RAC Horsepower
Note: This is a sub-section of the Royal Automobile Club (RAC)
This measure was instituted by the Royal Automobile Club in Britain and was used to denote the power of early 20th-century British cars. Many cars took their names from this figure (hence the Austin Seven and Riley Nine), while others had names such as "40/50 hp", which indicated the RAC figure followed by the true measured power.
Taxable horsepower does not reflect developed horsepower; rather, it is a calculated figure based on the engine's bore size, number of cylinders, and a presumption of engine efficiency. As new engines were designed with ever-increasing efficiency, it was no longer a useful measure, but was kept in use by UK regulations which used the rating for tax purposes.
RAC h.p. = (D x D x n)/2.5
where
D is the diameter (or bore) of the cylinder in inches,
n is the number of cylinders
See Also
Sources of Information