I would like some suggestion for a muffler design for my 99 cc single cylinder converted chainsaw engine.
I have made one with three simple baffles. Each at around 60 % of the full diameter of the canister section of the muffler. The engine is still very noisy with it, and has a sharp "bark" kind of sound. I required ear muffs while running it up on the bench and in the plane.
Can someone direct me to a source of design information?
Also, we are having difficulties with tuning this engine, and it has been suggested to me that this muffler has too little restriction.
Is this likely?
I will post a picture of it tomorrow.

Engine conversion thread here with some pictures:

http://www.rcuniverse.com/forum/engi...l#post12245172

A longer exit stinger and smaller diameter will make it quieter and give more back pressure. Filling it with glass or stainless steel wool will quiet it down too. Make sure it does not get back in the motor.

No muffler expert here but I have made a few custom ones over the years. All steel by the way as they were used on airplanes that needed nose weight. Simple rules I follow are to give the muffler as much volume as possible. That means make the can as large as possible! A large can also helps reduce the exhaust volume a tiny bit by slightly cooling the exhaust gases. I prefer a large diameter, long exhaust tube or tubes as these tend to produce what seems like a lower exhaust note. Smaller tubes produce a sharp cracking sound. I doubt the actual noise level changes much between the different tube sizes but the perceived sound level certainly changes. I've never used a baffle in the mufflers as I wouldn't know how to design them and would probably do more bad than good by installing them.

I have decided to make another one. I will use two concentric tubes, the inner one perforated with many holes and the gap in between packed with stainless scubbing pads. There is a name for this type of muffler, but I can't think of it.

Here are photos of a hobby king muffler I modified to fit into a plane. See the internal baffle plate. I have not flown these yet so I can't speak to effectiveness however I don't believe it will be great like a more expensive version.

Hi Jim i have made a few and i get a good simple muffling effect by running the stinger tubes all the way through to the base of the exhaust box and the tubes inside the box i drill large holes that breaks up the sound wave as it finds its way out.
Personally i would not use scrubbing pads as all exhausts suck and blow and any loose scrubbing metal may get sucked into the engine.
As stated get the main box as big as possible as hot exhaust gasses need somewhere to expand.
Dave.

Thanks Dave. Is there any chance you might be able to supply a simple drawing or diagram? It would make the explanation a lot clearer.

Hi Jim i do not have any drawings but here are some shots.
The aircraft is looking a bit used at the moment 3 years + old and 52 flights its not far off a strip and repaint.
The bent header is from hobby king the rubber joiner to the exhaust box is a car water heater hose this i got from Super cheap Autos they have a big box and you rake around until you find the correct size.
its a lot more durable then silicone tube , so far i have replaced the car water heat hose once its quite heat resistant.
The box i made from a large ally box section i had in my spare ally box the ends i added and used pop rivets and heat resistant silicone to seal.
The stinger you can see runs down to the bottom of the box and before brazing with ally brazing you drill the holes in the stinger i think i used about a 10mil. drill with plenty of holes, and you can see the thick bead of ally brazing round the stinger head.
The stinger rests on the bottom of the box as a support or you can fix it in someway perhaps split the ends and pop rivet it to the bottom.
The ally brazing are the rods you can buy i have it on my other computer and if you get stuck i will look it up later for you.
Its a fluxless rod ( see video below ) and it melts about 350C below that of ally about 500c and you clean the ally to join with a stainless steel brush very well and i clean as i heat the surfaces up at the same time and then coat both surfaces separately by applying the rod to the surface avoiding the rod in the flame or a bit will drop off.
By touching the rod on the heated surface the rod will melt from the contact with the heated surface then put them together and melt the two coated surfaces into one with heat away from the joint and use a bit more rod if needed, you do not heat the rod but apply the heat to the material you want to join. Apply the rod to the hot joint avoiding direct blast from the flame the heat from the join will melt the rod you can drag the puddle of melted rod with a stainless steel rod if needed , its a little bit tricky but thats the business we are in but naturally if you have a $2000 Tig welder, lucky you.

