changing gear ratios in outboard engines

Discussion in 'Outboards' started by rasorinc, Dec 6, 2011.

  1. rasorinc
    Joined: Nov 2007
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    rasorinc Senior Member

    I have never owned an outboard engine and need to know if changing gear ratios in a used 4 stroke engine is a big deal or not. And is it an expensive proposition? Want to buy a 60-75 hp. and have a 2.33/1 gear ratio. Thanks.
     
  2. thudpucker
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    thudpucker Senior Member

    Ras, I think you change that with the Prop.
     
  3. Submarine Tom

    Submarine Tom Previous Member

    I suggest you contact the manufacturer of the engine. That will give you an answer pretty darn quick. What ratios, cost, availability, shipping time, complexity, side effects, complications, detriments, etc, etc, etc...

    -Tom
     
  4. powerabout
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    powerabout Senior Member

    plenty of 60-70 have that ratio
    what model do you want to change?
    4 stokes generally have low ratios as they are gutless
     
  5. rasorinc
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    rasorinc Senior Member

    Yamaha Hi Thrust and Honda Power Thrust engines have this gearing and compared to their regular 50,60hp, models all other specifications are the same. So they want more money just for a different gear setting. I'm sure the props are different but engines are quoted W/O props. I want high thrust so if I buy used it appears if I install 2.33 to 1 gearing, I will hava a less expensive high thrust engine--with the right prop of course
     
  6. thudpucker
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    thudpucker Senior Member

    Good thought Ras!
    Remember not to load the engine down, or you'll burn up a Rod Bearing. Keep it singing hi "C" and it'll last longer.

    PS: That Locust Tree is still standing, but if it ever suffers any wind damage, your first on the list! :)
     
  7. rasorinc
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    rasorinc Senior Member

    Thanks, Stan
     
  8. powerabout
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    powerabout Senior Member

    are they just a gear ratio change or are they using the gearbox from a larger engine?
    Do they take different diameter props?
     
  9. Jimboat
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    Jimboat Senior Member

    Mercury, Tohatsu, Suzuki, Yamaha 70hp motors all have ratio 2.3:1. As interest, what is reason for specific gear ratio requirement? Torque/RPM is usually optimized by effective propeller selection? Also, remember that OEM designs their engine for particular gear ratio and propeller matches, so changes will also change performance of engine.
     
  10. powerabout
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    powerabout Senior Member

    you get a bonus if they install a larger gear case on a smaller engine so can get bigger diameter props
    and also a bit of longevity as the case is designed for a larger horsepower engines
    typical back in the day of OMC commercial engines
     
  11. Jimboat
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    Jimboat Senior Member

    Can't really imagine why we'd want a larger gearcase than OEM spec. If using larger props than the engine is designed for, then engine RPM will be reduced and gearcase under less load, so longevity not likely to be an issue.
     

  12. powerabout
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    powerabout Senior Member

    Jim
    they only make sense at displacement speeds
    As I remember OMC put V6 cases on V4 and the 3 cyl large case on the 2 cylinder engines etc
    regards
    Powerabout
     
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