Just to cut through the bullkaka...One last post to clear things up... 4 valves per cylinder and 2 overhead cams. This makes for a more precise and effective exhausting of the combustion chambers and intake of oxygen for combustion (making a happier combustion environment at all RPMs).

A really fun site to check out if you want to get into the whole thing more indepth is (this is the page for the DOHC info, you should also check the home page for other things from autos to zebras): http://auto.howstuffworks.com/question562.htm

7arpad7