As RPM's increase, so does friction and side loading of the piston skirt against the cylinder walls. The shorter the rod you use for any given stroke, you increase this side loading of the piston skirt against the cylinder walls, thereby increasing friction, heat, and piston/cylinder wall wear. Also, at higher RPM's, a longer connecting rod tends to "dwell" longer at the top/bottom of the stroke (i.e.-it has stopped moving at bottom dead center for a few micro seconds longer than would a shorter rod). This has advantages in certain applications where you are trying to fill the cylinder at higher RPM with a later closing intake valve, like if the piston were moving upwards ever so slightly at, say, 7,000 RPM's, it would be fair to assume that the piston which remained "at rest" longer (However slight) would allow more complete cylinder filling. However, in a low RPM "torque monster" engine, the short rod accelerates away from TDC more quickly, thereby exerting a more powerful "draw" on the intake charge sooner than a "lazy" longer rod would. You will almost always find engine builders favoring the longest rod they can reasonably fit into the engine. But the longer the rod, the shorter the piston skirt must be, and that allows for the piston to "rock" in the cylinder more due to less support of the short skirt against the cylinder wall.
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