PS.Also the tube joining the header has had the same treatment. IE. it go's all the way to the back of the box and inside the box it has been multi drilled so in effect there are two sound wave deadening parts to the box. where the exhaust go's in and then when it comes out.

Found this on my computer.
http://www.aluminumrepair.com/video_new.asp

Thanks Dubs.
What part is the "stinger"?

Hi Jim, the stinger is the part that the exhaust comes out.

Thanks Dubs.

Hi Jim. Did you get the Stihl MS660 running good yet?
[h=2][/h]Sincerely, Richard
[h=2][/h]

Richard,
Yes we did, finally. There were multiple reasons it was not running properly, and was difficult to start. I will list them:

1. The fuel metering pop off pressure was far too low. I adjusted that up to 25 psi, within manufacturers specification. Causing rich mixture.
2. The fuel metering lever was set too high. Also causing rich mixture.
3. The carb was hard mounted and subject to engine vibration and possibly aerating the fuel interfering with pumping and metering.
4. The welch plug was by-passing. This, combined with 1&2 would have caused the engine to run (still rich) with both tuning needles fully closed.

I have now mounted the engine on a rubber mount, contrary to all the advice from my mentors. We tested it out in the plane at the weekend, and the early indications are that this is a totally successful method to reduce the vibration down to a flyable level. This has turned around the viability of this engine to power the plane; I now think I will go ahead with it.

Video here of the temporary trial installation. The carb is not properly mounted. I am installing it permanently today. : https://vimeo.com/181367442

The carb is the original chainsaw carb. I will have to design and make a more suitable one after the plane is registered and test flown. However, this works ok and is quiet enough, just a bit ugly.

Congratulations on a good job sorting it out. The plane looks awesome, I look forward to some flight video. Sincerely, Richard

Thanks for that feedback and encouragement Richard!
Much appreciated.

Despite the fact that I have been busy preparing and packing gliders for our slope fest next week, I have been thinking about possible designs for a simple muffler.
As this engine will remain soft mounted, it means that the muffler must only be fastened to the cylinder head. That is how the original saw muffler is fastened; just by the two 5mm bolts into the cylinder head.
What I have in mind, with no proper understanding of the theory involved, is to make an aluminium one, round cheese shaped and with two concentric cylinders capped with a removeable round cap. The outlet, or "stinger" as it appears to be called, could be out of the back of the outer cylinder. The inner cylinder is perforated with multiple holes. Between the two cylinders, pack stainless steel scrubbing matt materials, more or less densely. This of course will slowly coke up and restrict exhaust. However, the removeable cap (held on with 3 or 4 screws), would allow it to be removed and replaced as required. The inner perforated part of the muffler would prevent the stainless from possibly going back into the engine, with catastrophic results.
I will do a small drawing of it in LibreCad when I get the time.

I have now made a new muffler. The pictures tell the story I think. I made it from some exhaust tube from the auto muffler shop in town and some scrap metal from the tip. It works good, about the same noise level as the standard one, but a lot better looking and around 80 grams + lighter.
I ran the engine with it and with the new prop. 6000 rpm on the bench. I will do a RPM check with the old muffler to compare, if I get around to it.

A good amount of noise comes from a two strokes intake believe it or not. Now that you have the exhaust sorted, you might try adding an air filter and see if that helps further. If the original was bulky or untenable, try one from one of the car guys suppliers. (You will have to re-tune the carb, but you will find it easier to tune, plus the engine/front of the plane will stay cleaner, and you will use less fuel.)

http://www.davesmotors.com/Products/AF-Air-Filters/

AV8TOR

Yes AV8TOR, I have fitted an air filter. You can see it in the attached picture. It consists of a stainless steel coffee or tea strainer covered with a self made foam sock. I made the foam sock by hot wire cutting 10mm foam sheets from some scrap. The ends are bevelled and glued with an acrylic glue. The round end was hand cut and glued in. Seems to work fine.
I have never run the engine without at least the fine stainless mesh strainer on, so don't know if the noise level would be higher with it off or not.