Less clearance is better.

Force the air to flow through the fins and the cylinder head. A classic cooling problem is the WW1 radial cowled engine with lots of air going in through the front and out the hole cut in the bottom of the cowl. However the poor old upright engine is sitting in stagnant hot air with little flow.

The image shows a typical pylon race set up. These engines run at 30,000 rpm and get HOT. Rule of thumb for all set ups, the outlet must be at least 3 times the inlet.