According to MotorTrend, the term “semi-hemi” more accurately describes the partially hemispherical shape of the hemi engine cylinder head combustion chamber. Geometric definitions aside, the hemi design provides some unique advantages and disadvantages over the more traditional wedge-shaped combustion chamber.
One of the hemi head design’s most significant advantages is the canted arrangement of the intake and exhaust valves. The dome-shaped chamber requires the valves to lay at an angle to the centerline axis of the cylinder bore, allowing valves with larger diameters to be used compared to those in a wedge-shaped chamber. In addition to the larger valve openings allowing more airflow through the combustion chamber, the taller chamber results in less flow interference from the cylinder wall.
Another hemi advantage is that its combustion chamber and related piston shape encourage peak compression earlier in the compression stroke. The increased efficiency promotes a quicker, more complete fuel burn earlier, transferring more power to the crankshaft than other traditional engine designs.
