For a stock 6.5, 3.5-4.5" exhaust is perfect. For the best flow make sure the tubing is the same diameter throughout and not small in the bends. With no muffler or cat it will be the best flow possible. If your state mandates that you have a muffler, I suggest getting an Aero-Turbine brand muffler. They are very good and have great flow qualities. Also, there are a few companies that make a larger than stock diamter exhaust crossover pipe, it helps immensely. With all of that done you should see a gain of 5-15hp.

 

so taking out the cat will make it a a 4 in exhaust truely with the acception of the 3.5" down pipe?

 

If the muffler is out as well, then generally speaking yes.

 

if you run 3.5 dp and 4" exhaust back with or with muffler there no diff in sound i have try it. it will sound just the same

 

Sorry to burst your bubble but, no. It depends on the muffler you use. Stock vs straight is a huge difference. But Glasspack vs straight is almost the same. Get what I mean?

 

i have disgree on that think glass pack will be loud but for one i wouldn't run one 6.5 it just not made for it