Check Your Engines Capacity & British RAC Horsepower Rating

If you have been involved in the restoration of British cars, you have more than likely been exposed to the term "taxable horsepower rating". This has often been a source of confusion and amusement in the minds of Americans, who are accustomed to large horsepower numbers and who haven’t had a tradition of petrol shortages and intrusive government policy towards their automobiles.

The first British road tax on cars was imposed by the 1896 Locomotives on Highways Act which imposed a fee of 4 pounds 4 shillings for a four wheeled car up to two tons. The Motor Car Act of 1902 established vehicle registration, at fee of 1 pound per year. From the 1st of January, 1910, a Horsepower tax was imposed, based on the British RAC formula, at the following rates:

The Royal Automobile Club formula for horsepower was not based on any practical bench test, rather it was based on applied theory and was derived from basic principles. It contained the following three assumptions, a mean effective cylinder pressure of 90 pounds per square inch, a mechanical efficiency of 75% and a mean piston speed of 1000 feet per minute. The calculation uses cylinder diameter squared, times the number of cylinders, divided by the constant 2.5.

From the 1st of January, 1921, the rate was set at a pound per horsepower. This started the light car boom and effectively limited large engines to luxury cars. However, by 1930, mean effective pressure was 125 pounds per square inch, and piston speed had doubled. This lead to a double rating system, as the Rover 10/25, where the 10 was RAC horsepower and 25 was indicated horsepower. In 1946 this was usally replaced by a flate rate. Note that the RAC formula favours cars with long strokes and small bores over cars with large bores and short strokes, and also a smaller number of cylinders.

For a more in depth article about the RAC formula, showing how it was derived from basic principles, refer to the document titled "The RAC HP (horsepower) Rating - Was there any technical basis?" by clicking on the following link.