All pre-war vehicles have an RAC horsepower
rating, which was linked to the Vehicle Excise Duty or VED, aka Road Tax,
Car Tax, Tax Disc and Road Fund Licence, and a figure for brake horse power.
When the horsepower tax was introduced on 1
January 1910 a formula, known as the RAC Rating, was used to calculate the
horsepower 1. The formula was based on applied theory and derived
from basic principles rather than from the results of a bench test. It had
three assumptions which never varied: a mean effective cylinder pressure of 90
psi; a mechanical efficiency of 75% and a mean piston speed of 1000 ft/min.
hp
= ( D2 x n ) / 2.5
where D = the diameter of the cylinder in inches and n = the number of
cylinders.
The rate was changed to £1per hp on
1 January 1921, encouraging an interest in light cars and effectively limited
large engines to luxury cars; ideal for the Austin Seven to have a low VED.
With RAC Ratings of 7.8hp and 7.99hp rounded up, perhaps the ‘Seven’ should have
been called the ‘Eight’
Years
1923 -1929
1929 - 1936
1936
1937 - 1939
Capacity
747.5cc
747.5cc
747.5cc
900cc
Bore – ins
2.208
2.208
2.208
2.235
RAC Rating - hp
7.8
7.8
7.8
7.99
Output - bhp
10.5
13
17
25
R J Wyatt writes 2 that in 1924
“Sir Herbert Austin proposed a new formula for taxation which took into account
the cylinder swept volume, rather than bore size only, which penalised engine
with a high stroke-to-bore-ration. His system was simple. He took the swept
volume of the cylinders, in cubic inches, and divided by 10, this figure would
be the unit of taxation, giving the Austin 20 a rating of 22 against the RAC 23;
the 12 would be 10 against 13; and the 7 would become the car to gain most from
his proposal dropping from an RAC 8 to an Austin 4, halving the annual tax to
£4. There is no doubt that the method was more logical, but in the light of a
decline in annual average tax and an increase in the number of cars that would
benefit most, it is not surprising that the Minister of Transport declared that
he would not consider any scheme which reduced revenue, and that re-imposition
of petrol tax was out of the question.”
By 1930, engine developments meant that the
effective cylinder pressure was 125 psi, and piston speed had doubled. Some
manufacturers, such as Rover and Wolseley, started to use a double rating system
and designated a car as eg. 10/25, where the 10 was RAC horsepower and 25 was
indicated horsepower 1. If Austin had used this nomenclature, with
the 8hp RAC Rating, our cars would have been the Austin 8/10; the Austin 8/13;
then the Austin 8/17 and the ‘Big Seven’ as the Austin 8/25 - which do not sound
so passionate as ‘Chummy’ or ‘Sevens’.
The RAC Rating was used until 1947 and
replaced by a flat rate of £10 per year for all cars.
References:
1. For an explanation of the derivation of
the RAC formula look at ‘The RAC HP
(horsepower) Rating - Was there any technical basis?’
byRichard Hodgson
at
www.wolfhound.org.uk
2. R J Wyatt. 1981. The Austin 1905-1952
pub. David & Charles.
This article, written by Doug Castle, originally appeared in CA7C Seven Focus in
May 2009
pp24-25.
