Wiki never lies
"The developer has stated that the game's proprietary IW 4.0 game engine is an improvement over the engine used in its previous game."
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.... The facepalm image is perfect for this post, it's the same engine, they just updated if for CoD4, like Treyarch did for CoD5, it's still the engine CoD4 and 5 used, they said this ages ago, it is CoD4 with different guns and a new campaign, I'll get Borderlands over this anyday.