silencing Weed eater engine

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

  1. Frosty

    Frosty Previous Member

    I was refering to your opinion that 4strokes do not have a scavanged air charge.

    Par --the internet is full of this stuff. Its like denying the world is round.

    Its possibly true that USA never really cared much about tuning, huge cubic inches were made to overcome any problem, stacking bigger and bigger carbs on single cam push rod engines even when Europe was twin cam multi valve 2 liter knocking out similar HP.

    Jaguars twin cam hemispherical combustion chamber engine was pre war when USA was still making flat heads.

    This along with some of the cheapest fuel in the wold resulted in huge American iron and is probably why you cant do Formula 1.

    This myth of back pressure required is well known and I have come across it often on USA motorcycle forums.

    Silencing a single cylinder is difficult because the shock waves!!!! are so separate. In a mullti cylinder engines where one cylinder can even scavenge another with the use of collectors with balanced pipes, infact thats the who idea of equal length pipes to time shock waves to assist other cylinders, meaning HP producing scavenging can be more RPM forgiving.

    So--- to keep a little 2 stroke quite restriction will work but will reduce power.

    Muffling with a perferated pipe wrapped in some heat proof sound absorbing material will absorb high frequency sound at least making the noise less offensive seemingly more quite.

    Modern system such as Toyotas today that you can stand by the side of it while it is running in complete silence use another way --I forget the name but it re directs the pulses into each other by chambers in the system, but done in such a way as to not destroy the whole HP producing system. If you think exhaust are just a pipe you could not be more wrong.
  2. ancient kayaker
    Joined: Aug 2006
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    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    Here it is Frosty again:-

    Zero marks for comprehension!

    - don't know what automotive engineers call it, probably pulse shaping, but a physicist would call it group delay. Darned thing shows up all over the palce and is usually a nuisance, messing up crisp short pulses in radar and sharp leading and trailing edges in robotic control systems - nice to know it has been put to use!
  3. Petros
    Joined: Oct 2007
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    Location: Arlington, WA-USA

    Petros Senior Member

    Ancient kayaker,

    You missed Frosty's point, with modern 4-stroke the intake and exhaust valves overlap, meaning both intake and exhaust valves are open the same time. In coming charges is happening the same time outgoing exhaust is occurring (they call that when valves are "teetering", it occurs at end of exhaust stroke and beginning of intake stroke). So YOU CAN over scavenge a 4-stroke so that unburnt fuel ends up in the exhaust system. The difference with a 2-stroke is it occurs every stroke, not ever other one.

    That "back pressure" wiki link is full of the same ignorant nonsense, with some stupid CO2 propaganda thrown in that is irrelevant to the issue. If I had more time I would fix that non-sense but little good it would do.

    I think we have a case of invincible ignorance, facts are just too confusing to use to educate those who refuse to learn.
  4. Frosty

    Frosty Previous Member

    Everyone knows what a turbo is or a supercharger, its rams more air in to the cylinder than it would do normally. Tuners match manifold the heads to incre4se flow, polish inlet walls to smoth flow, Increase carburation and aftermarket inlet manifolds 3 angle valve jobs cross flow head design all to get more air (volumetric efficiency) through the engine.

    Why or how could any one think that having back pressure what ever you want to call it could hep the engine to produce more power.

    The piston comes up and pushes out gases, how can one imagine that pushing out against back pressure could help when a perfect clean charge is what is wanted.

    Its a myth. In Singapore most people change alternators at 20,000km. Its the craziest thing Ive ever heard of but you see cars advertised with alternator changed. Its a myth that has traveled the entire Island and every one ( almost everyone) ---in there ignorance believes it.

  5. tom kane
    Joined: Nov 2003
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    Location: Hamilton.New Zealand.

    tom kane Senior Member

    Not many people realize that vacuum does not SUCK (another myth) but that atmospheric pressure PUSHES. A descending piston does not suck in the induction charge but atmospheric pressure pushes into the cylinder carrying fuel along with it.In a carb engine.To increase an induction charge we use Supercharges,Turbos Ram tubes etc.,
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