I want to get a Nelsons tune 87-93 and go w/ 70 flowmaster, nothing too loud just a mild tone. Would 3" be too big and hurt my torque. BTW 01 sierra 5.3 x cab

 

i would go with a free-er flowing muffler like magnaflow, dynamax, hooker, borla etc. 3" wont hurt performance at all.

 

all depends on what you want to do with the truck. with the mods you have listed that you want to get, stock diameter will be sufficient enough. also look into a different muffler other than flowmaster like stated above.

 

I too looked at a 3" system, but the backpressure loss seems insane! If you want a good tone just do a dumped Hooker Aerochamber muffler before the axle on the stock piping. Cheap and good sound. It'll be louder without cats, but with them it sounds fine.

 

Magnaflow, or borla IMHO. Stock diameter piping is fine for anything other than a max effort set up.

 

you cant keep stock diameter with an aftermarket muffler. Stock pipe is 2.75" and aftermarket mufflers are 2.5 or 3".