converting a yamaha 9.9 four stroke to 15hp?

Discussion in 'Outboards' started by Steve W, May 18, 2019.

  1. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    One other thing that comes into play on HP upgrades is that there is a 10% fudge factor in advertised/claimed HP, and actual HP.

    So sometimes the gap between the two models isn’t as big as it first appears to be.

    An over rated 9.9 and an under rated 15 may not be that far apart to start with.

    On Johnson/Evinrudes some years there wasn’t much difference in performance between the 9.9 and 15. People would complain if they upgraded and didn’t see an improvement, or very little.
  2. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    What you do see in the specs of identical CID motors, is say, 50 HP, max rpm range 5000-5500, 60 HP, 5500-6000 rpm, for example, which indicates at cruise speeds you won't notice much difference, the boat will get on plane much the same, it is really more a case of them being able to rev out further for the boy racers. And as ondarvr says, the precision in the nominal HP isn't there anyway.
  3. tpenfield
    Joined: Dec 2016
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    Location: Cape Cod, MA

    tpenfield Senior Member

    I just checked the 1989 model year Yammy 9.9 hp vs. the 15 hp

    Carburetor is the same P/N
    Crankcase is the same P/N
    Exhaust covers/gaskets are the same P/N's
    Crankshaft assembly is the same P/N
    Pistons are the same P/N
    Reed valve assembly are the same P/N

    I am thinking it may be as simple as a throttle setting :cool: . . . maybe worth putting a tachometer on the 9.9 and comparing the spec RPM of the 2 engines to see if the 9.9 p can be tweaked up to the spec of the 15 hp. If that does not seem possible, then I'd look for some sort of an RPM limiter on the 9.9

    On some of my home maintenance equipment (Tecumseh engines) the only difference between the 8 hp and the 9.5 hp was the RPM setting. Naturally, I remedied that with a screw driver :D

  4. 2senko6
    Joined: May 2021
    Posts: 1
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    Location: Alabama

    2senko6 New Member

    What was your solution? I was looking for info on the conversion.
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