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  • Actual Compression

    I know Lot's of send our heads of to be milled and reshaped, and when they come back the boat runs better. But I wonder how many actual check the comp before and after?
    I sent a 951 head to GROUPK and when I put it back on the machine had a little better low-end. I doing some more mods and checked it as was surprised to see I was only getting 130psi cold. I calibrated the gauge against a couple brand new 785 motors and it read 155 psi so I know it's close.
    Needless To say I'm having it shaved som more. I'm shooting for 150 psi Cold.
    98 ROTAX 951
    I intend to live forever - So far, so good.

  • #2
    Gypsy:

    The stock compression on a 951 is 150 psi @ sea level. Even at 550ft, I get 150 on both jugs. My gauge is spot on. The thickness of the head gasket will effect the compression.

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    • #3
      Gypsy,
      Maybe your battery was cranking the motor slowly. Recharge the battery and try again. Were both clyinders the same ?
      Bill O'Neal WCM
      <a href="http://www.watercraftmagic.com" target="_blank">www.watercraftmagic.com</a>

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      • #4
        Isn't Group K big on staggered compression? As in cutting the PTO side less (higher) to compensate for the difference in exhaust blow back charge.

        Gypsy, your head may be cut "staggered".

        Bill "O" or anyone, what do you think about staggered compression in a rec. application where the compression is only going to 160 or 165 psi?

        It is usually the rear jug that gets staggered on the 951 or is it not necessary?

        Thanks, Steve.

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        • #5
          I do not see the need to stagger the compression on any motor running pump gasoline that has the proper head design and chamber size.
          Harry does things his way. Others do it differently. We don't always agree.
          Bill O'Neal WCM
          <a href="http://www.watercraftmagic.com" target="_blank">www.watercraftmagic.com</a>

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          • #6
            OK Bill, thanks.

            I was just getting an opinion, but I'm going to have mine cut even at no less than 160 psi but not greater than 165 psi.

            But, I'm going to steal Group K's niffty idea about the two additional cooling head exit lines. Thank you HARRY K.

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