At around the time of the fall of France in May 1940, RAF pilots flying Merlin-engined fighters (i.e. the Hurricane and Spitfire) were at once gratified and mystified that the power rating of their aircraft enjoyed a (literally) sudden boost in power. The standard aviation fuel of the Luftwaffe and the RAF at the outbreak of the war was 87 (or thereabouts) octane. Jeffrey Quill, the notable Spitfire test pilot, wrote to the authors on 15 August 1988, in response to an enquiry about the introduction of 100 octane fuel:
I think it virtually certain that the Hurricane operated in France on 87 octane because I think it was only shortly before the Battle of Britain that we changed over to 100 octane. It had the effect of increasing the combat rating of the Merlin from 3,000 rpm at 6½ lb boost or 9 lb boost to 3,000 rpm at 12 lb boost. This, of course, had a significant effect upon the rate of climb, particularly as the constant speed propellers (also introduced just before the Battle) ensured that 3,000 rpm was obtainable from the ground upwards whereas previously it had been restricted by the two-pitch propellers. It also had an effect upon the maximum speed but this was not so significant as the effect upon rate of climb.
This increase of power in the British front-line fighters in the brief period between the Battle of France and the Battle of Britain came as an unpleasant surprise to Luftwaffe fighter pilots. It was not until late August that a force-landed Spitfire was analysed in detail, and the fuel was discovered to be ‘green’ and of much higher octane than the standard ‘blue’ 87 octane.
This ‘rich mixture response’ 100 octane fuel had been in regular use by the US Army Air Corps since 1938, on a top-secret basis. Under a cloak of security, the British Air Ministry began negotiations with the Anglo-American Oil Company (i.e. Esso) for the supply of this fuel, which became known in the trade as BAM 100, and a long-term contract was concluded between the Air Ministry and International Aviation Associates.
Unfortunately, with the outbreak of war in September 1939, the US Congress invoked the Neutrality Act, prohibiting the supply of this life-saving fuel. Several anxious months passed before a compromise was reached by the Roosevelt Administration and the British Government whereby the supply of this fuel could be renewed on a ‘dollar on the barrel-head’ basis.