I have mufflers on the exhaust system and they are right by the exit of the bottom of the cowl. That may be keeping the heat from exiting the bottom cowl, so now I am going with straight pipes.
I decided to take the mufflers off and just see what the pressure differential would be. We had one end of the line going through the heat vent on the co-pilot side and up and across the firewall to the back of cylinder 4. The end of the tube was facing the center of the engine to prevent ram air from entering since we were trying to measure pressure. I don’t have a pic of the other end of the tube, but it went under the oil cooler to measure the difference between the top of the cowling and the bottom.
Here is the difference in the two:
The difference ranged from 4-10″ depending on our altitude and speed, but it definitely passed the test.
We landed after the pressure test and Dale did all of the aerobatic testing so we could log it in the airframe book.
Here are a couple of pics of the air caps installed: