gives the formula used in various places to calculate tax horsepower for cars
The various car formulae were never applied to motorcycles. Instead, once things settled down around 1910, power was just linked to capacity. The main categories were:
250cc = 2 1/4 h.p., 350cc = 2 3/4 h.p., 500cc= 3 1/2 h.p., 750cc = 6 h.p., 1000cc = 8 h.p.
This scheme was used until the mid 1920s when there was a shift to a new scheme - I think of it as a penny per cc:
249cc = 2.49 h.p., 348cc = 3.48 h.p., 494cc = 4.94 h.p. and so on.
In the early scheme the h.p. ratings were always expressed as fractions; in the later scheme as decimals. This avoided (some) confusion.
It was extremely rare for a manufacturer to use anything other than these figures (like the actual h.p.) to describe a machine, in advertisements, factory literature, road tests etc. An exception was for racing machines.
For Douglas the change was made between the 1924 catalogue (when the 348cc TS and CW were listed as 2 3/4 h.p.) and the 1925 catalogue (when they became 3.48 h.p.). The RA went from 3 1/2 h.p. to 4.94 h.p., and the OB from "Six Hundred" (4 h.p.) to 5.96 h.p. The only "actual power" listed in either catalogue was 25 b.h.p. for the RA25.
In 1928 Douglas tried to buck the trend of two-decimal-place power ratings by calling the 596cc models 5.9 h.p. and the 348cc models 3.5 h.p., but I doubt this lasted. The EW and its variants are best described as 3.48 h.p.
Cheers
Leon
