In an interview with OPM (Original Playstation Magazine), Infinity Ward’s animation lead Zach Volker went into some details about the new game engine that will be powering the upcoming game "Call of Duty Ghost".
Here are a couple quotes from the interview to summarize:
Volker told me: “When we’re talking about a new engine we’re talking about upgrading significant systems within in that engine. We’re not talking about throwing it all away and saying we’re starting from the ground up”. The reworking of the old engine to create the new engine is, says Volker, because it’s, “impossible to develop a new engine from the ground up in a two year cycle”. Instead it come down to the amount that the current tech has changed. “What we do is we say, ‘okay what are the things that are significant and that we would say that are encompassing of the engine or its visual quality?” explains Volker. “Are those being upgraded in a significant way? Alright, then I think that warrants that we’ve got a new engine on our hands”.
OPM Can you say what tech the new engine’s built on?
ZV The original engine years ago was built on Quake engine. We’ve done so many new engines and overhauls on top of that I’m not sure any of it still exists.
OPM Is it still built from the original tech or is it built on something new?
ZV When we’re talking about a new engine we’re talking about upgrading significant systems within in that engine. We’re not talking about throwing it all away and saying we’re starting from the ground up. It comes down to the systems we’re augmenting and upgrading, and trying to decide what is the significance of this upgrade. So there is certainly going to be remnants here and there of our pieces of our last engine, where it was appropriate when, you know what, this doesn’t need any changing.
Infinity Ward has been significantly upgrading its existing game engine for the past 2 years. While it can be claimed that the end result is a new game engine, it is not a new engine built from the ground up. Which means that Ghost will be running an upgraded game engine that was most likely used in "Call of Duty: Modern Warfare 3"