"Hemispherical combustion" refers to the shape of the combustion chamber in an engine, which resembles half of a sphere. It is designed this way to optimize the burning of the fuel mixture, resulting in improved efficiency and power. Full definition
The HEMI engine that powers the Chrysler 300C uses aluminum cylinder heads with hemispherical combustion chambers for outstanding airflow leading to high power and torque. (topspeed.com)
If hemispherical combustion chambers are so great, why doesn't everybody make them? (articles.orlandosentinel.com)
Inspired by the original family of HEMI ® engines, the SRT 6.1 - litre HEMI ® employs the namesake hemispherical combustion chambers that provide power and efficiency, while retaining classic HEMI ® visual cues, including the orange - painted cylinder block and black valve covers. (topspeed.com)