[BruceAML]

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]

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

[BruceAML]

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

[tommays]

I am betting it fixed a balance issue

[BruceAML]

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]

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]

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.