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The BRM H16 Formula One engine in its final, 64-valve incarnation.

An H engine (or H-block) is an engine configuration in which the cylinders are aligned so that if viewed from the front, they appear to be in a vertical or horizontal letter H.

An H engine can be viewed as two flat engines, one atop or beside the other. The "two engines" each have their own crankshaft, which are then geared together at one end for power-take-off. This leads to a worse power-to-weight ratio than simpler configurations with only one shaft. One obvious advantage of the H configuration is to allow the building of reasonably short engines with more than 12 cylinders, their compact size being useful as aircraft engines where their small size allows for better aerodynamics. On the other hand, for automotive and especially race-car applications, there is also the obvious disadvantage of a higher center of gravity, not only because one crankshaft is located atop the other, but also because the entire engine must be sandwiched between the exhaust pipes, which places the center of gravity even higher. The U engine is a similar concept, except that it uses two straight engines.

Another advantage is in achieving excellent mechanical balance especially if only four cylinders are involved. ^[1]

[edit] Notable H engines

[edit] Aircraft engines

Napier Sabre H-24 engine. The two starboard 6-cylinder banks can be seen in this view

• Napier & Son, UK.
  • Sabre H-24, 36.7 litres 3,500 hp, 1940 - by 1944 in the Hawker Typhoon, the most powerful inline configuration piston engine ever.
  • Dagger H-24, 16.85 litres 890 hp, 1934 - air-cooled.
  • Rapier H-16, 8.83 litres 340 hp, 1929 - air-cooled.
• Lycoming H 2470 hyper engine.
• Rolls-Royce Eagle H-24, 46.2 litres, 3,200 hp - first ran 1944.
• Fairey Aviation Fairey H-16 "Prince", 1,500 hp

[edit] Other engines

A BRM H16 engine, mounted in the back of a BRM P83 Formula One car.

• The British Racing Motors (BRM) H-16 Formula One engine won the 1966 US Grand Prix with Jim Clark in a Lotus 43.[1] As a racing-car engine it was hampered by a high center of gravity, and it was heavy and complex, with gear-driven twin overhead cams for each of four cylinder heads, two gear-coupled crankshafts, and mechanical fuel injection.
• The Brough Superior Golden Dream motorcycle, first shown in 1938. An 1000 cc H-4 design and a few units were produced in early 1939. Any development planned was interrupted by WWII and subsequent years of austerity.
• Wooler built a motorcycle prototype with a similar configuration to the Brough Superior Golden Dream and exhibited it at the British International Motor Show at Earls Court in 1948 and again in 1951. This was replaced by a flat-4-engined prototype at the 1953 show.

[edit] Other uses of H term

Subaru produces water-cooled flat-4 and flat-6 "Horizontal" engines that are marketed as H-4 and H-6 (also thought to represent the configuration of the cylinders from a 'top down' POV as opposed to the traditional 'head-on' POV), despite the fact that their configuration has nothing to do with a real H engine.

[edit] References