We all do it: Browse through a motorcycle’s spec sheets to look for the engine’s power above everything else. But why do more and more spec sheets use the unit kW for engine power instead of HP? Why do not we settle on just one power unit?
The short answer is about accuracy and a standard measurement, as we shall see below.
Where did HP (horsepower) come from?
It all goes back to the invention of the steam locomotive.
Thomas Newcomen was the inventor in 1712, but it was James Watt (familiar name, is it not?) who improved the design in 1776. Then, Watt devised the method of comparing the power of his locomotive to the equivalent of how many horses to promote the power of his locomotive on a more relatable scale. Hence, horsepower.
Since then, this value has been adopted for rotary motion for trains and through the Industrial Revolution. Therefore, the value stuck for motorcycle and car engines.
Watt’s mechanical horsepower is defined as a horse lifting a 550lb. load 1 foot in 1 second, which equals 32,549 ft-lb of work per minute, or 4,500 kilogram-metres per minute.
Okay, so why Watt?
However, Europeans prefer to adhere to SI units or in other words, metric units. This is where the discrepancies creep up.
See, Imperial horsepower measures as 745.7 Watts, while the European SI metric horsepower unit also known as PS (Pferdestärke) or CV (Chevaux-Vapeur) is only 735.5 Watts.
This is why while some spec sheets pronounce an engine to produce 70 PS, it actually produces only 69 HP.
These different units i.e. mechanical HP, metric HP, PS, CV only created confusion to vehicle buyers, so in 1972, the kW replaced PS as the SI unit for engine power through EEC directives. But as of 1 January 2010, the EU only permits HP as a supplemental unit to kW.
How is kW calculated?
Kilowatt is a function of torque and revolutions per minute (RPM) and is calculated as following: Power (kW) = torque (Nm) x speed (revolutions per minute, or RPM) / 9.5488.
The calculation is actually the same for horsepower: Power (HP) = torque (lb-ft) x speed (RPM) / 5,252.
However, to convert published kW to HP: Horsepower = 1 kW x 1.34. Thus, an engine which produces 12 kW of power equals 16.1 HP.
