Gutting out the cat makes a good difference in most applications. Although, it does hurt the environment and can violate the rules. I would recommend buying a high-flow cat instead. High-flow cats have a larger ID (inside diameter) at the flanges, and often have less restrictive internals. In some instances, a high-flow cat is better than a gutted cat because the larger ID.Coyotekart63 said:My CAI I made myself works just fine. More power above 4500RPM and louder too. Between that and the cat gut I went from 300 miles per tank to 360 miles per tank.