I guess you can say he is running an off-road pipe just because he has gutted the cats. This does change the air-flow I'm not sure how much. The biggest difference is most off road pipe's are 2 1/2 inches compared to the 2 1/4 inch stock pipe. I gutted my cats before getting a 2 1/2 h pipe. There was a big diffence in torque. Also I did not like the sound of the gutted pipe it was real tingy sounding. If you get tested for emissions I would keep the factory cats and bolt them on for the test. Even an old cat h-pipe is better than none.
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