This rule of thumb is only the tip of the iceberg. It not just mean feet per minute, but also the peak piston speed that is critical. Is there anyone more concerned about reliability than the aircraft industry?
For purposes of rules of thumb, it is generally agreed that for an engine in aircraft service, 3000 fpm is a comfortable maximum MPS and experience has shown that engines having an MPS substantially exceeding that value have experienced reliability issues
Remember that we’re talking about cruising speeds
For further reading check out this link.
http://epi-eng.com/piston_engine_technology/piston_motion_basics.htm#mpsThere is another piston velocity which is used more as a "rule-of-thumb" in engine evaluations. It is called "mean piston speed", which is a calculated value showing the average velocity of a piston at a known RPM in an engine having a known stroke length.
Keeping in mind that every crankshaft revolution, the piston travels a distance equal to twice the stroke length, then Mean Piston Speed (MPS) is calculated by:
MPS (ft per minute) = RPM x 2 x stroke (inches) / 12 (inches per foot) = RPM x stroke / 6
The Mean Piston Speed at 4000 RPM for the example 4.000 inch stroke engine is:
MPS (ft per minute) = 4000 x 4 / 6 = 2667 feet per minute.
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Let’s see if this gets your attention
Note that R / S has no influence on MPS, although it strongly affects peak piston speed (4390 fpm for the example engine {R / S = 1.525} at 4000 RPM).
A rough calculator for peak piston speed is
PEAK PISTON SPEED = MPS X 1.5758 ( ½ X PI) = ( .5 X 3.14159) = 1.5758