It's my understanding that a conversion was needed to the engine to enable it to run on 100 octane. They even painted little '100' s on the engine cowling so that 87 wasn't put in by mistake. By May 1940 all Hurricane, Spitfire and Defiant Sqaudrons had been converted.
Here's what Jeffrey Quill said about 100 octane.
"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 3000 rpm at 6 1/2 lb boost (Merlin III) or 9 lb boost (Merlin XII) 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 was 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."
Have no idea what this means for CoD though..
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