Quote:
Originally Posted by mrg666
Knock is less likely after reducing exhaust back pressure by removing the second cat which reduces the cylinder pressure. We are talking about the second cat which has no oxygen sensors. So, AFR is controlled by the oxygen sensors on the first cat which is still there. Second cat shouldn't have much of an effect on AFR. Again, my engine is fine. But you should of course do as you feel less risky. I am just someone on the internet after all.
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More flow = more intake air = leaner mix. Remember, the engine/ecu has no idea there's more air going in. The tune works on the assumption that every car has the same flow with the same factory exhaust setup.