silverado 4.8
 

can i increase the size of exhaust tubing on my silverado 4.8? will it make a diffence?

From the cats back go with a flomaster 60 series duall 3" all way out the back.
Sure let it breath better ,and sounds mean. Give you a little more kick.

I thought these motors needed the backpressure supplied by the smaller tubes?

Not if done from cat back. But actually if you increased the size of the whole system
Would be a increase of 30 hp. But i would just go from the cats back on count of o2 sensors ect.

Do not go 3 inches all the way back. If you go form teh cat back you will LOSE TO MUCH BACK PRESSURE! If you want to do anything cut off the stock muffler and add from there. Trust me! Learned form my friend's experience. Oh and stay away from teh SUMMIT cat back system... you will not gain and if you do it will be very little. Better to remain stock than run that setup. That I learned from experience.

Talking about back pressure. What will mess with back pressure if you try to run straight pipes . As long as you use a muffler , back pressure will be fine. Everybody gets excited over back pressure. As long as you you use mufflers , you will not hurt it.
Reason is over a long period of time you might burn some valves.
I know a few ran straight from cats back , have close to 300k no problem.

It might not do any harm to the motor, but it will sound like ass and the performance will be horrible. If you like your truck to sound like it is doing 80 when your leaving your driveway and having all your neighbors mad at you then by all means runs a big open system on an otherwise stock motor. Back pressure is something to be concerned about. If you are wanting better sound with a performance boost on a 4.8 a performance muffler or high flow cats would be a good alternative. I will say from experience though - if you are doing it for strictly a performance gain it is really not going to make a huge difference.