Rod length changes the angle of the rod vs crank. It really changes piston speed from the 11-1 oclock postion. A shorter rod tends to stay at TDC a little longer and is lazy approching or leaving TDC. A longer rod keeps the piston moving faster in that area. Keep in mind that is the spark area on the compression stroke. On the intake stroke. a longer rod starts sucking faster as the piston moves quicker, which is needed with a longer stoke. Hope that helps.

I think I get it... but how does the faster piston have a dynamic effect of shortening the stroke? the accelerated piston physically cannot suck as much air?

mtgbiker, I pretty much did exactly what you suggested. I backed off the crane 731 a notch. If I achieve a reliable 475hp at 5000rpm I'll be happy. I plan to spend most of my time in the upper mid-range where I hope to stay above 600 torque.

I really struggled with the intake, but in the end I chose the Performer RPM dual plane (port matched is all). If someday I want more top end out of the engines I can get more flow with intake and heads. Probably not on this boat though. It's an old heavy scarab.

Maybe the longer rod affects dynamic comp. ratio,but it doesn't affect static compression. That is simply a fraction. Volume of cyl. and head, piston down over volume of cyl. and head, piston up. Merc. always used 8.8 to 1 with 89 octane, 9.4 with 91 octane seems ok but Merc. was always conservative with fuel/ air mix and timing. Don't use a thick head gasket. Keep the piston to head clearence at .040 for best performance and knock control. Using thicker head gaskets to cut compression will actually make knocking worse.

here is a web site that has some cool calculations.
http://www.golenengineservice.com/html/calculators.html
it will figure all kinds of things if you have the basic info.