Wings - The cowl in the pictures you posted does not enclose the engine, it only makes the front of the airplane "prettier" and more aerodynamic... the majority of the engine is still exposed to outside air for cooling purposes. The "cowl" the others are refering to is the type made from fiberglass or plastic, one that encloses the engine, either completely with the head and all under a cowl or mostly enclosed, meaning only the top of the head (or valve covers of a fourstroke) protrude from inside the cowl. I have included a couple pictures of my Funtana to help you understand. As you can see, most of the engine is enclosed. The two holes on the front are the cooling "inlet" holes, the large cutout in the bottom of the cowl which is visible in the bottom veiw of the airplane would be considered the "outlet" hole. The air coming into the cowl is getting forced in by airflow generated by the aircrafts forward speed and propblast coming off the propeller. Once inside the cowl the air gets slowed down by the engine, motormounts, fuel lines, carb and anything else you can see when looking through the cowl from the inlet holes to the outlet hole/holes in the cowl. So, if you loose air velocity as it travels through the cowl you need to increase the VOLUME of air that can exit the cowl so that the hot air coming off the engine is not permitted to circulate in a pressurized, turbulent area outside the most efficient path of travel that the majority of the air is taking through the cowl, hence an "outlet" that is at LEAST 1.5 times the size of your inlet, preferably twice the size. Hope this helps

Brian