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  1. #1
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    Default 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. #2
    Moderator JB's Avatar
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    Default 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.

  3. #3
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    Default 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

  4. #4
    Admiral tommays's Avatar
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    Default 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

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    Tommays
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  5. #5
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    Default 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.

  6. #6
    Moderator JB's Avatar
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    Default 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.

  7. #7
    Lieutenant Commander
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    Default 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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