July 1942, as per Jorge.....
It was now the turn of Fw 190 pilots to demand explanations from clueless intelligence officers, they had encountered Spitfires that out-performed them.... Rushed through development and into production, the Spitfire IX first flew on 20 September, 1941 and first went into action with 64 Squadron, based at RAF Hornchurch, on 28 July 1942...
As it looked just like a Spitfire V, the new high performance Spitfire IX was a surprise to the German pilots who encountered it... The Spitfire IX was able to achieve superiority over both the Fw 190 and the Bf 109G...
Originally designed for high altitude bombers, the Merlin 61 engine was the reason for the success of the Spitfire IX. Its two-stage supercharger essentially consisted of two supercharger blowers in series, one feeding the other, separated by an intercooler that reduced the temperature of the air and increased its density.... The Merlin 61 required a four-blade propeller to absorb the extra power and a second enlarged underwing radiator to cool the intercooler. Ideally, a larger-diameter three-blade propeller would have been more efficient, but limited ground clearance.... prevented this.
At sea-level, the Merlin 61 produced 1560 hp compared to the Merlin 45 powering the Spitfire V. But the real advantage was apparent at 30,000 feet, when the Merlin 61 was capable of 1020 hp as opposed to 750 hp... More power at high altitude delivered superior performance to that of the Bf 109G and FW 190A.
Rolls-Royce were able to incorporate the Merlin 61's additional blower in a slightly enlarged cowling. It could be installed in a Spitfire with minimum redesign. The Spitfire IX's structure was reinforced, weighing 600lbs more than the Spitfire V. Most Spitfire IXs were armed with two 20mm cannon and four .303 machine guns... US made .50-caliber Brownings were also used in place of pairs of .303 machine guns on some Spitfire IXs and later models...
..the Bf109 could still accelerate faster in a dive and maintain a steeper angle... but its stiff controls and the need to apply physical force to pull out of a high=speed dive meant that many less thoroughly-trained pilots were unwilling to use it.
