Thread: Fast Fred's OMC Mod 50 secrets Thread #2

  1. #51
    YARD BIRD
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    Default Fast Fred

    Like I said in the personal message I sent you, You ought to have your own LITTLE foundry to cast small aluminum parts .

  2. #52
    FFX-61
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    the straight in front 1/2 must do some thin for it, cuzz thay used it on the 31m

    Part one of "Secrets of the OMC Mod 50" : http://www.boatracingfacts.com/forums/showthread.php?t=2052

  3. #53
    Team Member BRIAN HENDRICK's Avatar
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    Default Triple Variable CRs

    A recent Yamaha catalog lists the CR of each cylinder in their triples.
    They vary, I suspect due to the nuances of common exhaust manifold pulse tuning. The 70" triple lists CRs of 5.76, 5.67, and 5.86 for cylinders 1 thru 3 respectively. The 158" V6 lists a CR of 6.7 for 1 thru 4,
    and 6.4 for 5 & 6. But odd how the triple's CR is highest on the bottom, the V6's[a pair of triples] has the lowest on the bottom
    I can only think the charge ramming effect is greatest on the cylinders with the lowest CRs, and this would corelate directly to piston temps.
    I suppose they develop the design by running the CRs up until the pistons start sticking or holing, and then reducing the cc's on the hot one until it cools down. I was wondering if any of the FEH or OPC triple guys are setting variable CRs ?, or jetting diffrently top to bottom.

  4. #54
    Sam Cullis Mark75H's Avatar
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    Quote Originally Posted by BRIAN HENDRICK View Post
    I suppose they develop the design by running the CRs up until the pistons start sticking or holing, and then reducing the cc's on the hot one until it cools down.
    And then dropping the CR 3 more notches to compensate for the fact that they can not control what boat the motor will be used on
    Since 1925, about 150 different racing outboards have been made.


  5. #55
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    can't say i've ever seen one (a yamaha tripple 70hp) test out like that, seems they test out like the V6. higher at the top.

    Fast Fred 5 star certafryed

    Part one of "Secrets of the OMC Mod 50" : http://www.boatracingfacts.com/forums/showthread.php?t=2052

  6. #56
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    Default

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    Part one of "Secrets of the OMC Mod 50" : http://www.boatracingfacts.com/forums/showthread.php?t=2052

  7. #57
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    E-tec certafryed

    trunk monkey

    Part one of "Secrets of the OMC Mod 50" : http://www.boatracingfacts.com/forums/showthread.php?t=2052

  8. #58
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    Default differnt CR's

    I guess lowering could be for 2 reasons?

    1) slows the pulse down from that cylinder so the next one (120ish degrees away) gets less return pulse and possibly later?

    2) Could have less CR due to the fact that it gets a very high return pulse and really stuffs the cylinder?

    2 guesses really?

  9. #59
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    Fueler Mod50
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    Part one of "Secrets of the OMC Mod 50" : http://www.boatracingfacts.com/forums/showthread.php?t=2052

  10. #60
    Sam Cullis Mark75H's Avatar
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    Quote Originally Posted by Powerabout View Post
    I guess lowering could be for 2 reasons?

    1) slows the pulse down from that cylinder so the next one (120ish degrees away) gets less return pulse and possibly later?

    2) Could have less CR due to the fact that it gets a very high return pulse and really stuffs the cylinder?

    2 guesses really?
    I had to think about it for a long while. I think it is most likely that the compression is varied from cylinder to cylinder due to uneven cooling in the head and block. Other manufacturers have sometimes varied the ignition timing to compensate for this.
    Since 1925, about 150 different racing outboards have been made.


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