List of Rolls-Royce Merlin variants
Introduction Rolls-Royce Merlin 23This is a list of '''Rolls-Royce Merlin''' variants. Engines of a similar power output were typically assigned different model numbers based on supercharger or propeller gear ratios, differences in cooling system or carburettors, engine block construction, starting system, or arrangement of engine controls. All Merlin engines were "right hand tractor", ''i.e.'' the propeller rotated clockwise viewed from behind, unless otherwise noted.
Variant table VariantTake-off PowerCombat PowerApplicationNotesPV-12  at  equivalentThe initial design using an evaporative cooling system. Two built, passed bench Type Testing in July 1934.First flown 21 February 1935.Merlin B at  equivalentTwo built, ethylene glycol liquid cooling system introduced. "Ramp" cylinder heads (inlet valves were at a 45-degree angle to the cylinder). Passed Type Testing February 1935.Merlin C at  equivalentDevelopment of Merlin B; Crankcase and cylinder blocks became three separate castings with bolt-on cylinder heads. First flight in Hawker Horsley 21 December 1935.Merlin E constant output maximum ratingSupermarine Spitfire prototypeSimilar to '''C''' with minor design changes. Passed 50-hour civil test in December 1935.Failed military 100-hour test in March 1936.Merlin F('''Merlin I''') Similar to '''C''' and '''E'''. First flight in Horsley 16 July 1936.This became the first production engine; and was designated as the Merlin I. The Merlin continued with the "ramp" head, but this was not a success and only 172 were made. The Fairey Battle was the first production aircraft to be powered by the Merlin I and first flew on 10 March 1936.Merlin G('''Merlin II''') Replaced "ramp" cylinder heads with parallel pattern heads (valves parallel to the cylinder) scaled up from the Kestrel engine. 400 Hour flight endurance tests carried out at RAE July 1937; Acceptance test 22 September 1937.It was first widely delivered as the 1,030-horsepower (770 kW) Merlin II in 1938, and production was quickly ramped up.Merlin I at 2,850 rpm Fairey Battle Mk.IFirst production Merlin; 172 built. Merlin I through III used 100% glycol coolant.Merlin II ''(RM 1S)'' at 3,000 rpm at 3,000 rpm at  with +  boostSpitfire Mk.I, Defiant Mk.I, Hurricane Mk.I, Sea Hurricane Mk.I, Battle Mk.IUsed 100% glycol coolant. First production Merlin II delivered 10 August 1937.Merlin III ''(RM 1S)'' at 3,000 rpm at 3,000 rpm at  with 100 octane fuel and + boost (5-minute limit).Spitfire Mk.I, Defiant Mk.I, Hurricane Mk.I, Sea Hurricane Mk.I, Battle Mk.IVariant of Merlin II with universal propeller shaft, able to be fitted with de Havilland or Rotol propellers. From late 1939, using 100 octane fuel and +  boost, the Merlin III developed  at 3,000 rpm at . Later developing  at 3,000 rpm, + boost at  for the Sea Hurricane. Using 87 Octane fuel the power ratings were the same as the Merlin II. First production Merlin III delivered 1 July 1938.Merlin VIII at 3,000 rpm at 3,000 rpm, + boost, sea level with 100 octaneFulmar Mk.IMerlin X ''(RM 1SM)'' at 3,000 rpm at 3,000 rpm, + boost, sea levelHalifax