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Thread: do cats affect AFR?

  1. #1
    Tuner in Training 346ci's Avatar
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    Aug 2008
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    do cats affect AFR?

    Will mounting a WB O2 in the post catalytic bungs affect afr readings? I would like to take a quick look at a friends truck without drilling and welding another bung in. Thanks ahead of time for any input.

  2. #2
    Tuner
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    I have found that, the bung in front of the CAT will produce most accurate readings. While using post CAT bung readings will be erratic during idle or light throttle.
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  3. #3
    Senior Tuner
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    Yes. Almost .25 to .50 difference. If you choose to use the bung before the CAT you will have to tune in OL (Open Loop) only.
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  4. #4
    Tuner in Training 346ci's Avatar
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    So my best bet would be use on of the pre cat locations and tune in open loop? This shouldn't be to big of a deal if I tune the VE before and then switch to OL to do WOT tuning?