Published: 03:53PM Oct 21st, 2010

Dear Classic American...I have noticed over the years, when it comes to pre-war cars, there seems to be a little confusion over the difference between brake horsepower and taxable or RAC horsepower.


The latest case in point was the beautiful Packard that won the Car of the Year heat at the Rally of the Giants. The text stated the car developed 39.2 BHP actually it was rated at 145 BHP, the 39.2 was taxable horsepower.

The difference between the two is BHP is the power the engine puts out. One British or American horsepower is 33000 ft lbs per minute; originally this was measured on a machine called a Pony Brake, thus the term ‘brake horsepower’. Incidentally a metric horse power is somewhat smaller which could explain the high ratings on Japanese and European cars.

Taxable horsepower is something that is calculated and has very little to do with the true power out put. The formula is DxDxN divided by 2.5. D is equal to the bore diameter in inches N is the number of cylinders. This means a big bore multi cylinder engine has a higher rating than a long stroke four cylinder. The system was devised about 1900 when it had certain relevance, but as engines developed it was left behind, except by the taxman. In 1920 horsepower tax was increased to £1 a horsepower, this was done with the intention of keeping American cars out of the country. The Ford Model T had over 50% of the U.K. market.

Mel Atkinson

Well, that certainly clears that up Mel, although you lost us with the algebraic formulas!

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