silencing Weed eater engine

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

  1. Frosty

    Frosty Previous Member

    They go to a lot of trouble to make volumetric efficiency, special 3 angle valve seats and head design, high flow carbs and inlet manifolds to get gasses through the engine and out..

    The last thing they need is a restricted exhaust.
  2. Frosty

    Frosty Previous Member

  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    You're right you don't want a restricted exhaust, but you also don't just dump out the exhaust ports either. You need a "head", which is the back pressure. Each of these arguments these clowns try to make about back pressure, assumes that more is a bad thing and less is a good thing and for the most part this is true in generally restrictive production applications, but not continuous duty cycle or high output applications, like that seen on a boat or a 2 stroke chain saw.

    You can play with back pressure all you want, but 14.7:1 is 14.7:1 and it doesn't matter any other way. These silly arguments about burning valves and the rest, are from bone heads that ran way too lean and deserve burned valves for no-braining themselves into something they didn't understand. The bottom line is it's a package deal and back pressure along with several other considerations have to be addressed or your engine isn't going to preform as well as it could, burned valves and all.

    You don't have to believe me, pull the muffler off your lawn mower, adjust the fuel mixture as best as you can and see how well it does in tall grass. Then go back and install the pieces I've suggested, more mixture adjustments, possably some timing and take on the tall grass again. I can assure you, if you brought me a 10 HP lawn mower, I could very easy make it 12 HP with just a few flow tricks (mentioned above). These same things work on any engine, though as displacement and cylinder count goes up, the preformance boost percentage goes down a bit.
  4. ancient kayaker
    Joined: Aug 2006
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    ancient kayaker aka Terry Haines

    I don’t know the theory but I do have some practical experience to recount.

    When I was a youngster back in the UK I strapped an old 50cc Trojan motor to my bicycle. The exhaust was a solid cast aluminum tube about 2.5" dia x 6" long, almost closed at the end - just a slot roughly 1" x 1/4" as I recall. Inside was a pierced steel tube from the port and the space between was packed with steel wool. It was a fairly effective muffler, certainly worked better than the average lawn mower.

    The quietest 2-stroke I have ever heard was the Velosolex bicycle motor which strapped onto the front wheel - very popular in France. It didn’t have much pop but made very little noise although the muffler wasn’t especially large as I recall; I believe it had a pipe between the motor and the muffler.

    Another quiet motor was the Cyclemaster which had the engine fitted inside the hub of a rear wheel, only 32 cc. It also had a pipe between the motor and the muffler.

    I suspect the pipes, which were fairly thick walled, contributed to the efficiency of the muffler. This might be a simple avenue to explore: lawnmowers and weed eaters don’t have space for such refinements . . .

    The NSU Quickly was a two-speed moped that was fairly quiet but the muffler was very long.

    Parts for strap-on bicycle motors are available here:
  5. Frosty

    Frosty Previous Member

    Your getting mixed up with backpressure and reversion again . A 2 stroke will barley run without and exhaust system . It only strokes twice therefor scavenging of the cylinder is by the exhaust. Speed of gasses should be over 250 feet per second to PULL the old gasses out.

    This type of exhaust tends to hit a point of efficiency which is why when a 2 stoke hits 3000 hold on.

    An exhaust system starts at the air filter.
  6. dorong
    Joined: May 2012
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    dorong Junior Member

    He is a just such an idea, only that the engine needs to be "hidden" from law enforcement, and a large and quiet p

    What do you think you can make something like my kayak? It would be awesome
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I know precisely what reversion and back pressure are, but you're still missing the point with is there's a target for each engine, which requires a mutli pronged approach, including sufficient back pressure, not too much or too little, but the right amount to hit the magic number. You can either tune an engine of fart around with it, maybe luckily getting it fairly close. Most wouldn't have a clue how to actually "dial in" and engine. Those that don't have an exhaust gas analyzer likely fit this bill.
  8. Frosty

    Frosty Previous Member

    No engine ever made in the world needs back pressure. It is a result of poor exhausts not a requisite. A good exhaust is clear zero if you can make one, and with reversion you can use this to suck out the gasses. Look at a new BMW or toyota with inlet manifolds reaching over the engine,--all that is breathing and getting the pipe length to work with the exhaust.

