Production of the LF Mk IXs, frequently referred to as the Spitfire IXB, initially ran in parallel with the Merlin 63 powered Marks. This version first became operational in March 1943 with the Biggin Hill Wing, comprised at the time of 611 and 341 (Free French) Squadrons. This type was by far the most produced of the Spitfire IX variants, with over 4,000 built. The maximum power of the Merlin 66 was 1,720 hp (1,280 kW) at 5,750 ft (1,283 kW at 1,752 m) and the maximum speed of the Spitfire LF IX was 404 mph (650 km/h) at 21,000 feet (6,400 m). The Merlin 66 introduced a new Bendix-Stromberg injection carburettor, which replaced the earlier S.U float carburettor.
The H.F IX was powered by the specialised high altitude Merlin 70 and entered service in the Spring of 1944. Serial listings show that the HF Mk IX was produced in relatively limited numbers when they were required, with priority being given to versions rated for low and medium altitudes[nb 6] Maximum power of the Merlin 70 was 1,710 hp (1,280 kW) at 11,000 ft (1,275 kW at 3,353 m): maximum speed of the Spitfire HF.IX was 405 mph (652 km/h) at 25,400 feet (7,700 m) at an all-up weight of 7,320 lb (3,320 kg).
The cockpit of a Spitfire IX showing the instrument panel and the Mk II Gyro gunsight.
Also introduced in early 1944 was a new Mark II Gyro gunsight. This gunsight calculated the correct angle of deflection to use when leading the target. Its introduction doubled the effectiveness of RAF gunnery and was a major factor in Allied air superiority.
The capacity of the main fuel tanks was 48 gal for the upper tank and 37 gal for the lower, for a total internal capacity of 85 gal. Jettisonable "slipper tanks" of 30, 45 or 90 gal could be carried under the centre-section. As an alternative a cylindrical 50 gal drop tank, adapted from those carried by long range Hawker Typhoons, could be carried on the fuselage bomb rack used on most Mk IXs of the Second Tactical Air Force. To further increase the combat radius some late production Mk IXs were fitted with additional internal self-sealing fuel tanks in the rear fuselage: the upper tank carried 41 gal and the lower 34 gal. When both were full this enabled a ferry range of over 1,200 miles (1,900 km), although they made the aircraft unstable in flight and only straight flight and gentle manoeuvres at low altitudes were recommended by the pilot's manual. The pilot was also warned to avoid instrument flying whenever possible.