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Napier Lion II at Canada Aviation Museum

The Napier Lion was a 12-cylinder broad arrow configuration aircraft engine built by Napier & Son starting in 1917, and ending in the 1930s. A number of advanced features made it the most powerful engine of its day, and kept it in production long after contemporary designs had stopped production. It is particularly well known for its use on a number of racing designs, in aircraft, boats, and cars.

Design and development

Early in the First World War Napier were contracted to build aero engines to designs from other companies: initially a Royal Aircraft Factory model and then Sunbeams. Both proved to be rather unreliable, and in 1916 Napier decided to design their own instead. Reasoning that the key design criteria were high power, light weight, and low frontal area, the engine was laid out with its 12 cylinders in what they called a "broad arrow"—three banks of four cylinders sharing a common crankcase. This suggested the design's first name, the Triple-Four. Today these designs, of which there were only a few, are sometimes referred to as a W-block, although that designation applies more correctly to an engine in which a common crankcase is shared by not merely three but in fact four rows of cylinders (since a "W" is made of four lines or bars). The engine was also advanced in form, the heads using four valves per cylinder with twin overhead camshafts on each bank of cylinders and a single block being milled from aluminium instead of the more common separate-cylinder steel construction used on almost all other designs.

Cutaway view showing the double overhead camshaft arrangement


Under A. J. Rowledge, the design of the newly renamed Lion was completed in 1917, and the first hand-built prototypes ran later that year. It was fitted to a de Havilland built DH.9 in early 1918, proving to have many cooling problems. In addition the milled block turned out to be difficult to build with any accuracy and they reverted to separate cylinders, although they remained aluminium. Both of these problems were worked out by the middle of the year and the engine entered production in June 1918. The first Lion I versions delivered 450 hp (335 kW) from their 24 litres. It then took the crown of the most powerful engine from the Liberty L-12, the excellent US wartime design of 400 hp (300 kW).

As the most powerful engine available (particularly after a turbocharger became an option in 1922), the Lion went on to be a huge commercial success. Through the years between the wars the Lion was ubiquitous, and Napier manufactured little else. They stopped making cars in 1925, and little thought was given to replacing their world-famous product. Between the wars it powered over 160 different types of aircraft.

The Napier Lion installed in the Napier-Railton car.

In highly-tuned racing versions the engine could reach 1,300 hp (970 kW), and it was used to break many world records: height, air speed, and distance in aircraft, boats, delivering 1,375 hp (1,025 kW) in a highly tuned Lion for a water speed record of 100 mph (160 km/h) in 1933. In land speed records, Lion engines powered many of Sir Malcolm Campbell's record breakers including a record of over 250 mph (400 km/h) in 1932 and John Cobb's 394 mph (634 km/h) Railton Mobil Special in 1947—a record that came well after the Lion had passed its prime and stood until the 1960s. The record had been held by British drivers for 32 years. Lions powered successful entrants in the most prestigious event in air racing, the Schneider Cup, in 1922 and 1927, but were then dropped by Supermarine in favour of a new engine from Rolls-Royce, the Rolls-Royce R which had been especially designed for racing.

During the 1930s a new generation of much larger and more powerful engines started to appear, and the Lion was clearly past its prime. Gradually, they fell further and further behind. By the time the Bristol Hercules and the Rolls-Royce Merlin arrived in the late 1930s, the Lion was too small and old-fashioned.

A marine version of the Lion, unsurprisingly called the Sea Lion, was used to power high speed air-sea rescue launches operated by the RAF.

Another adaptation for the Lion aero engine was propeller-driven motor sleighs, which were used for high-speed transport and SAR duties on sea ice by the Finnish Air Force and Navy.

Turning away from the broad arrow layout, Napier started on the design of two new engines using the even more compact H engine layout. The 16-cylinder Rapier produced 400 hp (300 kW), the 24-cylinder Dagger delivered just under 1,000 hp (750 kW). However these were both smaller than contemporary designs from other companies, and Napier had to start afresh with a new sleeve valve design, which eventually matured into the superb Sabre.


Lion models[1][2]
Model Date Works No. Power Notes Notable uses
I 1918 450 bhp (340 kW) at 1,950 rpm geared, also related IA and 1AY
II 1919 E64 450 bhp (340 kW) at 2,000 rpm
IIII experimental geared Gloster Gorcock
V 470 bhp (350 kW) at 2,000 rpm
500 bhp (370 kW) at 2,250 rpm
VA had increased CR to 5.8 Mainstay engine of the RAF in the late 1920s, replaced by Lion XI
VS E79 Turbocharged, intercooled
VIS 1927 Turbocharged Gloster Guan
VII 1925 700 bhp (520 kW) (racing) Gloster III (Schneider Trophy entrant)
Supermarine S.4
VIIA 1927 E86 900 bhp (670 kW) (racing) Golden Arrow
Blue Bird (1927)
Miss England I
Supermarine S.5
Gloster IV
VIIB 1927 875 bhp (652 kW) (racing) geared Supermarine S.5
Gloster IV
VIID 1929 E91 1,350 bhp (1,010 kW) at 3,600 rpm (racing) Supercharged, about 6-8 built Blue Bird (1931)
Fred H Stewarts Enterprise
Betty Carstairss Estelle V powerboat
Miss Britain III
Gloster VI (Schneider Trophy entrant)
Railton Special (John Cobb's land speed record car)
VIII 1927 direct drive Gloster Gorcock
XIA 1928 580 bhp (430 kW) at 2,585 rpm, 6:1 CR RAF production model Napier-Railton
Lioness E71 Inverted layout, for better visibility. At least some were built turbocharged, for racing.
Sea Lion 1933 500 and 600 bhp (370 and 450 kW) Marine version of Lion XI British Power Boat Company Type Two 63 ft HSL



Napier Lion at Brooklands Motor Museum

Other applications

  • British Power Boat Company Type Two 63 ft HSL
  • British Power Boat Company 60 ft 4 in[3]

Engines on display

Preserved Napier Lion engines are on static display at the following museums:

Specifications (Lion II)

Data from Lumsden[5]

General characteristics

  • Type: 12-cylinder water-cooled W-block (3 banks of 4 cylinders) aircraft piston engine
  • Bore: 5.5 in (139.7 mm)
  • Stroke: 5.125 in (130.17 mm)
  • Displacement: 1,461.6 in³ (23.9 L)
  • Length: 57.5 in (1460 mm)
  • Width: 42.0 in (1067 mm)
  • Height: 43.5 in (1105 mm)
  • Dry weight: 960 lb (435 kg)


  • Valvetrain: Two intake and two exhaust valves per cylinder actuated via double overhead camshafts per cylinder block.
  • Cooling system: Water-cooled


  • Power output: 480 hp (358 kW) at 2,200 rpm at 5,000 ft
  • Specific power: 0.32 hp/in³ (15.0 kW/L)
  • Compression ratio: 5.8:1
  • Power-to-weight ratio: 0.5 hp/lb (0.82 kW/kg)

See also

  • Napier-Bentley
  • Hispano-Suiza 18R




  • Lumsden, Alec. British Piston Engines and their Aircraft. Marlborough, Wiltshire: Airlife Publishing, 2003. ISBN 1-85310-294-6.
  • Vessey, Alan. Napier Powered. Stroud: Tempus (Images of England series), 1997. ISBN 0-7524-0766-X.

External links

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