It is an engine with a two-valve hemispherical combustion chamber within an aluminum head. it is more powerful and more economic then normal engines.

The earlier engines this type has been around since about 1953 in the 392 firepower hemi the head's were cast iron all early drag hemis were made from this engine including NHRA nitro engines the hemispherical combustion chamber is the real KEY not whether the matierial is iron or alum. the head was not in alum. untill about the mid sixty's and then only as a high dollar race head factory hemi's (MOPAR) on street car's didn't have alum. head's till the new hemi's came out

In addition to the combustion chamber, the 331, 354, 392, 426, and new hemis also had the main caps cross-drilled through the engine block from the factory, which makes the bottom end virtually bullet-proof when it comes to 10,000 rpms. Try that with any other Big-Block type motor.

In a hemi engine, the top of the combustion chamber is hemispherical. An engine like this is said to have "hemispherical heads." The spark plug is normally located at the top of the combustion chamber in a hemi, and the valves usually open on opposite sides of the combustion chamber. A hemi has a low surface area to volume ratio. This means you can stuff more gas and more air into a smaller space. High surface area can also cause heat loss. Fuel that is near the head walls may be too to burn efficiently, thus wasting energy and decimating gas mileage. A hemispherical design also allows the maximum cylinder pressure when it is needed. A hemispherical head is capable of having HUGE valves. Since the valves are on opposite sides of the head, there is more room for them. Large valves improve airflow, permitting the engine to perform better without nasty backpressure or a poor air-fuel mixture. This wastes as little of the engine's precious energy as possible when it is sucking air and fuel into the combustion chambers and when it pushes exhaust out. You may be asking why not every carmaker or hot rodder uses hemispherical heads, since they are so wonderful. Well the answer is practicality and design limitations. One thing that a hemispherical head will most likely never have is four valves per cylinder. The valve angles would be so crazy that the head would be nearly (if not completely) impossible to design without destroying some other distinguishing aspect of the engine. This means that an everyday hemi headed engine will never be tuned to produce a balanced amount of horsepower and torque throughout the power band, since cams can’t magically change while the engine is running. Smaller combustion chambers are becoming trendy as of today because they lose less heat during combustion, and also shorten the distance the flame must travel during combustion. The day that hot rodders begin living by that rule will be the day the hemi dies.