Need to start this off with the admission that I have no technical engineering qualification (ok, that should excuse anything really stupid that may follow in this post
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About the few pics we've seen of the Valencia exhaust. The most obvious difference is the slot before the exhaust outlet. The school seems to be out on whether it is now pointing a little more outward. There are definitely other differences, ie the channel on the floor and the shape of the bulge and cutaway of that.
Its stating the obvious that the objective is the channel the exhaust, but, the problem with a lot of images and drawings we see is that people show these flows as lines and arrows clearly heading in one particular direction. What I feel happens is, because these gases as so hot when they hit the atmosphere is they explode in all directions and some heads down the lines of the arrows. In looking at the slot ahead of the exhaust I don't think this is for cooling, it doesn't seem to be the right shape for that. I'm guessing/wondering if the subtle changes along with the slot are all aimed at containing the explosive behavior and shaping more of the gases to flow in the desired direction.