Hemi engine
A Hemi or hemisphere engine is an internal combustion engine in which the roof of each cylinder's combustion chambers is of hemispherical form.
Hemispherical combustion chambers, which had been used for centuries in mortars and cannons, were introduced on some of the earliest automotive engines, shortly after proving the concept of internal combustion engines themselves.
Hemispherical cylinder heads have been used since at least 1901 they were used by the Belgian car maker Pipe in 1905 the Peugeot Grand Prix Car of 1912, the Alfa Romeo GP car of 1914, Daimler, and Riley. Stutz built four valve engines, conceptually anticipating modern engines. The BMW double push rod design, the Peugeot 403, the Toyota T engine and Harry Arminius Miller racing engines are a few other examples.
With the hemispherical combustion chamber design, the intake and exhaust valves are usually on opposite sides of the chamber, allowing for the combustion mixture to flow directly across the chamber, commonly referred to as "cross-flow" heads. Significant challenges in the commercialization of hemi engines revolved around the design of the valve actuation, and how to make it effective, efficient, and reliable at an acceptable cost.
Porsche has made extensive use of hemi-head engines, including the air-cooled flat-6 engine in Porsche 911 models from 1963 to 1999. The 1973 2.4 L version generated 56 hp per naturally-aspirated litre of piston displacement.