It actually decreases air flow into the cook chamber on my build. If my cooker is running 20 deg or so too hot I can simply open that top vent and she will come right down where I want it. If it is more than 20 deg I also have to close off my smoke stack a little as well. I did not subtract the 2"x4" top intake from my overall required intake volume. Whatever the formula called for intake wise is in the lower intakes. The upper intake is just extra. Since I added a dual 25 cfm BBQ Guru system to my unit the top intake really works great.
Hi Jim, So your saying your not pulling more cool air in opening the top vent without stoking the fire? And yes I stand corrected if your closing the bottom air vent equal to opening the top air vent you are not increasing the air flow through the CC.