Originally Posted by dwfisk
OK I see you are leaving the 2" exhaust open, my bad. I would think you need at least a 3". Look for a custom muffler shop, they should be able to cut an sell you a 3 ft chunk of 3" pretty cheap. It will be galvanized or nickel plated but its OK for the exhaust.
One 3" will give you the same 3.7 square inch exhaust area you have with the four 3/4" inlets.
My math doesn't match your math. We may need a third opinion.
.5625 Sq. In. for the 4 3/4" ball valves
3.14 Sq. In. for the current 2" exhaust
7.065 if the exhaust is changed to 3"
After running these numbers I'm of the opinion that the OP doesn't have enough intake air. By my numbers NCoonis would have about the right intake by adding 2 1-1/4" intakes to the existing 2. NCoonis, make sure you have and get full flow valves. Not all ball valves have full sized holes in the balls.
My best advice would be to find a pit calculator and run some numbers thru it.