H formation engines are rarer than hens teeth – they are called H because they are designed like stacked flat engines. Most engines called H such as Subaru or SAAB engines are not really H formation, rather horizontal or straight. The Saab engine known as H was a replacement for their B engine and Subaru’s engine is more like a boxer engine.
H engines have been seen in aircraft where there was a need for huge amounts of power and stacking engines was one way of achieving the end result. Lycoming and Napier were producers of these engines – usually massive configurations, 32 litres and over 3,000 hp for example. Interestingly, Lycoming’s V8s ended up in some US cars in the 1930s.
So, an H formation uses two crankshafts that are connected and geared together. The power to weight ratio suffers but the concept works. In the 1960s, BRM developed a 3 litre H16 engine for Formula 1 and it was very complex and fairly unsuccessful winning only one race in a Lotus chassis – the 1966 US Grand Prix at Watkins Glen. It evolved into a 64 valve motor and had a habit of overheating or spewing out oil – not surprising really as the complexity of getting the mechanical timing right on 16 cylinders would have been quite a feat!
BRM had been campaigning a 1.5 litre 8 cylinder engine and when the F1 rules changed to 3 litres, they had the brainwave of stacking two motors together to get a 3 litre motor. Presumably they thought this would be cheaper than designing a whole new engine.
We’ll probably never see an engine like it again. These days with engines getting smaller and manufacturers trying to develop hybrids or turbo motors, the need for lots of cylinders or complex petrol powered motors is reducing. It was in reality, a branch of the evolutionary tree that has become extinct.