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  • 2 stroke dirt bike compression readings .. someone please explain this statement
  • kaiser
    Free Member

    been reading up a bit and come across the following info and wondered why the 250 differs ?
    ” An 80, 125,200 or 500 cc bike should have a reading of 150 to 190 .A 250 cc bike should have 170 to 230psi . ( these numbers are at sea level )
    this came from a 2004 motorcross and off rd performance handbook.
    could someone please explain?
    thanks

    Posted 12 years ago
    br
    Free Member

    differnt levels of tune and/or bore/stroke ratio?

    Posted 12 years ago
    TandemJeremy
    Free Member

    what engine?

    you can’t really take compression readings accurately from a two stoke anyway as it will vary with cranking speed due to the nature of the ports in the cylinder wall.

    Is it referring to a 4 stroke 250?

    Posted 12 years ago
    sugdenr
    Free Member

    http://www.your-adrenaline-fix.com/compression-test.html

    Nope, he read it right. WTF is that all about?

    Posted 12 years ago
    TandemJeremy
    Free Member

    Its a bit odd to say the least as I was under the impression 2 stroke compression readings are unreliable anyway as it depends on cranking speed and they are huge ranges as well- and different engines will have different readings.

    Posted 12 years ago
    nwilko
    Free Member

    uncorrected CR = {{Cylinder Volume + volume above TDC) / (volume above TDC;1 }}
    short stroke needed to maintain effcy of scavenging across ensgine speed range results in differing optimum CR’s…
    Assuming optimum CR regardless of volume were desired to be the same and as high as poss for effcy (ignoring knock). Youve still halved the volume but will hit a threshold where you cant reduce the vol above TDC without creating a large knock/ thermal problem..
    i’d guess the 80,125’s are small inefficeint singles..
    250 large capacity single, can run higher knock due to vol ratio (as above).
    500 a 4cyl engine, hence CR as the 125 ??

    Posted 12 years ago
    vinnyeh
    Full Member

    you can’t really take compression readings accurately from a two stoke anyway as it will vary with cranking speed due to the nature of the ports in the cylinder wall.

    Can you explain further please?

    Historically 2 stroke 250 mx engines tend to be long stroke in comparison to 125 and 500, which were short stroke- my guess is that this is the reason.

    Posted 12 years ago
    TandemJeremy
    Free Member

    as the ports are open as compression stroke starts gasses can escape both down the exhaust and inlet ports. the faster the engine cranks the less time for gasses to escape. the faster the engine turns the higher the compression reading will be unitl it reaches the speed at which it runs.

    Posted 12 years ago
    vinnyeh
    Full Member

    ah, thanks.

    Posted 12 years ago
    coffeeking
    Free Member

    To some extent the same could be said about 4 stroke engines, depending on how fast you crank you get better or worse cyl fill. Also depends on level of valve overlap.

    Posted 12 years ago
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