Mk.I, Wellington Mk.II, Whitley Mk.V and Whitley Mk.VII  at 3,000 rpm at  with maximum boost pressure +; this was the first production Merlin to use a two-speed supercharger; Used in Halifax Mk.I, Wellington Mk.II, and Whitley Mk.V bombers. First production Merlin X delivered 5 December 1938.Merlin XII ''(RM 3S)'' at 3,000 rpm at 3,000 rpm, + boost, Spitfire Mk.IICoffman cartridge starter. First version of Merlin to use 30/70% glycol/water coolant with reinforced construction, able to use constant boost pressure of up to + using 100 octane fuel. First production Merlin XII, 2 September 1939.Merlin XX ''(RM 3SM)'' at 3,000 rpm at 3,000 rpm, + boost, with 100 Octane fuel at .Beaufighter Mk.II, Defiant Mk.II, Halifax Mk.II, Halifax Mk.V, Hurricane Mk.II and Hurricane Mk.IV, Lancaster Mk.I, Lancaster Mk.III, Spitfire Mk.IIIHad a two speed supercharger giving boost pressures of up to + . First production Merlin XX, 4 July 1940.Merlin 21 at 3,000 rpm at 3,000 rpm, + boost, de Havilland Mosquito Mk.I, Mk.II, Mk.III, Mk.IV and Mk.VIMerlin XX with direction of coolant flow reversed for Mosquito wing radiator installationMerlin 22 at 3,000 rpm at 3,000 rpm, + boost, 11,000 ftLancaster Mk.I, York Mk.IMerlin 23 at 3,000 rpm at 3,000 rpm, + boost, de Havilland Mosquito Mk.I, Mk.II, Mk.IV, Mk.VI, Mk.XII and Mk.XIIIMerlin 22 with direction of coolant flow reversed for Mosquito wing radiator installationMerlin 24 at 3,000 rpm at 3,000 rpm, + boost, Lancaster Mk.I, Lancaster Mk.VII, York Mk.I and Halifax Mk.IIMerlin 25 at 3,000 rpm at 3,000 rpm, + boost, de Havilland Mosquito  Mk.VI and Mk.XIXMerlin 24 with direction of coolant flow reversed for Mosquito wing radiator installationMerlin 27 at 3,000 rpm at 3,000 rpm, + boost, Hurricane Mk.IVMerlin 28 at 3,000 rpm at 3,000 rpm, + boost, Lancaster Mk.III, Kittyhawk II (Curtiss P-40F)Built by Packard as the V-1650-1Merlin 29 at 3,000 rpm at 3,000 rpm, + boost, Hurricane Mk.XII (Canadian-built), Kittyhawk II (Curtiss P-40F), with splined propeller shaftMerlin 30 at 3,000 rpm at 3,000 rpm, + boost, Barracuda Mk.I and Fulmar Mk.IIMerlin 31 at 3,000 rpm at 3,000 rpm, + boost, Mosquito Mk.XX (Canadian), Mosquito Mk.40 (Australia), Kittihawk II (P-40F and L)Built in the United States as the Packard V-1650-1Merlin 32 ''(RM 5M)'' at 3,000 rpm at 3,000 rpm, + boost, Barracuda Mk.II, Seafire Mk.II, Hurricane Mk.V, Spitfire PR Mk. XIII A "low altitude" version of Merlin with cropped supercharger impellers for increased power at lower altitudes, as per the Merlin XXX; fitted with a Coffman engine starter; used mainly in Fleet Air Arm aircraft. First production Merlin 32 delivered 17 June 1942.Merlin 33 at 3,000 rpm at 3,000 rpm, + boost, Mosquito XX (Canadian), Mosquito 40 (Australia)Packard-built Merlin 23Merlin 35 at 3,000 rpm,  + boost at 3,000 rpm at Boulton Paul Balliol, Avro AthenaMerlin 38 at 3,000 rpm at 3,000 rpm, + boost, Lancaster I and IIIPackard-built Merlin 24Merlin 45 ''(RM 5S)'' at 3,000 rpm at 3,000 rpm, + boost, Spitfire Mk.V, Spitfire PR Mk.IG (later redesignated Spitfire PR.VII), Spitfire PR Mk.IV, Seafire Mk.IB, Seafire Mk.IICA variant of the Merlin XX fitted