Benefits and Drawbacks
Although a wedge-head design offers simplified valve actuation, it usually does so by placing the valves side by side within the chamber. This can restrict the flow of the air/fuel mixture into and out of the chamber without a compensating design (including designs utilizing overhead cams). With a hemispherical chamber, a straighter flow path is provided for the air/fuel mixture across the chamber. Engineers have learned though, that just passing gas through a chamber quickly does not provide the best combustion event from an emissions, efficiency, or power perspective.
Drawbacks of the hemispherical chamber include increased production cost, high relative weight (25% heavier than a comparable wedge head per Chrysler's engineers), poor low-rpm performance due to the large port size's turgid flow characteristics, and difficulty meeting emissions standards. These have pushed the hemi head out of favor in the modern era.
Read more about this topic: Hemi Engine
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