# RAC Horsepower Rating

An obsolete engine horsepower rating system that was used as a designation as to the power of an engine.

The indicated brake horsepower per cylinder can be calculated if the mean effective pressure in the cylinder is known. This pressure is the mean of the varying pressures acting on the piston. Being the mean value, it is that notional unvarying pressure which acts on the piston during the entire power stroke.

where
L = stroke [feet]
D = bore [inches]
N = engine speed [rpm]
Vm = piston velocity [feet min-1]
n = number of cylinders
Pm = mean effective pressure [lb in-2]

The RAC horsepower rating simplified this by making three assumptions:

1. Mechanical efficiency at Vm is 75%
2. The mean effective pressure Pm is 90 lb in-2
3. The mean piston speed Vm is 1000 ft min-1

Assumption 3 takes the stroke of the engine into account.

This then gives the simplified RAC horsepower rating often abbreviated to hp as:

where
D = engine bore [inches]
n = number of cylinders

In the very early days of engine design this formula worked OK. However, the mean piston speed soon more than doubled, the mean effective pressure increased and so did the mechanical efficiency. Therefore, although this rating was used as a way of determining the tax of a vehicle it was very inaccurate and did not relate to the actual engine brake horsepower which soon became 2 to 3 times that indicated by this formula.

Subjects: Mechanical Engineering