Compression Ratio of Current Yamaha 90 hp Two Stroke

BruceAML

Petty Officer 1st Class
Joined
Feb 26, 2007
Messages
371
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.
 

JB

Honorary Moderator Emeritus
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Mar 25, 2001
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45,907
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. :)
 

tommays

Admiral
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Jul 4, 2004
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6,768
Re: Compression Ratio of Current Yamaha 90 hp Two Stroke

I am betting it fixed a balance issue :confused:
 

BruceAML

Petty Officer 1st Class
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Feb 26, 2007
Messages
371
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.
 

JB

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Mar 25, 2001
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45,907
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. :)
 

Ray Neudecker

Lieutenant Commander
Joined
May 25, 2004
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1,656
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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