Actually the bore/stroke ratio is a variable that affects many aspects in the performance of any engine.
Volumetric efficiency and Thermodynamic efficiency included.
Generally a slightly oversquare design gives the best results.
Since Andy's question is only referring to Mechanical Efficiency, which refers to the difference between Brake Horsepower and actual output horsepower due to friction losses in the piston rings, bearings,oil pump etc.
Actually most of the friction losses occur between the cylinder and the rings.
The answer is that the short stroke is more efficient in this aspect as piston friction depends mainly on the contact area and the piston speed.
The contact area depends on the number of rings and the circumference of the rings (piston diameter * 3.14).
The piston speed depends on the stroke, the shorter stroke for given RPM has less piston speed and thus less friction.
Finally, the friction is much less for the short stroke, although the piston ring area is larger.
In fact the piston speed is the limiting factor for engine rpm, as there is a critical piston speed above which the rings will disintegrate. This limit changes with time, as metallurgical science is advancing, and the rings can stand higher speeds.
We should be talking about Average piston speed really, because the piston speed is changing from 0 at TDC, to max in the middle of the stroke to 0 again at BDC.
Last edited by ariszr7; 03-15-2002 at 04:11 AM.