With 100 octane fuel the supercharger of the Merlin III engine could be "boosted" to +12 lbs/sq.in., producing 1,310 hp (977 kW) at 3,000 rpm at 9,000 feet (2,743 m) with a time limit of five minutes. This increased power substantially improved the rate of climb, especially at low to medium altitudes, and increased the top speed by 25–34 mph up to 10,000 feet. During the Battle of France and over Dunkirk RAF Hurricanes and Spitfires were able to use the emergency boost. "In the first half of 1940 the RAF transferred all Hurricane and Spitfire squadrons to 100 octane fuel."