<Dl> <Dd> where D (\ displaystyle D) is the diameter (or bore) of the cylinder in inches and n (\ displaystyle n) is the number of cylinders </Dd> </Dl> <Dd> where D (\ displaystyle D) is the diameter (or bore) of the cylinder in inches and n (\ displaystyle n) is the number of cylinders </Dd> <P> The formula was calculated from total piston surface area (i.e., "bore" only). The factor of 2 / 5 accounts for characteristics that were widely seen in engines at the time, such as a mean effective pressure in the cylinder of 90psi (6.2 bar) and a maximum piston speed of 1000 feet per minute (5 metres per second). </P> <P> The system introduced a somewhat progressive way of taxing higher - value cars more than low - cost ones but was also introduced to protect the domestic British motor industry from foreign imports, especially the Ford Model T. Henry Ford's mass production methods meant that the Model T was competitively priced with British - built cars despite being a much larger, more durable and more powerful car than any similarly - priced model . In 1912 Ford opened a factory to build Model Ts in Manchester, to circumvent the import tariffs that, up to that point, had increased the effective price of foreign cars . Under the RAC's formula the Model T was now a 22' tax horsepower' car, making it much more expensive to run than its British - built rivals on sale for the same price . </P>

Who devised the unit of power called horsepower