silencing Weed eater engine

Discussion in 'DIY Marinizing' started by dorong, May 25, 2012.

  1. dorong
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    dorong Junior Member

    Hello everyone
    I wanted to ask how to reduce the noise of all 2-stroke gasoline engines or Weed eater engine?:confused::confused::confused:
    Reduce the noise of the engine power goes down?:confused::confused::confused:
    Thanks Doron:rolleyes::rolleyes:
     
  2. PAR
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    PAR Yacht Designer/Builder

    Air cooled engine are naturally much louder then liquid cooled, just for the lack of the insulating qualities of the water jackets and extra metal to contain them. The mufflers on most chain saw or weed eater engines isn't the most efficient of designs as much as making as small a foot print within the given package. You can reduce exhaust noise a good deal with a better muffler and still not rob much power, but the size of the muffler will increase dramatically, which is often the major objection. This change will not address machine noise from the reciprocating parts, but it will make a notable reduction in noise levels.

    If you just start putting different, more heavily baffled mufflers on, you will lose power, but a well thought out exhaust, can keep these loses to a minimum and still reduce noise.
     
  3. CDK
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    CDK retired engineer

    You'll never be able to make a silent Weed eater engine, but there is plenty room for improvement.
    The standard silencer is just a tin jar that expands and contracts with each power stroke. It does little to reduce absolute noise; it distributes noise in all directions so the reduction is relative. More wall thickness would improve the effectiveness but the manufacturer aims for minimum weight.

    For a boating application you could make a cylindrical silencer similar to the ones used for guns. It is a perforated tube within a larger, closed one, the space between the tubes filled with glass fiber or wire mesh. You need to experiment with the length of muffler to obtain the best performance without creating too much back pressure.
     
  4. Petros
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    Petros Senior Member

    the noise from a two stroke is typically the super sonic "pop" that occurs when the exhaust port is exposed. Any resistance added to the exhaust will of course reduce performance. However, containing the super sonic shock wave can be done without too much loss. You want to eliminate strait paths of the shock wave to follow from the exhaust port, and if it can be done without changing the direction of the exhaust stream too much, or restricting the volume, the loses will be minimal.

    The square metal box with vents as often used on yard equipment is light and cheap and accomplish those things, except is does change flow direction considerably. But any containment pipe, with vents to dampen the shock wave, will lessen the sharp pop. Also, if you put a long solid wall pipe on it, that leads to a large chamber that has fine vents in it directed away from the occupants, will reduce noise and have minimal amount of losses.
     
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  5. tom kane
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    tom kane Senior Member

    Exhaust back pressure on a two stroke engine can increase power by compressing induction mixture where it is needed ( in the cylinder) and a free flow exhaust may drag the induction mixture out. A longer Ram tube on the induction can give over 120 percent
    volumetric efficiency thus more power.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Tom is hitting the real issue with exhaust modifications. Both two and four stroke engines require a certain amount of back pressure to satisfy two basic functions, intake air/fuel mixture contamination (exhaust gases) and compressed gas containment. The higher the output of the engine the less back pressure you need.

    In the early days of 2 stroke diesel development a number of cute tricks were invented to address both issues and they've been used by hot rodders on 2 stroke (and 4 stroke) gas and diesel ever since. The basics are assumed, such as port matching and radiused flow paths. What isn't commonly discussed are the tricks, such as reversion lips or cones at the exhaust port (helps prevent exhaust gas contamination), back pressure controls and temperature controls.

    Anti reversion controls prevent exhaust gases from re-entering the combustion chamber when the port is exposed again, while gases are still evacuating. On small engines a lip or reversion cone can be installed at the very opening on the port. The idea is to prevent the slower moving exhaust gases that are clinging to the cooler exhaust port and tubing walls, from sneaking back into the chamber. Cones work the best, but small engines usually employ just a lip, which serves as a wall to slow the eddies of cooler exhaust gases, from curling back into the chamber.

    Keeping the flow centered in the outlet pipe is also quite important. Reversion cones help in this regard. The walls of the pipe cool pretty quickly just outside the chamber. The cooler walls slow the gas flow as it scrapes along, which also slows the more centrally located gases. Keeping the heat in the tube can make a significant difference in output. Thick wall pipe and 'glass wrapping are common tricks. This is one reason why manufactures use big, heavy cast iron manifolds, it keep the heat in the pipe.

    In smaller engines, pressure is generally handled 3 different ways; restriction, baffled or chambered. You'd thing these were the same, but they're not. The straight through "glass pack" style of mufflers are restriction and effectively narrow the flow path, while permitting some sound deadening to occur with the insulation within it. The louvers inside, squeeze the flow to a smaller diameter and the flow that eddies on the back side of each louver, are semi absorbed by the insulation.

    Chambered mufflers are the most common and as the name suggests, redirection of the flow to get the pulses out of phase for noise cancellation and flow restriction.

