Batmobile question

Really liked this but was a little confused about a scene with the Batmobile. This is a little spoilery:

After Batman's personality change and mass-duplication of himself, there's a scene where the Batmobile is towed out of a garage followed by a hot-rodded version driven out of the garage by Batman. I took that to mean that he traded in his old car for a new one, but why didn't he just hot rod the old Batmobile? The body for both seemed almost identical and it looked like all that was changed was a large engine sticking out of the hood and big tires. Or did I misunderstand that this is what he did? Maybe this was one of the Batman copies that did it.