I always wondered where tuners take the AFRs readings from, a sniffer on the tail pipe or from the O2 sensors on the vehicle.
I'd imagine that if you had a j-shaped O2 bung it would read slightly leaner than if the bung where right inside the test pipe. Therefore you would tune with a bit more richer AFR to offset that.
I'm pretty sure people using J-shaped O2 bungs on there test pipes are leaving power on the table because the O2 sensor is getting an in accurate AFR. Correct me if I am wrong here.
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