I have a stock 06 Duramax(Crew cab, short bed, 4wd). I am looking to increase economy at least a little as my primary goal. I am mostly interested right now in a cat back 4 or 5 inch system but have 2 questions.
1. Will this void my remaining warranty?
2. How much, if any, MPG gain can I expect?

Any input is greatly appreciated. I see most people on the forums have some sort of exhaust mod. I would like to know why that particular one was chosen for thier application.

Thanks.

No warranty issue if its cat back
Don't buy for a mpg increase.
Buy for lower egt's and a cool look....

I chose 4" duals and lost the cat, I did it mainly for the look and to allow future mods. I didn't notice a difference power wise, but I lost a little bit in mileage, maybe 0.5-1 mpg.

No warranty issue if its cat back
Don't buy for a mpg increase.
Buy for lower egt's and a cool look....
:exactly:

I like my cat back. Its nice and throaty but still quiet. It suits me perfect...

I think you can have a turbo back 4" exhaust and retain the stock cat if the installer trims back the cat inlet and outlet back until a 4" opening is available (the ends of the cat are tapered) and then welds or otherwise adapts that 4" opening to the 4" exhaust tubing. It was proposed to me by a local installer to do just that in the installation of a Magnaflow 4" system.

That could definately be done, but I dont think you would see much of an improvement over a standard cat-back kit. If warranty is the concern, buy a cat-back kit with the front-pipe (cat-delete) seperately. This will allow you to swap the cat in and out for dealer visits.