Re cooling all that has been said above is correct. Re exit "holes" on the cowl consider the following on most a/c fitted with piston engines, radials to be considered here, there were fitted things called gill cowls (or flaps) the actual cowl was usually set up for max cool when the flaps were open and closing allowed the engine to maintain operating temperature. On Russian engines that actually had louvres mounted on the front of the engine that stopped air flowing over the engine, if the place is that cold I don't want to go there. That being said I subscribe to the max airflow through repeat through the cowl. Big inlet smaller outlet will back up hot air in the cowl. Likewise if the air isn't directed onto the cylinders then it is doing little good, use baffles as required and of course have the right oil to fuel mixture and don't run lean.