The Weber lid has 4 each 3/4" Holes. That is Only 1.77 Sq Inches of Exhaust. The Weber lid works OK.
The 2" Bung is 3.14 Sq Inches, which also works just fine.
The Big Pappa UDS Kit has a Rotary style Exhaust with 8 each 3/4" holes 3.54 Sq Inches. It also works just fine.
I have also seen builds with 2 each 2" Dual Exhaust Pipes 6.28 Sq Inches, which also seems to work fine. I would not get any bigger than this. This is probably double what you really need for exhaust.
I believe as long as you have a little more Exhaust size than Intake size it will work fine.
From all of my research it looks like 3 each 3/4" Vents will work fine. I like 4 better.
I think most could get by with only 1 each 3/4 Full Flow Ball Valve. Leave 1 cap off, and fine tune with the One Ball Valve should work fine. Again I have 2 ball valves.
The Big Pappa Kit has a pair of sliding Intakes with 4 each 3/4" holes each. WAY more air than is needed for a Low and Slow cook. The Big Pappa UDS is set up to Grill also, so it needs more Intake Air for Grilling Temperatures. Using the Big Pappa Air Intake you need to only open it up 50% for Start Up.
If you are going with a Flat Lid with NO Bungs, I would get the Big Pappa Exhaust Vent for $17.
If you have a 2" bung add a 4 - 6 inch stack to it.
For the Air Intake I would put in 4 each 3/4" vents. Cap 3, and a Ball Valve on the 4th.
No expert, but that is what I would do.
Edited by Pappa Roach1 - 2/24/15 at 9:54pm