Joe Craig gave the mean piston speed of his 1938 TT winning Manx as 3700 ft/min, claiming it was an 'excessively high mean piston speed'. As the bore and stroke of those engines was 79 x 100, this equates to an engine speed of 6350 rpm. Here Craig would be using Frederick Lanchester's 'corrected' mean piston speed, which was the mean piston speed calculated by multiplying twice the stroke in feet, by the rpm, further adjusted by dividing the result by the square root of the stroke/bore ratio.
This gives a considerably more accurate representation of the stresses in the engine. The resulting values are higher than the uncorrected values where the engine is over square, and lower when under square. Thus the corrected value for the 1938 Manx was 3703 ft/min at 6350 rpm, and the uncorrected value was 4167 ft/min.
The Commando engine in 73 x 89 format, has at 6800 rpm an uncorrected mps of 3971 ft/min, and a corrected value of 3597 ft/min.
Dominators with 66 x 72.6 bore and stroke should be within 4000 ft/min, both corrected and uncorrected, at 8300 rpm, but as with the 750 Commando at 6800 rpm, who needs more for normal use (is there such a thing as a 'normal' Commando rider ?)
Todays top race engines can easily surpass a 4000 ft/min mps, whether uncorrected or corrected, but the most outstanding one in motorcycle racing that is available to the man in the street, if he wishes to purchase one, is the GM speedway engine. Turning out peak bhp at 10500 rpm, this is a 90 x 78 bore and stroke single, that has a mps uncorrected of 5408, and corrected 5791, but I have personally measured the rpm of a former world champion's engine when practicing before the start of a season, and with
data logging equipment installed, was astonished to see the engine speed at the start gate at 13200 rpm before the gate opened, a corrected mps of 7251 ft/min. Small wonder a few rods let go on the start line!
A check on many of today's engines will reveal some surprising results when calculating mean piston speeds. machines such as the Yamaha R1 or the BMW S1000RR are way over the 4000 figure, both corrected and uncorrected.
Back in 1965, Phil Vincent wrote two articles for the English magazine Motor Cycle, the first in issue for July 22nd, entitled 'Short and Fat' , and another in issue for September 23rd, entitled 'What - No Ceiling ?'. In the articles he discussed mean piston speeds at some length, but never mentioned Lanchester's 'corrected' version.