silencing Weed eater engine

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

  1. PAR
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    PAR Yacht Designer/Builder

    So which is it Frosty? You said no back pressure is best, now you suggest there's some back pressure. Then you list the classic and foolish comparison of compromised EPA conforming, automotive applications. It's a given that any automotive or production application will be overly restrictive.

    The bottom line still remains and not even remotely addressed in your discussion, or any of the links you've posted. Stoichiometry is stoichiometry, just because you don't understanding the principles, doesn't mean they're wrong, just that you don't understand them. It's foolish to continue this without an understanding of the dynamics involved.
     
  2. Frosty

    Frosty Previous Member

    Every manifold --header has back pressure if you can design one that does not you will increase HP.

    You can also lead a horse to water but you cant make it drink.

    Its a Myth due to people fitting open exhaust and then not modifying the fuel system to cope with it. Result is bad running but not from the exhaust.
     
  3. ancient kayaker
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    ancient kayaker aka Terry Haines


    - Oh I wish! It would have been more interesting. I was studying a hulking great stationary diesel and not even allowed to start it up!




    - I dunno Frosty, the hardware in those links didn’t look much like 2-strokes to me.



    There’s different 2-stroke types BTW. The one with the idler cylinder that acts like a kind of supercharger and requires no transfer port - not likely to be found on a Weed Eater - and there’s the common and cheap intake under the piston and the more efficient direct crankcase intake of which there are at least two kinds that I have seen. Their exhaust requirements are likely to differ and certainly will not be the same as a 4-stroke.



    The technical details (whichever version is correct) are not going to be much help to the OP, I suspect.

    Based on physics I think the simplest and most effective home-made muffler would probably consist of a small expansion chamber, a heavy gauge pipe, and perhaps a megaphone.

    That is the approach I would take; most of the parts can easily be made using plumbing parts. The expansion chamber is simply a larger diameter pipe. The size of the expansion chamber and the length of the pipe should be determined by the motor speed and exhaust temperature; start with too long a pipe and determine the quietest speed of operation then shorten the pipe and increase speed until an optimum is reached. Perform the final tests under load; this could be a large bin filled with water and a prop on the end of the shaft - nothing fancy.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Don't bother Terry, simple engineering concepts are beyond him. He doesn't understand that the moment you have gases leaving a port at 1,200 F, entering pipe, these gases cool (condense), causing them to move slower, plus frictional and turbulence resistance rises dramatically. All this couples to increase pressure in the "exhaust system", if it has a muffler or not. Top fuel dragsters have tuned pipes and a carefully calculated amounts of bend, to maintain the air/fuel ratio's "magic number". If this resistance wasn't necessary, nor desirable as Frosty suggests, there wouldn't be exhaust pipes on a dragster, just open ports. Empirical and logical evidence isn't enough, so he hunts down talking points about someone's F-150 and it's modified exhaust.

    Simply put, the original posters interest was a weed eater/chain saw engine, which is an all or nothing output engine. It's not designed to be flexible, doesn't need a mid range or low end envelope to perform in. It's just started, maxed out in RPM then loaded up. So, if you want to talk apples to apples, the ideal setup for the OP is a tuned pipe with a megaphone if desiring the last 2% of power. It'll produce enough pressure to get you 14.7:1, while keeping back pressure low enough, as to not restrict the engine unnecessarily. As to the length and diameter, well I'm not willing to do the math, particularly not fully knowing what engine he has, but the principles are well understood (by some).
     
  5. ancient kayaker
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    ancient kayaker aka Terry Haines

    For a tuned pipe it would likely be too long for convenience, which is why I suggested the expansion chamber to convert the pipe into a Helmholtz resonator at the noisiest frequency. My Physics is many decades in the past - the Physics hasn't changed but the efficiency of my memory has. However, it should work, maybe tuned to the third harmonic. One can get by with these simple systems with a fixed speed motor.

    I must admit I'm not clear on the principle of a megaphone in an exhaust system; it should act as a simple acoustic impendance transformer and I don't see what it does acoustically but it is used commonly on bikes. I'd expect it to make the exhaust louder . . . surely a regular labyrinth muffler would be smaller & cheaper, since it's only needed to knock down the harmonics that the resonator misses.
     
