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Compression Ratio of Current Yamaha 90 hp Two Stroke

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  • Compression Ratio of Current Yamaha 90 hp Two Stroke

    Can anyone explain the engineering theory behind the fact that the current Yamaha 90 hp two stroke motor has a compression ratio of 5.76:1 in cylinder 1, a compression ratio of 5.67:1 in cylinder 2 and a compression ratio of 5.86:1 in cylinder 3. The 70 hp two stroke which is also a 3 cylinder engine has a compression ratio of 6.1:1 in all 3 cylinders.


  • #2
    Re: Compression Ratio of Current Yamaha 90 hp Two Stroke

    I would be glad to if you first explain how you arrived at those numbers or where you got them, Bruce.
    sigpic

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    • #3
      Re: Compression Ratio of Current Yamaha 90 hp Two Stroke

      Yamaha "Outboards" 60 page brochure dated July 1, 2007. Published by Yamaha. This information may also be found at the Yamaha web site:

      http://www.yamaha-motor.com/outboard.../10/specs.aspx

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      • #4
        Re: Compression Ratio of Current Yamaha 90 hp Two Stroke

        I am betting it fixed a balance issue
        If a dirty bottom slows you down what do think it does to your BOAT

        1970 Cal 29 Sea Fever
        http://cal29seafever.blogspot.com/
        1981 J24 Tangent 2930
        http://s565.photobucket.com/albums/s...%20for%20sale/
        Tommays
        Northport Ny

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        • #5
          Re: Compression Ratio of Current Yamaha 90 hp Two Stroke

          Here is where it gets interesting. The part number for all three pistons is the same which leads me to believe that the crankshaft throws are different.

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          • #6
            Re: Compression Ratio of Current Yamaha 90 hp Two Stroke

            It is certain to be different timing on the ports to broaden the torque curve, Bruce. The 90 is a small displacement for 90HP and needs help getting both low and high speed torque. Ingenious idea.
            sigpic

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            • #7
              Re: Compression Ratio of Current Yamaha 90 hp Two Stroke

              The 90 hp set up is similar to that used in the old V-6 Special and the 2.6 liter 225 and Excell.
              On those models, both the port heights and head cc's are different to balance the differences in exhaust passage lengths from top to bottom.

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