I had read one that said that they had little or no effect on engine performance. I've also heard from a few people that they could net you some power. This is what I don't understand. If someone has a good read on the matter please post it up.
i use the dyno-tech cone inserts and they really do work. i installed mine right after the header on a 3inch exhaust system. super turbos, 3 in exhaust, 747 rwhp 719 rwtq 357 t-trim 22psi