making decent mufflers
 

I ran the echo 65cc conversion today. I polished the ports but made no change in timing. The timing was pretty aggressive already. Running gave interesting results. Bambula 22x8 for both runs. The first run was with a straight pipe of about 0.65" interior diameter. It is resting on top of the head in the picture. I got an impressive 7600 rpm for about 5.5 hp. It was PAINFULLY loud. It is not an option for real use. The second run was with the muffler attached to the engine in the picture. The box was gutted and a pipe of 0.65" ID was brazed on. I lost 900 RPM! I got in the area of 6700 RPM. The muffler box has a cylindrical coarse mesh screeon over the exhaust port. It is in there good and resisted all reasonable measure to remove it.

I know this engine will run good with a large volume can and a larger , about 0.75" pipe. I made a couple of such mufflers but the aluminum solder kept on melting in flight.

I want to make a steel body muffler. I was thinking of a propane cylinder. Those fat squat ones or the skinny ones for the cylindrical part of the body. The reason is so I can braze on the pipe or pipes.

Is there some formula or guide line on the expansion chamber volume or shape? I am guessing I was getting some inertia and laminar flow effect from the straight pipe. I bet that some expansion chamber volume might have a simmilar effect or even a resonate effect like a tuned pipe.

Any opinion on this?

RE: making decent mufflers
 

Match the ports...but polishing is a waste of time. It is proven too smooth a sufrace the fuel sticks more!!! This was done with a flow test device. Years Ago. My cousin had one. Capt,n

RE: making decent mufflers
 

First expansion volume at least 8 times engine displacement. 16x is better, and you then need only a small diameter stinger for good power and excellent silencing. This device is also known as MCM (mousse can muffler) It should be tuned with the header length. Header can be partly, or completely inside the can. Header and stinger not in line, but offset to each other for best silencing without power loss.

RE: making decent mufflers
 

Thanks for that.

I was looking at high temp silicone tubing today for that very thing. It is to connect the header to the can. I wanted some 3/4" (~20mm) ID. It is tough to come by. Is there an automotive application for this material? It woud be easiar to tell the auto parts something he understands. If I go in asking hight temp tubing to connect a header on a model engine to a metal Budwiser can with a folding chair led skewere through the midd he will be very confused.

Is there a metal flex tubing that will work?

I was reading up on aluminum beer bottles as mufflers. It looks promising.

I ran the Echo 65cc with an HDA carb and a gutted chainsaw muffler today. It go 7850 rpm with a Bambula 22x8. Things are looking up. :)

RE: making decent mufflers
 

3/4"' is 19mm... but Ive heard that the heater hose they use on big rigs works good.

RE: making decent mufflers
 

Heater and cooling hoses are as close as you can get in automotive supplies. They reportedly hold up well enough.

RE: making decent mufflers
 

I kept a sharp eye out for muffler cantidates. I have a bunch of old transformers. The shell halves on a less desirable
one worked great. I also used a piece of old shotgun barrel for the pipe. Now the solder won't melt!

This engine is running pretty good. IT tops out at 8K with a 22x8 Bambula and about 6800 with a menz 22x10. The idle is a bit rough. I can get it down to about 1800. I am questioning the ignition. I am absolutely certain the fuel side is good. The bearings and seals are new. The carb is a new HDA. The intake manifold is brand new, PFTE plastic, and fits perfect.

I carefully timed it to 29*. It did not give good power. I ajusted the timing by tach to, what turned out to be 36*, to give the better numbers. Could it be that it is not advancing properly? The ignition is a well known unit of which I own five. They are probably the most popular Chinese unit around. All work well. This one has less than an hour on it. Would this be a candidate for replacement by the vendor? I hate to tear into an airplane to get a known good unit but I guess I will late next week.

RE: making decent mufflers
 

I have used a standard electronic timing light, in the past, to see if the ignition is advancing OK. Use a 12v supply for the light with a common ground to ignition system. Haven't tried it with a shielded plug lead though, may not get enough induction. Good luck, red