[serialk11r]

I refuse to think about leverage differences, too complicated :P I prefer to just assume that metals have low hysteresis and that crank bearings have lower friction coefficient than piston skirts. Maybe someone can provide empirical evidence here.

I suspect combustion pressure is the biggest difference. A long rod gives more time between any given distance travelled around TDC, so your peak pressure is higher because more of the charge is burned by TDC, and the charge starts burning further up the compression stroke, so the piston doesn't push against a burning charge for as long. The net effect should be like an increased compression ratio in terms of cycle efficiency, without a higher physical compression.

Want.FRS, look at the Wikipedia article I linked you. In that case, a picture is probably worth a thousand words.
The piston's path is "linear", but its motion is quasi-simple-harmonic (aka sinusoidal) with respect to time. The rod adds some complexity to this, most obviously decreasing it around TDC. The Wikipedia article varies stroke and keeps rod length constant, if you change the rod length instead, you'll see that it's increasing BDC peak acceleration. The reason is the piston's postion at a given crank angle is the vertical distance between a circle of radius = stroke/2 and a tangent circle of radius =rod length, vertical meaning perpendicular to the common tangent of said circles. An infinitely long rod would make the piston's motion perfectly sinusoidal.

The rod transmits any force to the piston at the angle it makes with the piston's travel. The force on the side of the piston is completely determined by rod angle and force that the rod transmits. Increased force on a sliding surface increases frictional force.