Chrysler did not invent the hemispherical combustion chamber, but they did do more than any other company to promote it—to actually make the Hemi a thing. The use of a hemispherical, or domed combustion chamber, goes way back, to even before the inception of Chrysler. Its benefits were exploited on Fiat Grand Prix racers, Stutz sports cars and others well before WWI.
The advantage of the design over that of the alternative wedge head is in valve size and commensurate breathing. A wedge head’s valve size is limited to what can fit side by side within the space of the bore. A Hemi, on the other hand, spaces its valves on either side of the dome, allowing for bigger openings and better gas flow both in and out.
This video, from our friends at Hagerty, shows the breakdown and reassembly of a Chrysler FirePower Hemi that they say came out of a Chrysler New Yorker with a tree growing out of it.It gives an excellent overview of how a Hemi works and the ingenious head design that allows space for pushrods, rockers, big-ass valves and spark plugs all seemingly in the same place. We’ve seen their work previously on a Ford Flathead rebuild and this one is just as awesome.
Source: Hagerty