thinking of using: https://www.summitracing.com/parts/m...YaAlmuEALw_wcB

  2. #2
    Fuel Injected! ralmo94's Avatar
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    Factory OBD2 calibrations have an O2 after the cat to tell wether the cat is working, this o2 reads a different value than the front o2. You need to install Before the CAT.
    Best after the banks merge to get an average of both banks. You should be able to have another bung welded in. If going Open Loop you can just leave the narrow band out for now. I have $OD files setup already for Open Loop Tuning in a Write Up I did, if you choose to go that route. If you plan on leaving WB in permanent you should install an additional bung

    Hope that helps
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  3. #3
    LT1 specialist steveo's Avatar
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    1) does it matter where the Wide band is located?
    too close to your engine and it wont last as long.

    too far back and it'll respond slowly.

    after all your exaust/headers/ypipes merge together but before the cat would be the traditional place

    2) does it matter if it is reading before cat or after?
    in reality putting the wideband after the cat wont make a big difference for tuning

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    Quote Originally Posted by Sledhead2 View Post
    2) does it matter if it is reading before cat or after?
    A wideband O2 sensor after the catalytic converter is analyzing how well the converter alters engine exhaust.
    The entire point of a wideband O2 sensor is to analyze UNALTERED engine exhaust.
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  5. #5
    LT1 specialist steveo's Avatar
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    do you have experience with this?

    i do

    although not preferred it doesn't make a big difference

    ever had a dyno tune?

    most of those are a stick in the tailpipe - post cat

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    Quote Originally Posted by steveo View Post
    do you have experience with this? i do
    although not preferred it doesn't make a big difference

    ever had a dyno tune?
    most of those are a stick in the tailpipe - post cat
    Never had a dyno tune myself, witnessed several. True, most dynos use a tailpipe sensor.
    Always thought tailpipe sensors detect / analyze differently than wideband O2 sensors?

    Please, feel free to school me. I might be confusing emissions tailpipe sensors with widebands, or something ...
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  7. #7
    LT1 specialist steveo's Avatar
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    post cat afr reads just slightly leaner than reality

    so if you pretend things are normal, your tune ends up slightly richer

    which is fine for a street car

    some japanese ecms use the post cat narrowband o2 sensor for closed loop trims with a 1.002 lambda target

    if the only convenient place to throw the sensor was post cat, especially if its just temporary to do a road tune for a guy, id go for it

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    Quote Originally Posted by steveo View Post
    some japanese ecms use the post cat narrowband o2 sensor for closed loop trims with a 1.002 lambda target

    if the only convenient place to throw the sensor was post cat, especially if its just temporary to do a road tune for a guy, id go for it
    If govt emissions agencies mandated OEs to put post-cat O2 sensor at the furthest end of the car's butthole, they'd do it.
    Guess I just wonder why they don't ... ?
    Quote Originally Posted by steveo View Post
    post cat afr reads just slightly leaner than reality

    so if you pretend things are normal, your tune ends up slightly richer

    which is fine for a street car
    This part I have an incomplete understanding of.
    I understand that
    if the MAP is less than idle, it should be ok to go leaner (lean cruise) with proper timing adjustments, and that
    above a certain MAP, it's time to go richer than 14.67.
    Last edited by LeMarky Dissod; 10-29-2022 at 05:52 PM.
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  9. #9
    LT1 specialist steveo's Avatar
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    if the MAP is less than idle, it should be ok to go leaner (lean cruise) with proper timing adjustments, and that
    above a certain MAP, it's time to go richer than 14.67.
    i'll assume by the fact you're talking about AFR in HUNDREDTHS of points......you might want to think about what goes on in the reciprocating machine that you're feeding.

    on every huff of that piston, uncontrollable and somewhat random amount of your fuel and air charge doesn't end up viable, in fact it might not make it past the valve, and might even end up in your next cycle, as such the effective AFR on each firing stroke modulates wildly. this means you can't effectively tune that at a higher precision than +/-0.1 AFR (or maybe even less) outside of a lab

    if that doesn't put precision in perspective for you, your stoich ratio could go up or down by a random percentage too, depending on where your fuel is coming from, and how much corn juice is in it.

    it's chaos in these things. you need to start mentally placing words like 'approximately' and 'somewhere around' in front of your math

    that's why the +/-.182134xxx or whatever AFR difference at high airflows behind a cat isn't ideal, but is not a show stopper if you just want to get things 'close enough' especially for power enrichment which is a crap shoot anyway

  10. #10
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    WB should be around 6-10" from collector and before cats.

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