ORIGINAL: jetmech05
I wouldn't use extensions...cut the cowling holes bigger if possible. Alan is right about the holes for cooling the engine....Its not the size of the hole into the cowling thats so important its the size of the hole that lets the air out of the cowl that controls cooling of the engine.
Look at our full scale brothers they don't control the hole size into a recips cowl, they just close of the exit with cowl flaps.
Actually, there is more to it than that.
Actually cowling holes are usually as small as possible. But air doesn't like to turn, so the cowls are left open over the places the designers want it to go if possible. Like the cylinder heads. Or close to over them.
Look into the cowling of a T6, P47, Corsair, or almost any radial engined bird. Look closely and you will see that the cowling hole may be wide open but the inside isn't. Almost all the area is blocked to airflow. There are baffles that block the air except for VERY close beside the cylinders (and anything else the designers wish to air cool). The design is called, "pressure cowling". It's there on all the full scale birds. And there is more still.
Hot air is harder to move than cold. So the cold air coming into a cowl will go around the hot air that is next to the things that need cooling. And those things won't get cooled efficiently, if at all. So the colder incoming air has to be forced by the hot objects.
The percentage of air-in to air-out is important. There always needs to be more area for hot air exhaust than cold air intake. When air is heated it expands. You can think of it as getting fatter. Over the years, modelers have found that you need at least 1.4 or more exhaust area than intake. 1:1.4 works. Anything less is not good. And it helps to place the exhaust openings behind the hot objects as well. But all that is another topic.