i'm told the hd trucks, avalanche's, and 6.0 trucks have higher flow cats than my stock halfton 5.3. it makes sense, because my truck with exhaust is still tons of quiet, and i've heard plenty of loud 5.3 avalanches. the very first day i bought my truck , i cut off the stock exhaust and ran a straight through bullet muffler with a dump, just because i can't stand quiet. and even that was quiet. i was very disappointed. i keep asking questions on here about y pipes and cats, because everyone tells me it's my muffler, so i figured i'd ad my two cents. It's def not just the muffler. i am considering trashing the gibson, and going maybe flow. but.... i still know how quiet the truck was with basically just a cat, and a resonator. i'm thinking ASM long tubes and no cats, obdII o2 sims. but that cats gotta come off immediatly, headers are later... i'm just not sure if i should buy high flow cats. and if so.........then maybe the dynatech full system instead of asm's and fabbed y pipe, no cats, etc...