There's an interesting read if you can find it regarding stroke/rod length ratios and crankshaft/bore offset authored by Desaxe. In simple English, it discusses manipulation of the rod length to maximize the torque effect. It's a pretty complicated subject, but it's at least interesting to know what's going through the minds of the engineers when they choose certain rod lengths. Desaxe also experimented with offsetting the centerline of the crank as it relates to the bore, also attempting to get the most effect from the power stroke.

The theory goes something like this: After the piston passes TDC, the rod applies the pressure from above the piston to the crankshaft via the conrod. You're converting linear motion to rotational, so there are few things going on that can be optimized. Understanding that the highest level of engine torque is when the conrod is 90 degrees to the rotational axis of the crank, the rod length can be manipulated to slightly extend that period of optimal torque. It's not without its drawbacks because a rod that's too short will cause unnecessary side loading of the piston on the combustion stroke, and especially during the compression stroke. So, there's a trade off in changing the length of the conrod, and the Desaxe theory helps to arrive at what would be the ideal rod length. Offsetting the axis of the crankshaft isn't typically done because it increases the complexity of engine manufacturing to far to great an extent when weighed against the minimal gains, but this is an extension of the Desaxe stroke/rod length ratio discussion. Dave Gierke (a well-known engine guru who spends most of his time on the airplane side) explains the Desaxe theory in more detail. Although the book focuses on model airplane engines, all the same theories apply.

http://www.airagestore.com/planes/bo...rcraft-v1.html