Got a BCM muffler running on a ZDZ 40. I've noticed that the twin exhaust ports/tubes are crimped inwards on the end. Was wondering if there would be any performance to be gained by removing those slight restrictions. Appears to be a simple matter to reaming with a proper size drill bit or even a dremel sanding drum. Anybody experimented with this set-upd?

Even easier just to cut the crimped ends off with a saw.
Most people claim this adds power.

ZDZ 80, Bisson pitts also.....cut the tips and gained an instant 300 RPM...a bit more noise though.....real numbers, not " I heard from a friend of a friend of his" info. FWIW

ZDZ-40, cut 1/2" off ends and picked up at least 700 rpm. Major difference in power.

Louder, but not annoying in any way.

Another way is to cut the tubes to about 3/4" in length, then slip silicone tubing over them to extend to original length. This is a little bit quieter (not much) but you will not bend tubes if you nose over.

Thanks, I'll go for it!

Interesting. I am not happy with my performance and may try this if the muffler comes loose again.

Any restriction in the exhaust outlet tube(s) leads to higher muffler presure going to the fuel tank...and that's usually a good thing for reliable fuel draw....especially critical when using a Pitts style muffler. Pitts mufflers are notorius for reducing exhaust back pressure, and hence requiring a carefull retuning of the carb needles. Some installations simply will not run well with an unmodified Pitts muffler installed in place of the motors original stock muffler. There are threads on RCU citing the virtues of plugging one of the Pitts muffler exhaust tubes so as to re-establish sufficient back pressure and thereby improve fuel flow, thus eliminating fuel starvation problems. I have had to do exactly that on an OS46FX set up, and on an OS61FX too. Further, it would seem to me that a few hundred RPMs up or down, will not significantly, or even noticeably affect flying performance. So, bear this info' in mind as you modify your outlet tubes. There is merit in the old saying..."if it aint broke, dont fix it !". I hope this info' is helpful.
happy flying.

Jim, we are discussing Gas engines. The statement you made is good for glow. Reducing backpressure while keeping the noise of your engine low is of major importance to our ability to keep good flying sites and to fly strong maneuvers.

I guess I'm just gettin' too old....sorry for placing my lengthy disertation in the wrong forum. I appreciate your little note, and I'll just have to try to be a little more aware of where I am.
heh heh.
Have a great day.

Jim Bisson would tell you its a tradeoff.Bigger can+ crimped ends= less noise.Depends what you want.......there is no free lunch!

CUT them!!!! you will gain 300-500 RPM!

Ryan