An article from the Sydney Morning Herald comments on this years racing cars 1906 and notes the 6 cylinder Napier car.
Sydney Morning Herald (NSW, Australia)
19 April 1906
Page 10
THIS YEAR’S RACING MOTOR CARS.
According to the latest files to hand, this year’s
racing motor cars will present fewer novelties than
ever before. Transmission by propeller-shaft is gain-
ing in favour every day over chain drive, and all the
machines are being built with their centre of gravity
placed very low. One peculiarity which the new regu-
lations at home have engendered is that all cars will
have a large fuel capacity, provision being made for
as much as 45 gallons of petrol, and a proportionately
large oil supply, in order to cover the six rounds of
the circuit without stopping to fill tanks. Excepting
the Mercedes, and of course the six-cylinder Napier, it
is probable that all the starters will have four-cylinder
engines. The Richard Brasier cars, with four cylinders
of 165-m.m. bore, and 104 h.p., will have transmission
by cardon shaft. The Mercedes six-cylinder have 150-
m.m. bore, and 140-m.m. stroke, giving 135 h.p., four
speeds, with direct drive on the high, and ball bear-
ings throughout. Several of the French drivers have
already made acquaintance with the circuit, Hemery
visiting it on a 1905 racer immediately before his
triumphal return from Florida.
Reference: http://trove.nla.gov.au/newspaper/article/14766140