I measured my merge collector its 3 inch i want to get 3inch high flows 3 inch xpipe an so on.. Whats the best but affordable high flow cat out there an 3 inch xpipe ???? Thanks guys...

 

My 2.5 Borla Stingers are big enough for 600 HP (not that I have that much - just what they can easily handle); how much HP are you looking to have? 3-inch is overkill for anything less than a monster engine. If you don't maintain some backpressure, your HP gain will be too high in the RPM range to do any good.