To EstudioChispa: Horsepower - and horsepower!

[tyro]

Hello Russ,

Thank you for your kind remarks about this one - I'm pleased that you like it.

Ah, ha, I see I have caused some confusion when I say that this was a twelve horsepower car! But that's not my fault: that is the fault of a crazy system of classification which was used in Britain in pre-war days.

Now, you see, in Britain we pay "road tax", an annual fee to the government which nowadays is based chiefly on exhaust gas emissions. Prior to that it was based on engine capacity with people with large cars having to pay more than those driving cars with small engines.

But the first method of classification was devised as far back as 1910 and survived until just after World War II. The classification was based on "Horsepower" - but that wasn't real horsepower, it was "fiscal" horsepower or "RAC" horsepower, a crazy figure which was worked out by the formula: H.P. = the square of the cylinder diameter (in inches) x number of cylinders - all divided by 2.5.

Did you ever hear of anything so daft? But in 1910 when the system was devised, that H.P. was probably pretty near to b.h.p. so maybe it wasn't quite so crazy at the time.

If you look at my own little car, it has an engine of 1125cc, it has 4 cylinders and its cylinder bores are 2.5 inches. So you can work out that it was rated as a Ten Horsepower car :- 2.5 x 2.5 x 4 divided by 2.5 = 10. So, prior to WWII you would have paid an annual tax of £10 to the government to be allowed to drive it on the roads.

But my little car pumps out a staggering 20 b.h.p.! And this Wolseley, with its more sporty overhead valve 12 H.P. (rated) engine probably produced more like 50.

One of the main problems of this rating method was that it severely restricted the development of engines in Britain in the pre-war era. In order to keep road taxes down, engines were made of larger capacity and more powerful by increasing engine stroke rather than cylinder bore size - and that both severely limits engine speeds as well as severely restricting engine breathing because of restricted valve sizes. Interestingly, this "horsepower" rating also had an effect in reducing the popularity of Ford's "Model T" as it had an almost "square" engine with very large bores and short stroke for its era, having a "rated" horsepower of 22.5, so attracting a road tax of £22 and 10 shillings per year!

So now you know!

Take care,

Kind Regards,

John.

[EstudioChispa]

VERY interesting and informative, John! Tax policy has affected auto production for along time all around the world. Thanks for cluing me in about this one!
Russ