    Baffled mufflers are the "tunable" types, such as the Super Trap. Again the flow is redirected, but this time forced through a set of baffles. The Super Trap is one I've used many times and it's a clever setup, using perforated disks to redirect flow. The more disks installed, the more escape paths, so less back pressure, the less disks installed the more pressure and noise control.

    If you want to get a little engine's noise down, without robbing too much output, you need to control the flow, maintaining pressure, keep the exhaust hot and the flow centered in the pipe as best as possible and lastly reduce contamination.

    On a chain saw engine, I'd use a reversion cone at the port. I'd make a thick wall header, with a 45 degree bend and dump this into a Super Trap, if only because you can tune it to your needs. A few runs with an exhaust gas analyzer will get the pressure right, as well as the mixture. The heavy wall pipe will keep the heat in the tube so the gases flow better and the reversion cone will prevent contamination. You'll need to experiment with pipe length to match output requirements.

    This is a lot easier to do then describe. A stop down at the local "go-cart" racer shop will offer a lot of tricks. Most use chambered mufflers (big ones), but they also know how much pressure they need and tune with pipe length for the torque curve they desire. They can half the decibel level over an open exhaust engine, while keeping the output as high as practical.
     
  7. Frosty

    Frosty Previous Member


    Im familiar with this noise. A sledge hammer should have the desired effect.
     
  8. Frosty

    Frosty Previous Member


    So which one?
     
  9. PAR
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    PAR Yacht Designer/Builder

    Both are correct. A free flowing exhaust will suck out combustion gases, before they've had a chance to do all the work they can. Back pressure is required to keep the combustion gases in the chamber, long enough to serve a useful purpose.
     
  10. dorong
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    dorong Junior Member

  11. dorong
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    dorong Junior Member

  12. CDK
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    CDK retired engineer

    No Dorong, I don't think that's what you need. These are replacement parts in case the old one fell off and you can't find it anymore, or if rust did its job.
    What you need is something to dramatically lower the noise level, which automatically makes it much larger and heavier than the original one.
     
  13. Frosty

    Frosty Previous Member


    No back pressure is required by any engine in the world. You are talking about reversion, sonic and thermal shock.

    Back pressure as you call it is waves (not exhaust) going back to the cylinder,---and at other times pulling it out.

    Ive made many exhausts for bikes none of this is anything to do with silencing as that is shock waves like clapping your hands.

    To cure that is more difficult but there but 3 ways --restriction, muffling the high frequiency and I forget the name but you re direct the floe into itself to smooth the shock, as used today.

    A perforated pipe with asbestos or fibre glass wool is muffling. Retriction is the old tins of peas on the end.
     
  14. dorong
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    dorong Junior Member

    Thank you all, I think slowly I know what to do to reduce the noise level, add a bit of an insulating material to absorb noise, and perhaps more air exit sl

    Now I knew how to silence him I'm looking for my engine 2 or 4-stroke would be an output of 33 cc but a small will be important again, I had to fit a short curved tube end ProPElo T-8 and all business must enter PRO ANGLER instead of pedals, it will achieved great if I could get there
     

  15. PAR
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    PAR Yacht Designer/Builder

    Frosty, your understanding is either way above mine or something else, as your no back pressure comment, flies in the face of common engineering practices. Top fuel dragsters, producing 5,000 HP from their fire breathing V8's need back pressure, other wise their headers wouldn't have the curve in them. They don't need much back pressure, which is why the curve has been relaxed over the last 40 years, from90 degrees to the current about 30, but the curve exists to produce this pressure. It has nothing to do with sound, just chamber efficiency, though sound is affected a very small amount. Even miniature model engines need back pressure. In fact, one of the first things the "make and brake" engine tinkers noticed was pipe tuning, which directly affects back pressure and can offer more torque then an unrestricted engine. Anyone that's installed long tube headers on a car, will quickly notice what the difference a tuned pipe (and it's related back pressure) does to the seat of paints feeling, on an other wise on molested engine.

    The bottom line is, every engine has an ideal back pressure level, in order to get 14.7:1, good port flow with minimal contamination, which assumes your timing, fuel and air delivery is calibrated properly. With 2 strokes you can fool with it a little to gain something, typical emissions improvements at the cost of power or power improvements at the cost of durability and emissions, but we're not talking output optimization now, but some other form of compliance. In terms of maximum output engines, you're often looking to reduce back pressure, but not eliminate it, other wise there wouldn't be a need for exhaust pipes at all on certain engine installations, just open ports dumping into the air.

    It's actually fairly a complex puzzle and you end up balancing certain needs for some practicalities. For example you might have a need for high RPM output, so short tubes are employed, knowing full well you've lost low end torque, but gained HP (from less back pressure) in the the upper RPM bands. On the other hand you may have a need for an engine that's more "flexible" like that seen in a car, so you use longer tubes, knowing you've kissed off peak high RPM HP in favor of low and mid range flexibility (grunt).

    My point is you look at the the whole package and tune to it, not just to damn Stoichiometry.
     
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