World War II Ĭhrysler developed their first experimental hemi engine for the Republic P-47 Thunderbolt fighter aircraft. This complexity was referenced early in Chrysler's development of their 1950s hemi engine: the head was referred to in company advertising as the Double Rocker Shaft head. Significant challenges in the commercialization of engine designs using hemispherical chambers revolved around the valve actuation, specifically how to make it effective, efficient, and reliable at an acceptable cost. This adds to the overall width of the engine, limiting the vehicles in which it can be installed. The hemi head always has intake and exhaust valve stems that point in different directions, requiring a large, wide cylinder head and complex rocker arm geometry in both cam-in-block and single overhead cam engines (dual overhead cam engines may not have rocker arms). Thus, hemi-heads, because of their lack of squish, are more sensitive to fuel octane rating a given compression ratio will require a higher octane rating to avoid pre- detonation in a hemi engine than in some conventional engine designs such as the wedge and bathtub. However, if the hemi-head hemisphere is of equal diameter to the piston, there is minimal squish for proper turbulence to mix fuel and air thoroughly. The hemi-head design places the spark plug at or near the center of the chamber to promote a strong flame front. The result is a combustion chamber in the shape of the space between where the domed piston stops and the dome shape in the head receiving it. Since the combustion chamber is a partial hemisphere, a flat-topped piston would yield too low a compression ratio unless a very long stroke is used, so to attain the desired compression ratio the piston crown is domed to protrude into the head at top dead center. The intake and exhaust valves lie on opposite sides of the chamber and necessitate a ' cross-flow' head design. However, a hemi-head allows no more than two valves per cylinder, and these large valves are necessarily heavier than in a multi-valve engine. A hemispherical cylinder head ('hemi-head') gives an efficient combustion chamber with an excellent surface-to-volume ratio, with minimal heat loss to the head, and allows for two large valves.

6.1 hemi bolt torque specifications