Thread: Johnson "70mod"

  1. #51
    Team Member hupiveneilija's Avatar
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    Damaged propeller gets a new job. Cut the plades few cm, can use as a testprop on boatramp.
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  2. #52
    Team Member hupiveneilija's Avatar
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    Here it is! https://youtu.be/oPcqyNwOrxE
    Funny how much better it works when wires are in right places. Tomorrow to the boatramp with that "testprop" and see how it run.
    1. 145psi 2. 148psi 3. 146psi. Cyl.head temp aprx.60*c. Ign timing 2*btdc.

  3. #53
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    it is running hooray.
    sounds nice and crisp 2 stroke ..good.

    not high compression though ? why seems very strange unless the head was from a 50hp 3 cylinder ????

  4. #54
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    Quote Originally Posted by filthy phill View Post
    it is running hooray.
    sounds nice and crisp 2 stroke ..good.

    not high compression though ? why seems very strange unless the head was from a 50hp 3 cylinder ????
    Porting, ex timearea. With stock porting supposed to be higher. Correct?

  5. #55
    Team Member zul8tr's Avatar
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    Higher exhaust porting yields less static compression pressure because less cylinder volume from TDC to top of port. Most stock 2 cycle outboards are about 6.5 compression ratio (CR) relative to top of exhaust port, if you raise the exhaust ports that lowers the CR relative to exhaust.
    " Three may keep a secret if two of them are dead" Ben Franklin
    " ------- well Doctor what have we got a Republic or Monarchy? A Republic he replies if you can keep it"
    Benjamin Franklin, 1787 Constitutional Convention, as recorded by signer James McHenry's in his diary at the Library of Congress

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  6. #56
    Team Member hupiveneilija's Avatar
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    Thats the case. And scavening & cylinder filling when running rpms is another, it might have quite high dynamic compression.

  7. #57
    Team Member zul8tr's Avatar
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    Yes much greater dynamic compression that cranking, engine is dealing with a hopefully controlled burning fuel air mix that expands rapidly and if ignition timing correct and other factors the max pressure on the piston will occur near 10-15 degrees ATDC to push it down. Depending on a lot of conditions dynamic pressures could exceed 800psi
    " Three may keep a secret if two of them are dead" Ben Franklin
    " ------- well Doctor what have we got a Republic or Monarchy? A Republic he replies if you can keep it"
    Benjamin Franklin, 1787 Constitutional Convention, as recorded by signer James McHenry's in his diary at the Library of Congress

    Location: SW Orlando, Fl

  8. #58
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    Zul8tr
    you said
    "Higher exhaust porting yields less static compression pressure because less cylinder volume from TDC to top of port. Most stock 2 cycle outboards are about 6.5 compression ratio (CR) relative to top of exhaust port, if you raise the exhaust ports that lowers the CR relative to exhaust. "

    to counteract the raising of the exhaust port (40 thou) many things have been modified.
    The idle reliefs have been closed off ? this should raise compression.
    the head being skimmed ? this should raise compression.
    and the combustion chamber itself being reduced to about 2/3rds its normal size ? should dramatically raise compression on its own..
    these 3 factors should of raised the compression to far exceed that of the OMC SST60.

    I think another known to be reliable compression gauge would be a good idea to know what is realy going on, simply because 145 psi is not
    even close to what is considered a high compression OMC 56.

    I would of been expecting atleast 175 psi, then bed in and make around 180 to 190 psi .

  9. #59
    Team Member hupiveneilija's Avatar
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    Good morning!
    Engine starts nice and idles like nothing with forward gear. My "testprop" is awfully heavy when buried down deep, had to lift so that a/v plate is level with water surface. Ign on idle 4*atdc, max 17* now. 800rpm with gear. That prop shakes alot. Better have official version. Reacts nicely to throttle input especially over 3k, instant 7k when smash the pedal😬. Picture for plugs after few minutes in 6000rpm, not wot, then shut.
    Sorry no video, alone in a ramp.

    Hope it works when on the move and gets plane.
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  10. #60
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    Quote Originally Posted by zul8tr View Post
    Higher exhaust porting yields less static compression pressure because less cylinder volume from TDC to top of port. Most stock 2 cycle outboards are about 6.5 compression ratio (CR) relative to top of exhaust port, if you raise the exhaust ports that lowers the CR relative to exhaust.
    This is 5.9 stock and with that small chamber head 7.2 BUT with stock porting!

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