with  single-stage, single-speed supercharger for low altitude Spitfire use. First production Merlin 45 delivered 13 January 1941.First of specialised engines for Spitfire Mk V variants and early Seafires.Merlin 45M at 3,000 rpm at 3,000 rpm, + boost, Spitfire LF Mk.VVersion of Merlin 45 with "cropped" (smaller diameter) supercharger impeller allowing greater boost at low altitudes.Merlin 46 at 3,000 rpm at 3,000 rpm, + boost, Spitfire Mk.V, Spitfire PR Mk.IV, Spitfire Mk.VII, Seafire Mk.IB and Seafire Mk.IICMerlin 47 ''(RM 6S)'' at 3,000 rpm at 3,000 rpm, + boost, Spitfire HF Mk.VI high-altitude intercepterAdapted with a Marshall compressor (often called a "blower") to pressurise the cockpit. First production Merlin 47 delivered 2 December 1941.Merlin 50 ''(RM 5S)'' at 3,000 rpm at 3,000 rpm, + boost, Spitfire Mk.VLow-altitude version with supercharger impeller "cropped" to  in diameter. '''Merlin 50''' series was first to use the Bendix Stromberg "negative-g" carburettor.Merlin 50M ''(RM 5S)'' at 3,000 rpm at 3,000 rpm, + boost, Spitfire LF Mk.VMerlin 55 at 3,000 rpm at 3,000 rpm, +, Spitfire Mk.V and Seafire Mk.IIIMerlin 55M at 3,000 rpm at 3,000 rpm, + boost, Spitfire LF Mk.V, Seafire Mk.IIIVariant with "cropped" supercharger impellorMerlin 60 at 3,000 rpm at 2,850 rpm, + boost, Wellington Mk.VIFirst variant fitted with two-stage, two-speed supercharger; rated for high altitude.Merlin 61 ''(RM 8SM)'' at 3,000 rpm at 3,000 rpm, + boost, Spitfire Mk.IX and Spitfire PR Mk.XIProduction variant of Merlin 60 fitted with a new two-speed two-stage supercharger providing increased power at medium to high altitudes. First British production variant to incorporate two-piece cylinder blocks designed by Rolls-Royce for the Packard Merlin. First production Merlin 61 delivered 2 March 1942.Merlin 62 at 3,000 rpm at 2,850 rpm, + boost, Wellington Mk.VIMerlin 63 at 3,000 rpm at 3,000 rpm, + boost, Spitfire Mk.VIII,  IX, PR.XIReplaced Merlin 61Merlin 63A  at 3,000 rpm  at 3,000 rpm, + boost, Spitfire PR Mk.XIMerlin 64 at 3,000 rpm at 3,000 rpm, + boost, Spitfire Mk.VIICabin pressure blowerMerlin 66 ''(RM 10SM)'' at 3,000 rpm at 3,000 rpm, + boost, Spitfire LF Mk.VIII LF Mk.IXFitted with supercharger rated for low altitude; Bendix-Stromberg anti-g carburettor Merlin 67 at 3,000 rpm at 3,000 rpm, + boost, Merlin 68 at 3,000 rpm at 3,000 rpm, + boost, Mustang III (North American P-51B and C)Packard V-1650-3Merlin 69 at 3,000 rpm at 3,000 rpm, + boost, Mustang III and Mustang IV (North American P-51C,D,F and K)Packard V-1650-7Merlin 70 at 3,000 rpm at 3,000 rpm, + boost, Spitfire HF Mk.VIII HF.IX, PR.XIBendix Stromberg anti-g carburettor Merlin 71 at 3,000 rpm at 3,000 rpm, +, Spitfire HF Mk.VIICabin pressure blower, Bendix Stromberg anti-g carburettor  Merlin 72 at 3,000 rpm at 3,000 rpm, +, de Havilland Mosquito PR Mk.IX, B Mk.IX, Mk.XVI and Mk.30.Welkin Mk.IMerlin 73 at 3,000 rpm at 3,000 rpm, +, de Havilland Mosquito Mk.XVI, Welkin Mk.ISame as Merlin 72 with a cabin pressure blowerMerlin 76 ''(RM 16SM)'' at 3,000 rpm at 3,000 rpm, +, de Havilland Mosquito PR Mk.XVI, Mk.30, Welkin Mk.I  at ; Fitted