Also the tax HP of 10 would be a desirable car.
h.p. = (D2 x n)/2.5
This was based on engineering assumptions that screwed up engine making for decades.
The RAC formula, to simplify down to the above formula, made three assumptions which do not change, whatever the engine under consideration:
(1). Mechanical efficiency is 75% max
(2). The mean effective pressure is 90lb/sq.in max
(3). The mean piston speed is 1,000 ft/min max, i.e. 2 x L x N = 1000 ft/min
This meant that British engines were long stroke and small bore.
After the war these assumptions were nonsense but habits, will to change and money to build new engines meant engines were a bodge till after the sixties.
Jon the Chief
