Grace's Guide To British Industrial History

Registered UK Charity (No. 115342)

Grace's Guide is the leading source of historical information on industry and manufacturing in Britain. This web publication contains 150,094 pages of information and 235,418 images on early companies, their products and the people who designed and built them.

Grace's Guide is the leading source of historical information on industry and manufacturing in Britain. This web publication contains 147,919 pages of information and 233,587 images on early companies, their products and the people who designed and built them.

RAC Horsepower

From Graces Guide

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

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