tubing and flow to the mufflers. The bigger the tubing the more air flow pushing out. Then if you have gutted cats pre or main, there is nothing really stopping the air flow to the mufflers. The way i see it is that you will gain the most HP if you have bigger tubing like 2.5" or 3" and with no air restrictions back to the mufflers. I have heard good things about the hyper and thermal exhaust and of course from personal experiecnce the B&B. I guess alot of it comes down to price and noise level. Just my .02.