ORIGINAL: Mike Connor
Lots of good info here and it seems the right size in the right place is the key. However, form all the pictures, there is more then one way to get the job done. Here is a picture of my latest attempt and it may get mods depending on what Andy lennon's cowl chapter says or what you guys say.
I used contest balsa and wrapped it around the front half of the cyl. There is a medium size opening in the front with a height of the cooling fins. The cyl head is partially exposed and the tapered outlet to the rear is at least 1.4 times the fin area. This cowl design when finished will be tested on a inexpensive engine before going on a speed engine.
If the speckled grey stuff is your cowl you need to consider streamlining the area around the carb. Streamlining is not just putting a slick surface on the shapes you've got to enclose. What you've got down around the carb isn't really going to do much of anything in the way of streamlining that area of the airplane. You've still got basically a large flat area almost perpendicular to the airflow. Look back at that yellow speed plane's entire front end.