The British RAC Horsepower rating was used for decades to rate vehicles for excise purposes. The motor tax paid depended on the rating of the engine. The engines used at Brooklands were usually rated in litres and horsepower (h.p.). The delivered power in b.h.p. was usually considerably above the h.p. rating.
The formula is: D2 x N / 2.5 where:
D = diameter of a piston in inches. 1 inch = 25.4 cms
N = number of pistons
The constant, 2.5, took into account Pi (3.14), a piston speed of 1000 feet per minute, a compression ratio of five-to-one and a mechanical efficiency of 75%. Increases in these factors may give rise to an increase in power of three to five times.