converting a yamaha 9.9 four stroke to 15hp?

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

  1. Steve W
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    Steve W Senior Member

    I have one of the older (1989) motors that is 232ci, the same as the 15 of the same vintage that I would like to upgrade to 15 hp. I have the owners manual in front of me and it gives 3500-4500 as the full throttle operating range which seems quite low compared to most outboards. At some point Yamaha tuned up the 212 ci 8hp to become the 9.9 that is currently sold but so far I have not been able to find what year that took place. I am wondering if anyone here has done this conversion and can enlighten me as to what is involved, ie, is it as simple as a carb swap or is there something else involved.
    Background, I bought this motor, model # FT9.9 EXF which is the 25" leg high thrust model with the 2.92:1 gear ratio and huge prop and had it on a 36ft lightweight cat and it was adequate but it is now on a shorter heavier catamaran and while it has great thrust a bit more hp would be nice as I find myself running it at wot much of the time. Any help would be appreciated.
     
  2. JamesG123
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    JamesG123 Senior Member

    Maybe compare parts diagrams to see if there is any obvious differences in intake/exhaust or valve train. This and improved metalurgy might have enabled raising the redline and placing the powerband higher in the rev range. I would be careful with trying to "hot rod" an older engine. Crunchy noises are likely to result.
     
  3. messabout
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    messabout Senior Member

    The motor as it is will probably be capable of developing 15 HP. The parts of my 1999 Johnson 9.9 are identical to the 15. The difference is merely the RPM at which the HP was rated. That was a sneaky way to satisfy the under ten horsepower rule on so many regulated lakes and streams.

    That is an old ploy first used by Kiekhaffer Mercury motors way back in the 1950s. But for a different reason. The Merc ten was under rated and could embarras the Johnson and Evinrude 10s that were rated more realistically. That was a strong sales advantage for the Mercs.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    The exploded parts diagram is instructive in these matters, but often times there are ignition timing setting differences as well. Carb jets are a common difference. If you had to buy all the necessary parts, it would be uneconomical, not worth the bother either.
     
  5. ondarvr
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    ondarvr Senior Member

    To start with, get the parts diagrams for both motors and see what is different.

    I’m not aware of any Johnson/Evinrude 9.9 and 15s that were identical except for RPMs. The RPMs were different because at the bare minimum the carbs were different, with the 15 being larger. There were other assorted differences depending on the exact year.

    Carb jets are only different if there are other differences in the two motors. You can’t increase the HP rating by just changing jets.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    There can be a variety of differences, and the parts diagram will show it, if there are differences like engine porting, tuned exhausts, it is madness to be bothering with it. Carb jets are almost certain to be different. But the throats may be different diameter, too. It is pretty silly to get involved in trying, I think, you will have to get the motor re-tuned, as ignition timing will also likely be different, there may even be differences in the reed block on some engines. They are not dumb enough to have you changing a 9.9 to a 15 easily, or at minimal cost, were it so the word would get around, and those that were prepared to shell out extra for a 15, would just buy the 9.9 and make the cheap and easy change.
     
  7. DCockey
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    DCockey Senior Member

    I looked at parts diagrams for a Yamaho 1999 9.9HP and 15HP engines, and the carburetor assemblies and reed valve assemblies were the same part number. I didn't look at other parts.
    You might want to check some parts diagrams because I've found several instances of Mercury engines with identical mechanical specifications other than power and max rpm ratings where the only differences were the addition of restrictor plate in the intake manifold and different decals for the lower power versions.
     
  8. ondarvr
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    ondarvr Senior Member

    Tohatsu has one with a different cam on the throttle linkage to restrict travel.

    Sometimes you get lucky.
     
  9. ondarvr
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    ondarvr Senior Member

    “I looked at parts diagrams for a Yamaho 1999 9.9HP and 15HP engines, and the carburetor assemblies and reed valve assemblies were the same part number. I didn't look at other parts.”

    It can be timing and/or linkage restriction on some.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    You would need to be Sherlock Holmes to thoroughly investigate the differences between the engine HP classifications, most people who get these ideas will say "the only difference is X and Y", when almost always there is a Z, and often more.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    I think what can be said about different HP ratings on the same block, particularly with carb 2S, is that the engines with the lower HP was generally to be preferred, because the differences in fuel burn were usually stark, and the only advantage was in top-end speed for the higher HP engines. Another reason to leave them alone.
     
  12. DCockey
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    DCockey Senior Member

    Do you have any actual examples of multiple part differences between outboard (not car, truck or inboard) engines with the same mechanical specs but different power ratings, or are your claims speculation? The Mercury examples I mentioned above with the only differences being a restrictor plate and the decals are based on a complete comparison of the parts listings for the engines. Have you done any actual comparisons?

    Why would two outboard engines with the only differences between them being a restrictor plate, throttle stop, or similar burn different amounts of fuel except when the higher horsepower rated engine is operated at the higher power ratings?
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Larger cylinder ports, addition of tuned exhausts, larger reeds, different carbs (not just the jets), different timing, maybe even different compression ratio with some, these were common. And yes, I have experience with engines with widely varying HP ratings on the same CID block, a good one would be the old OMC V4 that at one stage was 85, 115, 140 hp, the 85 burnt about half the fuel of the 140 doing much the same job, by which I mean not running around at full throttle. You sacrifice fuel economy for reserve top end speed.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    With the old 2S bangers, it was particularly the case with the simple cross-flow engines, that a big cubic capacity, and not trying to squeeze too much power out of it, that produced the better motor. When the loop-charged 2S became the norm, it seemed better to keep the cubes fairly modest, and squeeze more from them, rather than increase CID. Or perhaps more accurately, keep the displacement per cylinder low. Four strokes of course, which now dominate, the whole picture has changed.
     

  15. sailhand
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    sailhand Junior Member

    The main difference is the exhaust as far as I know. A friend who owns a yamaha dealership says the rest is identical. Its probably easier to simply buy a 15 and swap the whole powerhead or just the leg. Parts are very expensive and there may be more involved. Buying a second hand fifteen, prefer ably from a fresh water area, not saltwater, would give you a better motor and heaps of parts. The other option is to put the 9.9 and shortshaft together and sell it as a complete motor.
     
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