From what I have figured out about engine air flow. Since the bore of the plenum intake is 52MM and anything larger than 52MM requires a chip or the computer being flashed 52MM is the ideal size for the throttle body. I also read that long runners give more low end torque and shorter runners take away from that, Why do it? After all it's a truck not a race car. But, Getting the engine to breath in more air is immaterial if it's got no where to go. So it makes since to me to add a catalytic converter with the two 2" ins and a 3" out instead of a 2 1/2" out. then a 3" exhaust pipe back to a 3" in 3" out muffler. My other question is about tail pipes. I have two Mangaflow 3 1/2 X 16 angle cut Stainless tips that have 2 1/2" ins, What would running 2 1/2" tail pipes do instead of running two 2 1/4" tail pipes?

 

Nothing noticeable.

Dyno tests show that the exhaust after the cat has very little effect on power output. Running too big, or too small though, can hurt you.

 

so should I leave the cat a 2 1/2" out with 21/2 exhaust pipe and 2 1/4 tail pipes?

 

In regards to the racecar comment, there's a good amount of guys on here who use their trucks for other purposes than hauling, moving, and pulling. I built my motor and trans to produce and handle more horsepower and higher RPM's because I personally like mud and rocks :P Some guys just want to use their trucks for general "truck" purposes and some don't. To each their own haha.

 

That would probably be adequate to the task. Not sure if going to three inch after the cat would improve anything.