Hello, i am trying to get a pitts style muffler to fit under the Cowl of my F-1 Kyosho Kelly. Is it all right if i shorten the exhaust tips, will it have any affect on the engine??? Any help would be appreciated

Flyboy76

you can cut off the exhaust extensions with a hack saw. The engine will not care. However, sometimes it makes for a better install if the exhaust is directed out of the cowl via these extensions. In other words, you cut holes in the cowl to clear the extensions. The exhaust has to come out somewhere.

Right i was hoping to be able to cut the tips off enough so that i could just attach a dubro exhaust deflector to them and then have a couple of hole in the cowl and trim the deflectors off so they just poked through the holes. Nice neat and simple. I know some people would say to put a tuned pipe on it but i have the pitts so that is what i'm using. thanks for the info.

Flyboy76

G'day Mate,
If you cut the tubes it will change the tuning of the engine, usually makes them sensitive to needle valve adjustments.
Make the hole bigger in the bottom of the cowl, you will need to to keep it cool anyway.

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.

Here are just two examples.

On one, I cut the pipes at the cowl. The cowl was straight and that's where the cuts had to be. The original length of the pipes was gained back with the extensions. I didn't choose the length however to match that length.

On the other, I left the pipes the original length because the angled cowl would reach around them.

On two-cycle engines, very often you can change the length of the exhaust pipes without any measurable effect on the engine. It depends on the design of the muffler and how the exhaust pipes play in that design. Lengthening does increase backpressures but we can almost always get away with short or moderate increases.

There is a silicon tubing sold in hobby shops that works better than the lighter, special purpose exhaust routers. It's the blue stuff in one of the pictures.

BTW, the picture of the Sukhoi shows the blue stuff holding the cutoff pieces on the muffler. The muffler for that engine has restrictors in the ends of the pipes. I wanted to test to see if the restricted outlets made any difference. They didn't seem to affect the power of the engine but may or may not have affected the sound. I wound up using longer pieces of silicon and left them off. The inner diameter of the silicon matches the id of most muffler pipes, btw. It provides a very good replacement for the aluminum, btw. It gives in noseovers for example.

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.

I used to crew recips

I do what Da Rock does, exactly. Cut'em off. On an OS .61 FX with Bisson pitts muffler I did restrict one of the tubes furthest from the exhaust port. I tapped the tube ID and inserted a set screw, with threadlocker, that I had drilled a 1/8 dia hole through. Made for a much better running engine. I have other engines with the same set up that don't require the restriction.

BTW I have the same Hobbico Sukhoi shown in DR's picture....many many flights, I love that plane.