Rolls-Royce Merlin - Variants

Variants

This is a list of representative Merlin variants, describing some of the mechanical changes made during development of the Merlin. Engines of the same power output were typically assigned different model numbers based on supercharger or propeller gear ratios, differences in cooling system or carburettors, engine block construction, or arrangement of engine controls. Power ratings quoted are usually maximum "military" power. All but the Merlin 131 and 134 engines were "right-hand tractor", i.e. the propeller rotated clockwise when viewed from the rear. In addition to the mark numbers, Merlin engines were allocated experimental numbers by the Ministry of Supply (MoS) - e.g.: RM 8SM for the Merlin 61 and some variants - while under development; these numbers are noted where possible.

Data from Bridgman (Jane's) unless otherwise noted:

• Merlin II (RM 1S)

1,030 hp (775 kW) at 3,000 rpm at 5,500 ft (1,676 m) using + 6 psi boost (41 kPa gauge; or an absolute pressure of 144 kPa or 1.41 atm); used 100% glycol coolant. First production Merlin II delivered 10 August 1937. Merlin II used in the Boulton Paul Defiant, Hawker Hurricane Mk.I, Supermarine Spitfire Mk.I fighters, and Fairey Battle light bomber.

• Merlin III (RM 1S)

Merlin III fitted with "universal" propeller shaft able to mount either de Havilland or Rotol propellers. From late 1939, using 100 octane fuel and +12 psi boost (83 kPa gauge; or an absolute pressure of 184 kPa or 1.82 atm), the Merlin III developed 1,310 hp (977 kW) at 3,000 rpm at 9,000 ft (2,700 m); using 87 octane fuel the power ratings were the same as the Merlin II. Used in the Defiant, Hurricane Mk.I, Spitfire Mk.I fighters, and Battle light bomber. First production Merlin III delivered 1 July 1938.

• Merlin X (RM 1SM)

1,130 hp (840 kW) at 3,000 rpm at 5,250 ft (1,600 m); maximum boost pressure +10 psi; 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, 5 December 1938.

• Merlin XII (RM 3S)

1,150 hp (860 kW); fitted with Coffman engine starter; first version to use 70/30% water/glycol coolant rather than 100% glycol. Reinforced construction, able to use constant boost pressure of up to +12 psi using 100 octane fuel; Used in Spitfire Mk.II. First production Merlin XII, 2 September 1939.

• Merlin XX (RM 3SM)

1,480 hp (1,105 kW) at 3,000 rpm at 6,000 ft (1,829 m); two-speed supercharger; boost pressure of up to +14 psi; Used in Hurricane Mk.II, Beaufighter Mk.II, s, Halifax Mk.II and Lancaster Mk.I bombers, and in the Spitfire Mk.III prototypes (N3297 & W3237). First production Merlin XX, 4 July 1940.

• Merlin 32 (RM 5M)

1,645 hp (1,230 kW) at 3,000 rpm at 2,500 ft (762 m); first "low altitude" version of Merlin with cropped supercharger impellers for increased power at lower altitudes; fitted with Coffman engine starter; used mainly in Fleet Air Arm aircraft, mainly the Fairey Barracuda Mk.II torpedo bomber and Fairey Fulmar 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)

1,515 hp (1,130 kW) at 3,000 rpm at 11,000 ft (3,353 m); 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)

1,415 hp (1,055 kW) at 3,000 rpm at 14,000 ft (4,267 m); 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 3,800 ft (1,158 m); low-altitude version with supercharger impeller "cropped" to 9.5 in (241 mm) 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)

1,565 hp (1,170 kW) at 3,000 rpm at 12,250 ft (3,734 m)

1,390 hp (1,035 kW) at 3,000 rpm at 23,500 ft (7,163 m); fitted with a new two-speed two-stage supercharger providing increased power at medium to high altitudes; 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. First production Merlin 61, 2 March 1942.

• Merlin 66 (RM 10SM)

1,720 hp (1,283 kW) at 5,790 ft (1,765 m) using +18 psi boost (124 kPa gauge; or an absolute pressure of 225 kPa or 2.2 atm); low-altitude version of Merlin 61. Fitted with a Bendix-Stromberg anti-g carburettor; used in Spitfire L.F Mk.VIII and L.F Mk.IX.

• Merlin 76/77 (RM 16SM)

1,233 hp (920 kW) at 35,000 ft (10,668 m); Fitted with a two-speed, two-stage supercharger and a Bendix-Stromberg carburettor. Dedicated "high altitude" version used in the Westland Welkin high-altitude fighter and some later Spitfire and de Havilland Mosquito variants. The odd-numbered mark drove a blower for pressurising the cockpit.

• Merlin 130/131

2,060 hp (1,536 kW); 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. Maximum boost was 25 psi (170 kPa gauge; or an absolute pressure of 270 kPa or 2.7 atm). 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

2,030 hp (1,514 kW); derated 130/131 variants used in Sea Hornet F. Mk. 20, N.F. Mk. 21 and P.R. Mk. 22. Maximum boost was lowered to +18 psi gauge (230 kPa or 2.2 atm absolute).

• Merlin 266 (RM 10SM)

The prefix "2" indicates engines built by Packard, otherwise as Merlin 66, optimised for low-altitude operation. Fitted to the Spitfire Mk.XVI.

• Merlin 620

1,175 hp (876 kW) continuous cruising using 2,650 rpm at +9 psi boost (62 kPa gauge; or an absolute pressure of 165 kPa or 1.6 atm); capable of emergency rating of 1,795 hp (1,338 kW) at 3,000 rpm using +20 psi boost (138 kPa gauge; or an absolute pressure of 241 kPa or 2.4 atm) ; civilian engine developed from Merlin 102; two-stage supercharger optimised for medium altitudes, and used an S.U. injection carburettor. "Annular" radiator installation development of 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. Used in Avro Tudor, Avro York, and the Canadair North Star.