I'm finishing up a Carl Goldberg Super Chipmunk and I'm mounting an OS 91 Surpass.
I followed the directions for mounting the engine horizontal. In hind site, inverted may have been better.

But my question is, if I can squeeze the muffler in the cowl - should I do it? Some have concerns of over heating - or the carb sucking in hot air. I test fit it, and I would have to do a little cutting to the bottom of the cowl (along with the hole I already made on the side for the top of the engine) - but it looks like it will fitm, and point downward. Or should I just have the entire muffler assembly out of the cowl - pointing out toward the wing.

Should it look pretty - or be plain old functional.

I did that with my Chipmunk, and it works fine, BUT..........
you have to cut a pretty fair sized hole (mine is a 1"x 2" oval) in the bottom, and it helps to use some sort of an extension on the muffler. That 4-stroke exhaust is pretty hot and will easily melt the covering off the part of the fuse under the cowl if you let it impinge. Don't bother asking how I know all this, lol.

My Chipmunk is several years old and I ran the muffler outside til I recovered the plane and put on a new fiberglass cowl. It flew fine the other way but looks much cleaner with it enclosed.

Strat, thanks for your input.

Nice to hear yours is several years old. Can't wait to fly it, from what I read I should enjoy it.
It's been a good kit to build too - not too complicated.

I hope to post some pics when it's done.

Catdog,
Built the Chippy myself with the .91 Surpass with the pump, I purchased two 90 deg. muffler elbows from OS and was able to place the muffler with the exit pointed out the bottom back of the cowl. End of the muffler is even with the bottom of the cowl, nothing sticking out and look clean.

WYo-flyer,
I see that Tower does stock the 90 degree headers. I couldn't find them earlier. Maybe that's the ticket. (not cheap though).
I see OS also makes a 4.72" flex pipe (listed at $28.99!) That could be even better, but I wonder if you still have enough back pressure from the muffler for the fuel tank.

Anyone ever try the flex pipe?

I have a buddy that had a Goldberg chipmunk that had his muffler in cowl I'm not sure what type of muffler it was but it strapped on to the engine and it looked like one of the long muffler extensions.

I bought my Chipmunk already built from a flying buddy of mine. I originally put a 61 FX in it. Last summer the engine died on takeoff (hole in pressure line) and I nosed it in. Not much damage done but it totaled out the cowl. I decided to take out the 61 (it went quite nicely in a Venus) and put a 91 FS back in its place. I ordered a new cowl from precision-fiberglass-prod.com and it arrived today. (Very nice btw and good price). Does anyone have any pictures of how they cut their cowl? There has to be a way to get the cowl over the cylinder and needle valve without cutting it to pieces but I can't see it. I'm sorta thinking about cutting the right side off, hinging the two pieces together at the top and figuring out a way to attach the two pieces at the bottom (maybe a strip of lite ply overlapping the two pieces and then screw into the lite ply). This seem like a lot of trouble but it would be easy to put on and take off.

Just wondering what to do,

Thanks for the help,
Tim