I had some minor upgrades for me a few years back. Cam, valves, Flowtech headers, 3"exhaust, 2chamber FM's, street tune, K&N cold air intake, and couple other little things. Don't know what worked or didnt specifically but all together has lots more power. Forget about MPG...doesn't matter lol Mainly for towing my camper but its fun when not towing too.
About the only way you are gonna get any more performance out of that old 6.0 is to get a custom tune, not a commercial tuner, but a custom tune. I recommend contacting Justin over a BlackBear Performance. They are out of California, but he travels the country custom tuning GM pickups and SUVs. They also have a handheld available from EFI Live called the Autocal. Ive run several of the GM trucks with LS motors, and it was by far the best mod I ever did to them. I had an in-person tune once, where I met up with Justin in Houston (The Woodlands actually), he plugged his laptop into my OBD II port, and within a few minutes, its was like I had a whole new truck. The second time I went with the Autocal to allow for updates for future mods. If you have the Autocal, just email Justin any changes and he can write a new tune for you and email it to you. Don't waste your money a handheld tuner. I had a Hypertech on an 06 2500 with the 6.0, and I think all it did was lighten my wallet. Also had anintake and exhaust on all 3 trucks, but didn't notice any changes with those either. But, the intake and exhaust definitely wont hurt if you plan on getting the tune.
Other than that, a set of long tube headers, cam, and maybe a lower gear ratio. Also, if you have a truck with the 4L60E, a TransGo shift kit and maybe a Corvette servo will definitely clean up the shifting. IF you have the 4L80E or any of the 6 speeds, you probably wont need any transmission work. Any other mod will just be a waste of money, or will require some pretty extensive engine and transmission work.
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