H16 Formula One engine in its final, 64-valve incarnation.]]An H engine (or H-block) is an engine configuration in which the cylinder are aligned so that if viewed from the front appear to be in a vertical or horizontal letter H. An H engine can be viewed as two flat engines, one atop 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. The only 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 - see Lycoming. The U engine is a similar concept, except that it uses two straight engines. Examples The H configuration is very uncommon. Known examples are: Road vehicles Formula One