Exhaust Muffler Design

[kerf]

The carb side of the silencer is sealed with a piece of 5/8" round foam caulk backing. This effectively seals the silencer to reduce noise from being directed toward the rider. I went to Wally World looking for something to use for my silencer, finally found a three piece canister set, regular price $6.95, on clearance for $2.00.

There is about a 1 1/2" space between the end of the air filter cover and the canister. I tapped the air filter screw hole with a 1/4 x 20 tap and used a short piece of threaded rod with nuts to hold the parts in place.

I did a 20 mile run this morning, if anything it's running better, go figure. On the down side, I'm suddenly very much aware of my chain.

 

[DetonatorTuning]

kerf,

in persueing the sound issue you may have dovetailed over to another that you had previously been concerned about.

at times you have mentioned that under low speed ,high load conditions you have strong inlet back pulsing from the carb that soaks the filter and causes rich running.

the new chamber MAY be adding just enough extra resistance to cancel some or all o fthe intake reversion pulse.

keep track of your filter and let us know if this is true. pretty awesome if it is.

also keep a close eye on your plug, you may suddenly move leaner as a result.

steve

 

[kerf]

Thanks, I'll do that.

 

[DetonatorTuning]

any changes at the plug or mileage to report ?

steve

 

[kerf]

New Direction

Many of you are aware of my battles with the R460 and the associated noise this little engine produces. I've tried many different muffler designs and my last one has proved the most effective. It also brought to light another noise source, intake noise. My solution was to put a silencer over the air filter to gain a little edge on the noise, the muffler and silencer are documented in this thread. Not totally satisfied (who would have thought) with my results, I added some fiberglass air filter material between the stock air filter and the end of my silencer. This helped a great deal but decreased air flow as evidenced by an over rich condition, which I corrected by adjusting the carb. Again not content, I began research into another method method to cut intake noise.

I began searching intake noise and found an interesting article about a guy with a Tomos 55 moped. He had installed a HP air filter and gotten pulled over for illegal muffler, which I recount to illustrate the significance of intake noise. What I was actually after was information on the Helmholtz Resonator and how I could employ it to help my problem. The two methods that I found to correct this issue were pre air filtration silencing and post air filtration silencing, the Helmholtz Resonator or as it's more commonly known an air box. Air boxes are widely used on engines today, especially in the automotive industry and I thought this might be the answer.

During my searches, I found an intake silencer for a Rotax ultra lite engine (IMG 1), this seemed pretty simple to me, an air box between the filter and carb with the filter at 90 degrees to the carb bore. Working on this principle, I began looking for a suitable box and settled on one of my old friends, a 4 x 4 x 2 inch electrical box from Home Depot. This plastic box has no ports or openings (drill your own) and has a "marine" rating, due I guess to the O ring sealed lid. After determining how I wanted to mount and brace the air box, I did a little drilling and painting (Krylon Fusion) and put it together. I have photos of the mounted box (IMG 2), the brace (IMG 3) and the assembled unit (IMG 4 & 5).

My first test ride was amazing, first off I had to lean the carb back out and for the first time I realized just how effective my muffler is. This has reduced my engine noise by at least 50%, absolutely unbelievable!

 

[happycheapskate]

I'm running a lawnmower box muffler because I don't care about the noise (it wakes up drivers!).
I think if I cared about noise I would look for some kind of large chamber with baffles in it to change the noise to a deep echoing sound.

Has anybody tried running their exhaust through a truck heater core or oil cooler? That would change the direction of the exhaust many times, cool the exhaust (which might reduce sound wave volume at exit), and would provide variable back pressure (a valve like on gas or compressed air pipes might even be incorporated)

 

[professor]

Today I learned what happens when you take a muffler off a 98cc two stroke.
I drained the gas after pulling the engine (flipped it upside down and shook it around). Then decided to pull start it as it sat on a chair to get all the gas out before putting it away.
At the idle stop, the engine raced at probably 3000rpm for a good couple of minutes ( with me hanging on to it real good). I have to think the 3hp cigar muffler (and piping) was a big hp killer.

 

[happycheapskate]

Thats just silly to fire or start any kind of engine without it mounted to something.

 

[happycheapskate]

Thats nice but seriously complicated. What is the metal round part that is on the outside of the home depot box, before the air filter (nice filter!)

Why not just use a 4x2 box with a oiled foam filter in it, with 25 cent size holes on the outside, like a briggs and stratton lawn mower filter box. The foam filter will absorb fuel that blows out of the intake on downshift/decelleration and should cancel a lot of intake noise.

 

[kerf]

Thats nice but seriously complicated. What is the metal round part that is on the outside of the home depot box, before the air filter (nice filter!)

Why not just use a 4x2 box with a oiled foam filter in it, with 25 cent size holes on the outside, like a briggs and stratton lawn mower filter box. The foam filter will absorb fuel that blows out of the intake on downshift/decelleration and should cancel a lot of intake noise.

Actually it's incredibly simple. The R460 is a high performance engine and to that end, is provided with a high flow gauze air filter similar to a K&N. The trick is to reduce the intake noise while not substantially increasing air restriction. The Rotax unit, I modeled mine after, list a 2% reduction in power, in my case that would be almost .1 horse power. So far I can't tell any difference in performance but I've only been up to about 8400 rpm (35 mph), but throttle response feels good. Having a solid surface directly across from the throttle bore must reflect some of the sound waves back toward the source, whatever, it works.

The round metal object on the side if the air box is a velocity stack.