Originally Posted by fastforward

...the two features that earn the new 5.7-liter HEMI power plant its HEMI name are:
The hemispherically shaped combustion chamber designed to accommodate large valves and put the spark plugs close to the center of the combustion chamber
The cross-flow valve arrangement that is perpendicular to the crank-shaft centerline

http://www.swiftindavis.com/sblog/index.php?itemid=285
or
http://www.allpar.com/mopar/new-mopar-hemi.html
or
http://auto.howstuffworks.com/hemi.htm

Originally Posted by link2

Today's Hemi is that (a "Hemi") in name only. [Editor's note: Bob is referring to the head design. It is not a true hemispherical head, but looks vaguely like a hemispherical head with parts filled in.]