This is probably suited to the Engine Area of the forum but oh well.
What Engine is COD4 and previous COD titles on? I know that COD1 was Quake 3 but are they still using Quake 3 as a basis for COD even now? You can get crap loads out of an engine with it's source code and constant upgrades so I'm thinking it's still a big ass modified Quake 3?