I believe the over(!)-simplified idea is that what cools the engine is the movement of cool air coming in to the upper side of the engine compartment and flowing down through the cooling fins, carrying heat away out the cowling exit area that is shared by the exhaust pipes. In order for downdraft flow to happen there needs to be a pressure differential between the top and bottom of the cowl...high pressure where the cool air comes in on top of the engine and lower pressure beneath the cylinders. If the fiberglass ramps on the inside top of the cowl aren't sealed it allows a very big "leak" of air between the upper and lower cowl and it will reduce the needed pressure differential.
A lot of us that have battled cooling problems have been on a seemingly never ending quest to seal the upper plenum as much as humanly possible. The usual leak culprits are ill fitting rubber/silicone baffles, the ramps we're talking about, the big hole that feeds the oil cooler and loose fitting curved aluminum around the cylinder fins. The search for leaks includes turning the hangar lights off and having a friend shine a bright flashlight all around while maneuvering your head in every possible way to spot light leaks that imply air leaks.
# 3 is frequenly the hottest and if you search these forums you will see a lot of discussions. The 2 most common solutions include either putting a washer as a spacer between the cylinder and the baffle or pound a curve in the baffle there. Both solutions are in order to allow air to travel from above to below, around the cooling fins. All this said, 380 degrees is not at all hot ond many would call it very close to ideal.
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