How the Speed Spitfire’s Racing Merlin led to the development of the Merlin III powered by 100-octane fuel, the engine fitted to Spitfires and Hurricanes during the Battle of Britain

How the Speed Spitfire’s Racing Merlin led to the development of the Merlin III powered by 100-octane fuel, the engine fitted to Spitfires and Hurricanes during the Battle of Britain

By Dario Leone
Oct 12 2023
Sponsored by: Mortons Books
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The Speed Spitfire

Conceived to capture the world speed record just before the outbreak of World War II, the “Speed Spitfire” originated from a standard Spitfire Mk I, K9834, that was taken from the production line and rebuilt by Supermarine in order to make the aircraft competitive with the German Messerschmitt and Heinkel contenders.

As told by Calum E. Douglas in his book The Secret Horsepower Race Western Front Fighter Engine Development, on Apr. 26, 1939, Germany broke its own speed record with a highly specialised new bespoke race-aircraft: the Messerschmitt Bf 209. In Southampton, Supermarine and Rolls-Royce had progressed a long way in plotting Britain’s record attempt but the companies were intimidated by the development work now needed to convincingly beat the Bf 209 by a good margin—the new record set at an incredible 469mph.

They correctly calculated that squeezing the necessary power from the 27-litre Merlin was likely to require more time than was available and backed out.

How the Speed Spitfire’s Racing Merlin led to the development of the Merlin III powered by 100-octane fuel, the engine fitted to Spitfires and Hurricanes during the Battle of Britain
Speed Spitfire

Racing Merlin engine

However, independent of the efforts at Derby, RAE engineers had learned a great deal about increasing the power of the Merlin and had boosted it to nearly 1800bhp, ironically using special racing sparkplugs developed from those taken from a German Auto Union racing car as no others would stand the temperature.

The Speed Spitfire was test flown and, on May 30, Muir, the head of Farnborough‘s engine department, reported to Jennings, the aerodynamics chief, that 408mph had been achieved at ground level. Muir reported that this was very close to the estimate made by Jennings on Mar. 6.

Further tests were carried out to determine the influence of the special racing Merlin engine on the speed of the aircraft and, on Jun. 7, tests confirmed that only by increasing supercharger boost from +6.25 to +23.5lb/sq inch could the speed of the aircraft be increased from 312 to 400mph at 3000ft altitude.

Spitfire Mk Vb print
This print is available in multiple sizes from AircraftProfilePrints.com – CLICK HERE TO GET YOURS. Supermarine Spitfire Mk Vb – W3257 E-FY – 1941

The version which was being developed that would have taken advantage of 100-octane fuel was the Merlin III, which would be fitted to most Spitfires and Hurricanes during the Battle of Britain. The close link between the work being carried out on the Speed Spitfire project (which was abandoned with K9834 being later modified for other uses) and the real fighters about to be used in battle was highlighted when, on May 16, 1939, the following letter was sent to Farren at the Air Ministry. Farren had wanted to know what fuel octane numbers were needed to fully open the Merlin throttles well below their rated height of 15,500ft -which would have the effect of considerably increasing engine power at low altitude.

Fully opening the Merlin throttles

“Dear Farren,
At 5000ft the octane number of the fuel would be 120 (on the BMEP scale), which may be obtained by using 30% D.T.D.224 + 60% benzol + 20% methanol + 4ccs tetraethyl lead. This is what is called the racing fuel, and I should imagine that reasonable quantities of this are available. An alternative fuel is the 100 octane fuel supplies as such + 6ccs tetraethyl lead. This, also, I would imagine would be reasonably available.
Question 2. What octane fuel is required to enable the Merlin II engine in Hurricane or Spitfire aircraft to be opened up to, say 1400bhp at 5000ft?
Answer. At 5000ft the octane number required would be 106 on the BMEP scale. Alternative fuels are: –
D.T.D. 230 + water. This, however means operating the bi-fuel system…
Question 3. What will the aircraft speeds and the engine rpm be compared with the standard aircraft at various heights from 5000ft upwards?
Answer. These are tabulated below.”

How the Speed Spitfire’s Racing Merlin led to the development of the Merlin III powered by 100-octane fuel, the engine fitted to Spitfires and Hurricanes during the Battle of Britain

The Secret Horsepower Race Western Front Fighter Engine Development is published by Mortons Books and is available to order here.

Photo credit: Unknown

Spitfire
This model is available to order from AirModels – CLICK HERE TO GET YOURS.

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Dario Leone

Dario Leone

Dario Leone is an aviation, defense and military writer. He is the Founder and Editor of “The Aviation Geek Club” one of the world’s most read military aviation blogs. His writing has appeared in The National Interest and other news media. He has reported from Europe and flown Super Puma and Cougar helicopters with the Swiss Air Force.
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