Look the area of the exhaust will not affect how much air flow gets in or out that is controlled by the inlets.
if you have 3 3/4" inlets and you have 1 open and a ball valve partially closed you will only flow as much air as will flow thru those openings.... you will not draw a vaccum on a self powered air pump which is essentially what a UDS or any smoker for that matter is. The engine and fuel are the heat and the coals/air, the throttle is the intakes and the exhaust is the smoke stack. You will not draw a vaccum (you can not exhaust more exhaust than the air you draw in) but you can cause a restriction by under sizing your exhaust..... which will result in stagnat air and rancid acrid smoke or an inability to reach temps...If you can't get the air out you can't get the air in... the longer stack creates a restriction making the exhaust speed up on it's way out of the longer stack and the effect is intensified if the stack is tapered from bottom to top.....
If you don't believe this just please stoke you up a really hot fire put in about $100.00 worth of meat and slap that domed lid on your WSM and close the exhaust and wait about 5 hours when you take the beautiful meat out of the pit it will be nice and foul tasting with about a 1/4" of soot and raw right in the middle...
Thats why most never choke a fire with the exhaust, they starve it with the inlets....Basic BBQ knowledge.....
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Last edited by razrbakcrzy; 02-15-2013 at 10:41 AM..