  6. Petros
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    Petros Senior Member

    PAR and CDK,

    Frosty is correct, both of you have fallen for the myth of "back pressure". There is NO amount of back pressure that is ever good for an engine.

    What the average person does not understand that with a tuned exhaust system the back pressure is actually lower than with a short open pipe. confusing this is why the people who have not studied the dynamics of exhaust headers think that you need "back pressure" to make more hp. The typica engine builder repeats this sillyness out of ignorance, I would expect better for you two.

    I had worked many hours professionally in an engine lab in a previous career. I have seen how with a tuned exhaust system, when you put a pressure tap just down stream of the exhaust valve, you will actually measure pressures below atmospheric. The harmonics of a tuned system wll create a low pressure node right at the exhaust valve, so when the valve opens the engine "sees" manifold pressure below atmospheric pressure. This only works at the correct rpm of course, that is why the whole system has to be tuned to the rpm range you want the peak hp to occur (cam timing and intake runner length must also be matched to the exhaust header for an optimum system).

    The term "tuned" exhaust is meaningless unless you build the whole engine around the same rpm you want to optimize the output around.

    The lowest pressure you can get in a short pipe, or with no exhaust pipe, is atmospheric pressure. With a properly designed system you can actually get well below atmospheric pressure, at certain rpms. Hence the myth that the longer pipe (that must have more "back pressure" than the short pipe, right?) makes more hp because it has back pressure.

    As pointed out however, sometimes the tuned length is too large to be useful, so the next best thing is to make a resonator chamber.

    If you are so inclined, here is an exhaust pipe length calculator:

    http://www.mez.co.uk/mezporting/exhaust_length.html
     
  7. ancient kayaker
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    ancient kayaker aka Terry Haines

    Petros, how much of your work was on two-strokes? Two-stroke are quite different from four-strokes.

    http://en.wikipedia.org/wiki/Back_pressure

    The need for back pressure in a two-stroke is a recurring theme in the web pages I have found that refer to two-strokes as distinct from four-strokes.

    There is talk of the “back pressure myth” . . . it may well be a myth that two-strokes must have back pressure in order to run, but it is true that the majority of them are designed to operate with back pressure and that removing it on such a design will impact its operation negatively.

    In my experience a two-stroke can and will run perfectly well without an exhaust system of any sort. I have tried it myself and have never had a problem getting it to run and keep running. In addition to a whole series of two-stroke motor bikes all subjected to my youthful curiosity, in my model aircraft days I had many midget two-stroke engines for which the exhaust was simply a hole in the cylinder.

    For racing model aircraft I teased extra revs mostly by abusing the exhaust with a file and adding metal polish to the fuel for a while to loosen them up. They went like stink but not for long and were very difficult to start, and you could smell the unburnt fuel they chucked out from a long distance despite their small size.

    No doubt tuned exhausts can be used to increase two-stroke performance but the engine and exhaust should be designed together as a matched pair. Add a tuned exhaust - or a megaphone exhaust for that matter - to a two-stroke not designed for it and you will run the risk of creating a fuel-guzzling polluting monster. That may be acceptable on a dirt bike, or a tiny model aircraft that only runs for a few minutes at a time, but not on a lake or river. Do something like that with an engine used on a boat and you will be responsible for polluting the very water you ride on and the air above it.

    A four-stroke is a quite different beast, there is no fresh fuel entering the cylinder during the exhaust stroke to be sucked out of the exhaust port and a scavenging exhaust system can be used to increase power without reducing efficiency in such an engine.
     
  8. Frosty

    Frosty Previous Member

    Im sorry but thats not correct. A 2 stroke has no exhaust stroke its charge is from the transfer port that was compresed in the crank case on the down stoke of the power stroke.

    With 'back pressure ' under these circumstances a 2 stroke would barely run as they were in the 50's a gutless machine. The exhaust on a 2 stroke is of paramount importance. Unless its a turbo.

    If back pressure was required we would all be sticking a oily rag up the pipe to increase power

    The four stroke does have a fresh charge entering the cylinder
    at the end of the stroke when the inlet opens to take advantage of the shock wave that should have been timed to be in negative pressure in the header at that time. ( see petros post) This is called valve overlap that most modern engine have today ( except a Harley that is soo agricultural can run on a single carb with 2 pots).

