The RAC Formula

In the early days of steam power, engineers devised a method of rating steam engines. It was deemed that the average horse could raise a weight in pounds multiplied by a height in feet, to a total of 33,000 in a minute. Therefore 1 horsepower = 33,000 ft pounds per minute. This formula was adopted for the internal combustion engine in due course and the term horsepower came into common use. The power of an engine was determined using a measuring devise of one form or another and expressed as "Brake horse power".
In 1906 the Royal Automobile Club, devised a formula to estimate the power of an engine without the use of a measuring devise. The formula was supposed to be a simple way of estimating the power of an engine when used in competition. All that was needed to determine an engines horsepower rating by this method was to know the diameter of the cylinder bores in inches “D” plus the number of cylinders “N”. The formula is, D2XN over 2.5, here is an example. 4 inches X 4 inches X4 over 2.5 = 25.6 Horsepower.
The formula was out of date as soon as it was devised, but was retained in use a measure of comparison. Why didn’t they just use the cubic capacity, I don’t know?  The French used a similar system, Cheval Veur, (Steam horse.) shorted to CV. I. E. 2CV. In Germany the term PS was uaed.
 On January the 1st 1910 the R.A.C. formula was adopted as a means of determining the rate of taxation for motor cars in Great Britain. An unfortunate consequence of this was that for the next forty years engine design in Britain was retarded. This was due the fact that the smaller the bore of the engine the less tax paid. This led to small bore long stroke engines and resulted in high piston speeds that limited engine speed, thus power output. Cars where classified by their horsepower rating, i.e. 7hp, 9hp, 11hp. And usually named using the horsepower rating and engine power output in brake horsepower .i. e. 11/20, 15/40. In 1921 tax rate was set at £1 per Horsepower. This had a big influence on the cars people purchased. Particularly in the case of American imports that even after excise duty was added were relatively inexpensive but the annual tax made them expensive to own. This continued until 1948, when all cars where subject to the same rate of tax of £10 per year except cars rated at 6hp and 7hp registered before that date. In time even this exception disappeared. One of the next changes in taxation was that cars with an engine capacity of less than 1549cc paid a lower rate of tax than cars with engines greater than 1549cc. This was the first time that anything approaching Light and Heavy cars classifications where adopted. In March 2001 cars registered after that date where subject to a new method of determining the tax due, an emission based system of taxation. The emissions measured are CO2 carbon dioxide, and the unit of measurement is grammes of CO2 per Kilometre travelled. This is another method of differentiating light cars from heavy cars as this system penalises weight. It will also show up inefficient engine designs. The lighter designs available are in the lower bands, Less than 150 and 151-165 g/Km, and the bigger cars are in the top bands of 166-185 and Over 180 g/Km.
By 1914 there where seventy different light car models available to purchase in Britain, with a similar number of models available in continental Europe, from 7hp to 12hp.
The ultra light car was slow to make a significant impact on the market. By 1926 the up to 8hp sector made only seven percent of sales in Britain, and that went up to eleven percent by 1928. At that time the up to 10hp sector which included the smaller car's, made up twenty five percent of the market, but by 1933 was up to sixty percent of all sales.
In 2003 there are approximately two hundred and forty models available on the British market with an assortment of engine and trim options. Over thirty nine percent of these are available with an engine of 1600cc or under. Models available with only engines under 1600cc amount to fifteen percent of the overall total and those with only petrol engines of 1600cc and under with larger diesel engine options amount to another eight percent. That leaves sixteen percent of the models available that can be purchased with an engine each side of the arbitrary line. Some are light cars that can have a large engine fitted to give a high performance and some are medium sized cars that can be purchased with a modest sized engine to improve economy. This illustrates the problem of defining a light car.

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