The Mark IX also added 70mph to the Spitfire’s top speed and 10,000ft to its fighting altitude. ‘The performance of the Spitfire IX is outstandingly better than the Spitfire V,’ the official test report read. ‘Spitfire is considerably faster and its climb is exceptionally good.’5 The only structural alteration from the Mark V had been to elongate the body by nine inches to accommodate the Merlin 61. But what a difference those nine inches made. With its two-stage supercharger, in which air was doubly compressed before entering the carburettor, the Mark IX was given an astonishing 20 per cent
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