Well have I heard having an exhaust too big can cause eddies, or swirling masses of a fluid, in the exhaust which do not help the cause. I don't know it a 6" will cause it or not. Also most stack kits are only 6" on the stacks themselves, the extension pipe and down pipe are usually 4" or 5"
imagine a hose that's 4" and water running through it at 10psi at 50 mph, now increase the size to 6" but keep the psi the same, the water would move slower b/c it has more room to disperse it's energy, thus slowing it down and making it less effective.
there is a happy medium when it comes to power and exhaust size. now while i don't agree w/ the over 500hp go to 5" and anything under stay w/ 4", i know from personal experience that the 6" loses power on an under rated truck.
i mean look at some of the people out there running 800-1000hp they only have a 5".
These trucks contrary to popular belief like a little back pressure. I had a straight through muffler on my 5" exhaust and put a hush power on and it helped spool up, throttle response, lowered cruising boost and egt, and you could hear the motor not working as hard. My buddy has a smarty, power puck, tst, sb66, stock intake, and stock exhaust on his 04. He straight piped it and actually lost spool up. So don't go with a 6".
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