altitude" version of Merlin with cropped supercharger impellers for increased power at lower altitudes and a maximum boost pressure of +18 psi; fitted with Coffman engine starter; used mainly in Fleet Air Arm aircraft, mainly the Fairey Barracuda Mk.II torpedo bomber and Supermarine Seafire F. Mk.IIc fighters. Also Hurricane Mk.V and Spitfire P.R. Mk.XIII. First production Merlin 32, 17 June 1942. Merlin 45 (RM 5S) at 3,000 rpm at ; used in Spitfire Mk.V, PR.Mk.IV and PR.Mk.VII, Seafire Ib and IIc. Maximum boost pressure of +16 psi. First production Merlin 45, 13 January 1941. Merlin 47 (RM 6S) at 3,000 rpm at ; high-altitude version used in Spitfire H.F.Mk.VI. Adapted with a Marshall compressor (often called a "blower") to pressurise the cockpit. First production Merlin 47, 2 December 1941. Merlin 50.M (RM 5S) 1,585 hp (1,182 kW) at 3,000 rpm at ; low-altitude version with supercharger impeller "cropped" to in diameter. Permitted boost was +18 psi (125 kPa gauge; or an absolute pressure of 225 kPa or 2.2 atm) instead of +16 psi (110 kPa gauge; or an absolute pressure of 210 kPa or 2.08 atm) on a normal Merlin 50 engine. Merlin 50 series was first to use the Bendix-Stromberg "negative-g" carburettor. Merlin 61 (RM 8SM) at 3,000 rpm at , at 3,000 rpm at ; fitted with a new two-speed two-stage supercharger providing increased power at medium to high altitudes; +15 psi boost; used in Spitfire F Mk.IX, and P.R Mk.XI. First British production variant to incorporate two-piece cylinder blocks designed by Rolls-Royce for the Packard Merlin. Reduction gear ratio .42:1, with gears for pressurisation pump. First production Merlin 61, 2 March 1942. Merlin 63 & 63A at 3,000 rpm at , at 3,000 rpm at ; strengthened two-speed two-stage development of Merlin 61; +18 psi boost; Reduction gear ratio .477:1; Merlin 63A did not have extra gears for pressurisation and incorporated a strengthened supercharger drive quill shaft. Used in Spitfire F Mk.VIII and F. Mk. IX. Merlin 66 (RM 10SM) at using +18 psi boost (124 kPa gauge; or an absolute pressure of 225 kPa or 2.2 atm); low-altitude version of Merlin 63A. Fitted with a Bendix-Stromberg anti-g carburettor; intercooler used a separate header tank. Used in Spitfire L.F. Mk.VIII and L.F. Mk.IX. Merlin 76/77 (RM 16SM) at ; Fitted with a two-speed, two-stage supercharger and a Bendix-Stromberg carburettor. Dedicated "high altitude" version used in the Westland