HEMI is short for hemispherical combustion chamber and refers to a line of Chrysler engines starting with the FirePower engines of the 1950’s. The concept of hemispherical heads allowed for larger valves in the dome shaped combustion chambers for better air-fuel delivery and became synonymous with the Dodge and Chrysler entries into the muscle car market of the 1960s.
The angled valves of the hemispherical chambers necessitated a “double rocker arm” arrangement which added another level of cost and complexity. Chrysler’s HEMI engines have progressed over successive generations leading to significant improvements in both performance and efficiency. The best-known HEMI today is probably that of the Dodge Hellcat which produces 707 horsepower with the help of a supercharger.
The idea of a hemispherical combustion chamber actually dates back much further, to at least the early 1900’s. Several other manufacturers have used a hemispherical combustion chamber designs, though typically in much less well-known engines and certainly not with the notoriety that the HEMI name tends to garner. Some, but not all HEMI engines used domed pistons, particularly for increased compression in higher performance applications.
The hemispheric combustion chamber is often regarded as a good design for a two-valve engine but it does have its limitations with an increasing number of valves. Many modern engines frequently feature 4 or 5 valves per cylinder, so the hemispherical combustion chamber is no longer particularly common.
The modern HEMI engine is more a tribute than a true testament to the original design. For example, in the current 5.7L HEMI, the heads lack the distinctive hemispherical dome shape that gave the original HEMI its namesake. These modern systems use more advanced technology and systems to improve performance. Utilizing two spark plugs per cylinder ensures more even combustion and improved emissions control, while the variable valve timing makes up for some of the restrictions of utilizing only two valves per cylinder.
Through it has evolved from those early Chrysler days, the HEMI V8 engine still holds a special place in the hearts of many automotive enthusiasts. Whether in its early iteration powering a 1966 Dodge Charger, or in one if its modernized forms driving the HEMI-powered Hellcat, the HEMI engine stands as a symbol of horsepower, technology and automotive prowess.