Dave- please elaborate on your data set and figuring principles that allowed you to work up your 4 h.p. figure.

We're talking the difference here between

**brake** horse power (bhp) and horse power as used, years ago, for taxation purposes.

The bhp is a measure of how powerful an engine is - so, for example, a basic Villiers 197 might give about 9 bhp, but could be tuned, in racing format, to give about 30 bhp.

When we talk, though, of Austin 7, 12, 16, etc, Morris 8, 10, 12, Lanchester 14, and so on, then we're referring to the "horsepower" by which those cars were taxed. The formula for calculating that hp (the "RAC" horsepower) takes no account of how much power the engine actually produces (!), but concerns only the area of the top of the piston:

RAC hp = bore (in inches) squared, times number of cylinders, divided by 2.5

For the 328cc Berkeley engine, that's 2.283 squared, times 2, divided by 2.5.

Since the RAC hp system continued for many years as the basis for annual car tax in the UK, it greatly influenced the production of narrow-bore, long stroke engines - nicely torquey, and easy to drive, but neither spritely nor particularly economical!

Britain stopped using the RAC hp to determine car tax at the beginning of 1947, but the public (and motoring magazines) continued into the 50s sometimes to talk of engines in those terms.