    A good system can pull as hard as a turbo at certain RPM. A turbo is more accomodating but not perfect and yes --even a turbo is a restriction but the result is a benefit.

    The exhaust starts at the air filter
  9. dorong
    Joined: May 2012
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    dorong Junior Member

    The problem with law enforcement authorities with no exhaust, gasoline engine is the problem, to sail with him must bureaucrat of certification that it is impossible to describe, not motor gasoline this problem but another world, so I want a powerful engine and quiet, would love to get ideas
  10. dinoa
    Joined: Oct 2007
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    dinoa Senior Member

    A cheap way of ensuring you don't seize an engine while experimenting with exhausts and fuel mixture is to use an EGT (exhaust gas temperature)sensor. They go for about 100bucks. Most 2 stroke cylinders will not tolerate working EGTs above 1200F( a few even to 1300F) and lead to piston seizure almost always on the exhaust facing side of the piston. This is easy to inspect as well as stuck rings by just removing the exhaust manifold and peeking inside.

    Two stroke weed wacker engines run well rich of stoichiometric at fuel air ratios of 12:1. The extra fuel helps to lower the EGT and cool the engine, hence the high fuel consumption.

    Other ways are to read the spark plugs for a nice tan color at the electrode (no white or ash grey which is too lean or sooty black which is too rich) and exhaust finger print in piston crown but you may seize the engine before getting a reading. Remember a 2 strokes run best just before seizure (closer to stoichiometric 14.7:1) which you want to approach cautiously.

  11. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Interesting to see how this thread steers away from the OP's question.

    As a student in mechanical engineering I joined the contest to get the most from a 50 cc "moped" engine. We applied what we had just learned and kept modifying the little engines even though the rpm was already well over 12.000 and life expectancy was almost nil.
    It became clear that the intake should have minimal resistance, but the exhaust should have some back pressure. The optimal muffler was a long, straight tube with a trumpet horn at the end. In the horn was a perforated disc that could be pulled in with a Bowden cable. At full throttle the top speed could be marginally increased by pulling the disc in the horn.

    Dorong, have you considered extending the exhaust of your hidden engine so it ends under water? I think that's about as silent as possible.
  12. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    CDK, I'm reasonably confident that most engineering students have gone through just what you describe. I remember pretty much the same "problem" and our results where similar, with a long, straight tube, utilizing a single near 90 degree turn and the pipe running nearly the full length of the vehicle and a megaphone at the end. The length of pipe, diameter and megaphone received a lot of experimentation, as did the location of the bend, which was limited but compartment variables (hard to clear the frame and get it under the vehicle). All this talk of no back pressure is just nonsense. The moment the flow exits the port and enters the pipe, it cools and slows, forming back pressure. Again, each engine, regardless of type, will have a back pressure requirement that best suits efficiency. Design limitations and configuration choices often force a less than desirable arrangement, but this is irreverent in a conversation about ultimate efficiency, much like flow dynamics assuming a perfect fluid medium for data. Compromises are usually forced on us in the process, but the physics are fairly straight forward.
  13. Frosty

    Frosty Previous Member reversion shock

    Exhaust basics
    The exhaust system has some resistance to the exhaust gases being pushed in because of the diameter of the tubing. This is called backpressure. With significant amounts of backpressure the piston will have to push harder against the outgoing exhaust, and horsepower will be lost. Conversely if the piston doesn?t have to push as hard to get the same amount of exhaust gases out of the cylinder, you will gain horsepower. Backpressure is decreased. How do we decrease backpressure to take advantage of this horsepower? By making our exhaust systems flow more.

    Destroying a myth.

    Some say that "an engine needs backpressure to work correctly." Is this true?

    No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.

    The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.

    The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.
  14. Frosty

    Frosty Previous Member

  15. hoytedow
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    hoytedow Carbon Based Life Form

    Excellent posts, Frosty.
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