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Shhhhhh! Muffler options


Pitabu

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Hi All- I just replaced the muffler on my old B&S 8 hp with the factory stock. While quieter, I'd love to notch down the decibels down a bit more-any ideas? I realize there are some back pressure considerations, bit anything else I can bolt on to take it down to dull roar? Thanks! Sean
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Several people have looked into building a quiter muffler. I designed and built one. It ended up being louder than stock at idle and getting quieter as the rpms were brought up higher. Even though it is quieter than a stock muffler at WOT, I was not really happy about the low rpm noise. I have another design idea, but haven't fabricated one yet. By-the-way, the designs I am looking at do not produce much backpressure, because the gas flow area within the muffler is never restricted to less than the area of the inlet pipe. In fact it is normally at least 25 percent larger. The mufflers I am working on are designed to be fabricated with common tools (drill, hacksaw, grinder, welder). The parts are relatively easy to fabricate and assemble. Welding of the muffler case the is most time consuming part of the assembly. If my next design is as good or quieter than the stock muffler, then I will post the design for everyone on the site. For those of you Techies, it is based on the premise that a standing wave cannot be maintained in a vortex. Is that clear to everyone? LOL
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In order to make a silencer work, you need some back pressure to break down the pulsations. The engine people guard that information like it was Fort Knox material. To get a good silencer you almost have to run an engine on a dyno so you really get every ounce of power out of it and work on your muffler designs from there. Sometimes like Rod has got in his muffler, you give up one noise for another. Rod has almost has what he wants and a little more back pressure on the low end might take away his annoying noise.
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I have spent literally hundreds of dollars on this same problem and have yet to find a solution. Not gonna spend the time in the archives but to save you some time the problem is the engines in these machines, not the mufflers. I have tried stacks, supertrapps and john deere mufflers. I have also tried lengthing the total exhaust piping and cheap cans. I have even taken measurements of all these setups with a dB meter. No success. Only chance is to change out engines to a QS series and go from there. Brent
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I agree with Brent. The biggest problem is that there is only one cylinder firing every other revolution. The more cylinders that you have, the easier it is to deaden the pulses and produce a smoother flow. With a one cylinder, you just don't have too much that you can do without creating lots of back pressure. Two cylinder engines are much easier to quiet down. I had a V-twin 14 Hp B/S Vanguard that was so quiet at idle you had to check closely to see if it was running. Even at full throttle the mower made enough noise that you could not hear the engine. This was with the factory muffler on that one.
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Okay, stupid question time. If a twin is quieter than a single---because of the 2 input pulses, which I suppose are out of phase with each other (Assumption)---Question: then why can't the smart folks teach us to build a muffler for a single that splits the input into 2 out-of-phase paths which cancel the noise? As an aside, I also tried a stack muffler---way too loud---gave it away, and went back to the B&S little square box muffler. /r David in Denver No snow yet down here, but it's snowing in the mountains. Let is snow, let it snow, let is s.n.o.w. oooo!
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I've always noticed that everyone else's engine seems quieter than mine. Sort of like kids; if they belong to you their crying seems the loudest. I have a KT-17 twin on my Deutz-Allis and a 17 hp Kawasaki single on my (ahem) JD walk-behind. They seem equally loud, but I'm usually wearing plugs, so my perception is probably warped. The vortex effect makes sense. I know that cars equipped with turbo have very quiet exhausts. Still, when you mess with air flow, you're creating or losing back pressure at different rpms, which will affect hp or torque. Like someone said, the engineers who designed these things probably would have made a quieter muffler if they could have. I'll stick with my ear plugs!
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David's idea is very interesting to me: [url]If a twin is quieter than a single---because of the 2 input pulses, which I suppose are out of phase with each other (Assumption)---Question: then why can't the smart folks teach us to build a muffler for a single that splits the input into 2 out-of-phase paths which cancel the noise?[/url] I'd like to see more discussion on David's idea of splitting the pulses into two halves, and delaying one half (via a longer exhaust path) in an effort to produce a reduction in sound, similar to the Bose Corp.'s ANR (Active Noise Reduction) technology. However, I believe issues exist as to whether: a) the pulses could be effectively split, and b) whether this would cause excessive backpressure. Of course, the biggest issue, IMO, is whether simply delaying one half of the exhaust (via a longer route) would actually result in creating the "opposite" of the other half of the sonic output of the pulse. Bose's technology, as I understand it, is based on the theory that reproducing the exact opposite of a sound (and I don't know if that's measured in frequency, tone and/or?) will result in a cancellation of the original, unwanted sound. The Bose units sample the ambient noise, and allegedly reproduce it's exact opposite, for your ears to hear, in combination with the original, resulting in the cancellation of the original. I had the opportunity to test these headphones, at the Bose booth at the Reno Air Races a few years ago, as the Blue Angels made a low-level flyby, and I was stunned at what a good job they did! While I could still hear the jets, they were muffled far better than they would have been with any non-ANR phones I've ever used, including my 30 dba-reduction Peltor units. So, are there any acoustic engineers in the house who could flesh this out for us? I'd love to hear more, and ultimately be able to build such a muffler, so I could hear less, when I'm tractoring. Thanks, Peter
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Sound canceling on headphones is done by sampling what is sound is present and then adding the same sound back in that is phase shifted 180 degrees. Basically sound is a pressure wave comprised of positive and negative pressures. To cancel them, when there is a positive pressure point in the pressure wave, an equal negative pressure point in a phase shifted wave is applied by a speaker. The addition of the two results in a cancelation effect. As for the a sound cancelling scheme for an exhaust system, companies like Flowmaster are making mufflers with this. Basically they take a portion of the exhaust flow within the muffler and then divide it and at a later point in the muffler the exhaust is combined. When the two exhaust flows are combined sound cancelation takes place. For single clyinder motors, I'd think the best approach would be to do something like what flowmaster has done for sound canceling and use the sound from the same exhaust pulse to cancel itself instead of that from one pulse canceling sound from another exhaust pulse. Now for instance our single cylinder briggs motors are approximatly 32 cubic inches. The twin kohler motors on our tractors are 40+ cubic inches. Each kohler cylinder is ~20 cubic inches. Generally larger cylinders take in more air/fuel than smaller cylinders so more energy is present to make sound. This is one reason a single cylinder motor is louder than a twin.
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There are several ways to quiet an exhaust. Problem comes from the size of the muffler and the complexity of the internal parts. As you may have guessed, If the muffler was large enough, and cost didn't matter, then you could have a very quiet engine.
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One of the quitest 'mufflers' I have had, was a rather long, about 8" pipe screwed into the exhaust with a punctured steel can wired ovet the end. Had a bunch of holes in the can at the bottom, away from the pipe inlet. Was quieter than stock on a 5 HP Clinton.
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