Test Pipe or HFC?

Originally Posted by FuTuRe is Z

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.