I swapped my 4.8 to a LQ4 and used the exhaust I had off the 4.8 which was catless, 2.5" stock piping, magnaflow muffler out to 3.5" tailpipe. With the 4.8 it sounded decent, now with the 6.0 it just sounds like garbage. Not loud, or deep. Completely different from the 4.8.
I read the stock 6.0 exhaust is dual 3" into the muffler, then it comes out at 3.5", could the smaller 2.5" stock exhaust be causing the sound difference ?

 

The 6.0 in the denali, ss and eskys used 2.5" duals.... The 2500s used a 2.75 or 3" into a single muffler...

 

my 4.8 stock came out of the manifolds 2.5" into a y-pipe, and remained 2.5" all the ways to the muffler. Not sure if its a huge difference, but it sounds bad.

I have tried several mufflers and all sound horrible - flowmaster 50, flowmaster 10, magnaflow. I put the mufflers on my father in laws 03 2500HD 6.0 and the FM10 was loud, the FM50 sounded pretty decent, but on mine it was a totally different sound. Only difference between his and mine is the exhaust piping being dual from the manifolds to muffler - where mine is a single 2.5" pipe.

 

Most aftermarket manufacturers seem to botch the flange/slip fit part of the 6.0 exhaust. But one side is 3 and the other is 2,75. I see 2.5% fuel trim diffs all day long. Going to be going dual 3/3 soon.

FWIW my stock GXP came with 2,25 inch duel setup and the ls3 had 420 ponies from the factory. I've heard that 2,25 is only good for 300 horse though. Not sure about the conflicting info there.

 

If youre seeing that much of a fuel trim differnece sonethings leaking..... Not going to start on aftermarket botching exhaust!!!

 

Since making this post I have ordered everything I need to do a full 3" dual exhaust. Started at the manifolds, welded 2.5" vband clamps to it, then 2.5 > 3" adapter, then to a 3" X pipe, out to the dual mufflers, then out the back. Just getting over the flu so I should be finishing it this weekend. Went with some Jones Full Boar mufflers off Summit for temporary.

 

Pics..... We need pics... Flu or not.... No get better lol

 

Lo . 10-4. I need to order some exhaust tips, but all other parts are in I've got a couple spare resonator pipes that I'm undecided if I'm putting in before the mufflers or not.