Last time I heard someone was running without cats, was Chesbosto & Techno. which had fabbed up a simple open pipe with the sensor mounts to take the place of the original cats. Supposedly it did make some *easily noticeable* extra power (which would make sense with all normal engines). I have only removed my 2nd muffler and my car and put a straight y pipe on it, and I have noticed some gains as I pull and keep pulling on all stock Z's I've street raced. I would be most interested in purchasing a set from technosquare if they do release the open pipe to replace the cats. It's much better than paying well over a grand for the "high flow" cats offered by nismo. Just a thought. But in essence to answer the first question, the less restricitve the exhaust, the more power to be gained, and that I can only prove from racing and pulling on stock Z's, not butt dyno's which is more than enough proof for me

I've not heard that using open pipes (w/o sensor's mounted or sensor fakers in place) to replace the cats will not kick off the CE light in an OBDII sys. Most interesting! hmmm....

 

Has anybody taken pressure readings before and after the cat to determine how much of a restriction it creates. It would require punching some holes in the exhaust pipe, but it would be nice to know.