with a two-speed, two-stage supercharger and a Bendix Stromberg anti-g carburettor. Dedicated "high altitude" version used in the Westland Welkin high-altitude fighter, and some later Spitfire and de Havilland Mosquito variants.Merlin 77 ''(RM 16SM)'' at 3,000 rpm at 3,000 rpm, +, de Havilland Mosquito Mk.XVI,  Welkin Mk.I, Spitfire PR  Mk.X Same as Merlin 76 with a pressurising blower Merlin 85 at 3,000 rpm at 3,000 rpm, +, Lancaster Mk.VI and Lincoln Mk.IMerlin 130/131 at 3,000 rpmde Havilland Hornet F Mk.1, PR Mk.2, F Mk.3, FR Mk.4.Redesigned "slimline" versions for the de Havilland Hornet. Engine modified to decrease frontal area to a minimum and was the first Merlin series to use down-draught induction systems. Coolant pump moved from the bottom of the engine to the starboard side. Two-speed, two-stage supercharger and S.U. injection carburettor giving a maximum boost of +. On the Hornet the Merlin 130 was fitted in the starboard nacelle: the Merlin 131, fitted in the port nacelle, was converted to a "reverse" or left-hand tractor engine using an additional idler gear in the reduction gear casing.Merlin 133/134 at 3,000 rpmde Havilland Sea Hornet F Mk.20, NF Mk.21 and PR Mk.22Derated 130/131 with maximum boost lowered to +.Merlin 224 at 3,000 rpm at 3,000 rpm, +, Avro Lancaster Mk.I, Mk.III and Mk.XPackard-built Merlin 24Merlin 225 at 3,000 rpm at 3,000 rpm, +, de Havilland Mosquito Mk.25 and Mk.26Packard-built Merlin 25Merlin 266 ''(RM 10SM)'' at 3,000 rpm at 3,000 rpm, +, tSpitfire LF Mk.XVIThe prefix "2" indicates engines built by Packard, otherwise as Merlin 66, optimised for low-altitude operation.Merlin 620 continuous cruising using 2,650 rpm at + Avro Tudor, Avro York, and Canadair North StarCapable of emergency rating of  at 3,000 rpm using + ; civilian engine developed from Merlin 102; two-stage supercharger optimised for medium altitudes, and used an S.U. injection carburettor. Universal Power Plant (UPP) installation similar to that used on Avro Lincoln. The Merlin 620-621 series was designed to operate in the severe climatic conditions encountered on Canadian and long-range North Atlantic air routes.Preserved Merlin XX at the Royal Air Force Museum LondonAudio of RR Merlin 66/266 starting.Parallel valve Merlin cylinder head
References ===Notes======Bibliography===*Air Ministry. ''Pilot's Notes for Spitfire Mark F.VII - Merlin 64 or 71 engine; Mark F.VIII-Merlin 63,66 or 70 engine. Air Publication 1565G & H -P.N.'' London, UK: Air Ministry, December 1943. * Bridgman, L. ''Jane's fighting aircraft of World War II.'' London: Crescent, 1998. * Harvey-Bailey, A. ''The Merlin in Perspective - the combat years.'' Derby, England: Rolls-Royce Heritage Trust, 1983. * Lumsden, Alec. ''British Piston Engines and their Aircraft''. Marlborough, Wiltshire: Airlife Publishing, 2003..* Price, Alfred. ''The Spitfire Story''. London: Jane's Publishing Company Ltd., 1982..* Robertson, Bruce. ''Spitfire: The Story of a Famous Fighter''. Hemel Hempstead, Hertfordshire, UK: Model & Allied Publications Ltd., 1960.Third revised edition 1973..
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