The UK's old RAC horsepower formula was this (I went and looked!): Diameter of cylinder (in inches) squared, times number of cylinders, all divided by 2.5. Hence, long strokes were 'favoured' in taxation terms versus short stroke engines, by the hp rating being lower for the same capacity. The logic of the formula was good, however. The consequence was that legislators and administrators inadvertently promoted less efficient engines. But then we didn't have a global fuel crisis and all the rest of it in those days, nor did we have roads where we could cruise along at any speed you like.
If anyone wants to pursue, just google 'RAC Horsepower Formula', and read away! In those days, it led to engines of certain capacity loosely being described as Trevinoz says. Or 15/50, 16/60 etc, where the first number is the RAC hp, the second the bhp. Wolseley cars were some of the last to maintain this dual description, until the end of the '50s. Not all countries did it the same way, but for many years tax discs/registration fees were priced according to (arbitrary) power output in many places, and even today in France at least, the 'Fiscal Horsepower' dictates the registration fee payable when getting a log book. An A 10 is rated at 7, a B31 at 4 (if I understand the papers correctly). Which ties in not too badly with 100cc to the horsepower. Amazing how these legacies from the earliest days linger on.