    This valve overlap means exhaust and inlet are open --negative headers pull through the fresh charge right through the engine AS good as a turbo at certain rpm hence the bunch of bananas over modern engines. All these shock waves can be timed to pull through air mix. This also works on the inlet manifold if the length are timed to the exhaust and valve overlap.


    Its complicated stuff,-- I dont blame you for not understanding, as for CDK leaning back pressure at college,, should go ask for your money back.
     
  9. CDK
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    CDK retired engineer

    No chance at all Frosty.
    I was a bright student with a scholarship....
     
  10. ancient kayaker
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    ancient kayaker aka Terry Haines

    Well, Frosty, most of your post was fine but what about my previous post was not correct? Or did you read it a little too quickly perhaps?
     
  11. CDK
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    CDK retired engineer

    College was just like that. Tuning small engines was a hobby among the students.
    My Husqvarna 49 cc moped did almost 120 km/h with the exhaust properly adjusted. The hobby ended abruptly when I caused a traffic accident with it and the police forced me to scrap the machine under their supervision.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Again you're talking apples and oranges. There's a huge difference between a continuous duty single cylinder engine and a mutli cylinder engine designed to be more flexible.

    If no back pressure was desired, not a single high output, continuous duty engine would have exhaust pipes. The exhaust would directly dump. Show me one high output, continuous duty engine, that doesn't have two things; first an exhaust pipe and second that the pipe isn't forcing some level of restriction on the flow.

    I was just looking at a the Packard built Merlin II at it has a 6.5 sq. inches of exhaust port opening, yet dumps into a cone that decreases the outlet opening to just over 3 sq. inches across a short sweeping curve of about 45 degrees. In fact, I can't think of a single continuous duty engine that doesn't restrict the exhaust flow, even if just a little, if for no other reason then to not cold shock the valves, which isn't as applicable on a 2 stroke, but still would effect efficiency.

    So, lets skip all the BS about port scavenging, because it's not relevant on a continuous duty, single cylinder engine (none that I know of use exhaust gas scavenging). Every high output continuous duty engine I've ever seen has a short, usually slightly bent and slight smaller tube. Or they save the tube diameter an work with just flow, such as that seen on dragsters. I guess these guys just don't get it, right? BTW, the Merlin received 3 different exhaust pipe changes, two of which directly effected efficiency, the other to add forward thrust to the aircraft. The efficiency changes used a longer or more curved pipe! Than again it was a crap, low output, junk engineered motor anyway . . .
     
  13. Petros
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    Petros Senior Member

    PAR, it is like you did not even bother to read my or Frosty's posts. I know you are smarter than that, it seems you refuse to even get the slightest understanding. Tuned exhausts have LOWER restrictions than no pipe at all.

    LOWER BACK PRESSURE OCCURS WITH LONGER EXHAUST PIPES! when property sized and tuned to the exhaust cycles of the engine. The harmonics of the exhaust length and dia will result in LOWER back pressure. It does not matter if it one cylinder or 12, it does not matter if it is 4 cycle or two. IT does not matter if it is "continuous cycle" or not. The physics does not change.

    The lower the back pressure the more the power output. Short pipes do not take advantage of any dynamic effects of the exhaust stream and have HIGHER back pressure than longer tuned systems. Short or no exhaust pipes have HIGHER back pressure than longer tuned pipes.

    I have measured pressures BELOW atmospheric pressures in exhaust pipes of 1000 hp engines running at 10,000 rpms. It has a tuned header and a 10 foot long exhaust pipe.

    How can you get below atmospheric pressure without a resonating exhaust system? You can not.
     
  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    Dorong: have any of the posted ideas been of use for you?
     

  15. tom kane
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    tom kane Senior Member

    Gosh.. we are learning a lot about exhaust systems and have come a long way from one of the very first internal (Crosley) combustion engines which had an exhaust 60 feet long to assist in induction.Just imagine a "plug" of hot gasses hurtling down 60 feet of tubing,I guess there would be an interesting amount of pressure back pressure waves behind that pulse.The would have been a nice "chump sound " Google Pulsing for Power.

    Science was not on our minds when we tried removing exhaust tubes and mufflers from our two stroke motor bikes in the 60`s and found they were gutless and lacked speed without the nice chrome.
     
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