A horsepower rating for automobiles, according to a specification adopted by the Association of Licensed Automobile Manufacturers, and later by the newly-formed Society of Automotive Engineers (and called “S.A.E. horsepower”). Originated around 1908. It is found in manufacturer’s literature of the period, sometimes accompanied by a more accurate rating made with a dynamometer. The same formula was used by the Automobile Club of Great Britain and Ireland (later the Royal Automobile Club).
where D is the diameter of the cylinders in inches and N is the number of cylinders. For example, a 4-cylinder engine with a 3-inch bore would be rated at 14.4 horsepower.
The constant 2.5 was a judgment call by the Association’s Mechanical Branch, based on a piston speed of 1000 feet per minute, about 1500 revolutions per minute. The absurdity of this technique as a measurement of power must have been apparent at the time (Henry Leland increased the actual horsepower of the “Merry” Oldsmobile’s engine by 25%, simply by better machining of the identical design). It did, however, put all manufacturers on an equal footing, eliminating wild advertising claims.
Fred H. Colvin and Frank A Stanley.
American Machinists' Handbook. Second Edition.
New York: McGraw-Hill, 1914.
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Last revised: 2 September 2005.