The areas for the intake and exhaust are this way:

The intake area is usually sized to the frontal area of the cylinder and head. That's for sport airplanes. An engine down in a Cap 232 cowl would want all the holes in the front of that cowl to have a areas that add up to an area the size of the front of the engine's cylinder and head. One of my engine's has a cylinder about 2" wide. And the cylinder up to and including the head is about 2" also. That's 4 sq inches of intake area. I actually closed some of the big gaping holes in the cowl of a recent ARF. I left the holes in front of the engine and closed off all the rest.

Since that cowl had about 4 square inches of intake, I made sure that I had 6 square inches of area behind the engine. I actually stood off some of the cowling away from the fuselage to do this. In your case, you've got the right idea for where that is going to be and you've got to make it yourself.

Serious speed planes aren't like sporters. If you're actually trying to get the airplane up near it's top speed, you do need to consider the opening size. What a lot of people did was start with the general size for sport planes and do some test runs. They usually found that the engine wasn't running warm enough and that the opening was holding the top speed down. So they'd reduce the width of the intake opening and test again. They were matching the size of the opening to the speed they were getting and considering the problem of engine heat and tuning the drag of the opening all at the same time. It is fun to do.

That is a little confusing to me. If the size of the holes in the cowling is equal to the size of the cylinder and head then the entire front would be open. From the pictures I am looking at, that is not the case.

I attached my picture again with yellow outline to distinguish between the fuse, cowl and spinner. There is a little flat area (less the 3/8")in front of the carb that could not be avoided on this engine. The front of the carb comes up to the nose ring and spinner and I saw no other options. My Jett engines will work out a little better in this area.

I have tested different props over and over before and this may end up being similar to get the best results. I read that a cowled engine can give you up to 12% increase in speed. If I get 1/2 that and not burn up my engine I will be happy for awhile.

Thanks for your input

Sorry, I've been typing about this issue so much that I'm getting sloppy. The opening you have looks suitable for your engine for maximum speed.

The sizing equal to the cylinder is a starting point for general use models. Everyone interested in maximum speed expects to work down from there, and most have some experience and rely on that to figure their first opening size. And often expect to work that down.

As for the average model that's not a pure speed model:

ARFs often come with pre-cut openings. They're often larger than would be required for the engine you would select. So you close off enough of the openings until the right amount of opening remains. You do that in such a way that the opening that remains is in front of your engine.

BTW, ARFs also usually come with little or no exhaust openings directly behind where the engine cylinder winds up.

The Sukhoi31 below came with a solid fake engine plate. It's painted flat black in the picture. There are actually two holes cut into it. There is the large one directly in front of the cylinder. There is a smaller, harder to see one to feed cold air directly at the muffler. What isn't shown are a couple of foam baffles I made to route the air inside toward the opening for the hot air exit.

The picture of the Cap's red cowl shows the two large openings the ARF mfg cut into it. At first, the opening on the right might not look open at all, but it is. I glued a foam baffle just inside the opening and angled it to deflect incoming air toward the crankcase and the muffler. The total openings are way more than the suggested size because the airplane's purpose does not include "fast" or "speed".

Da rock
I did the same thing to my GP Skylane as you did to your cap speed was not a design goal but I wanted the motor to get better cooling than just cutting out the cowl openings and dumping the air into the cowl. A bit of foam and balsa made for light baffles and worked well.
My cub cowl is a bit more aggressive...
This was not designed for speed just wanted it to look good, and work well, as I told Mike the hardest part was setting up the exhaust which I ducted through the fuse.
Mike,
That chapter has been sent to you.

Iron Eagle,
Looks good. I would make one suggestion. The engine part that has to dissipate the most heat is the heat. That should always have access to cooling airflow.

Doing your own sculpting is rewarding isn't it? Not only do you wind up with something unique, but you have control over how it works and can make it work better.

BTW, a flying buddy just finished an Edge ARF with a BIG 4 stroke in it. The cowl had two huge openings. The engine happened to be mounted at 180degrees however. And he had only cut the cowl to clear the front part of the head. Basically, the front of the engine was shielded from the airflow. The engine ran like crap. He wanted to know if I thought he had the wrong prop for that engine or if it just wasn't a big enough engine. It was touchy, ran rough and didn't seem to have much power for a 1.50FS. My advice was to open the cowl in front of the engine and close off most of the gaping holes that weren't doing spit. Darned if he didn't take the advice. Bloody engine runs like a locomotive now and the airplane is trimming out to be an excellent flier. And incidently, he's running the same prop.

Da rock
It is hard to see in the photo because of the angle but the top of the head is even with the bottom lip of the cowl intake. I also have the top of the cyl and head in front of the exit to the cooling exhaust duct so it should get the best flow, at least that was the idea. Here is a side shot where you can see the cooling exhaust. You are right it is a lot of fun carving out your own designs.

Mike
Sorry for the hijack....

Not a problem. This is an engine cowl thread and I am learning.
Thank you everyone for your input.

Mike was saying in his post in the speed forum he seems to be getting a little better than 120 mph passes with the beast. He has not had any know cooling issues with it thus far, not bad with a .46 I would say.

I finally remembered to touch the cyl head right after a landing and it was too hot to leave my hand on but I could touch it briefly without problem. It was a 90*F day. During flight there is no sign of overheating. I left the cowl on for all flights on day two. I think I am OK. In time I will get a speed check without the cowl to see if it does anything other then make it look good.