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he is talking the tail of the car, right?
so the pic of the van is misleading.
we can argue about the transition from the body to the kammback, but suffice it to say if we are going from straight to 20 degrees, I'm willing to bet a 4 inch radius woudl be great, and if you had no radius, just a straight angle, you would lose a very small amount of efficiency.
On the sides and top of the Kammback, there will be no vortex created if the air was perfectly proportioned between sides and back. However, we all agree the odds of that are about zero.
However, if the top and sides are both at 20 degrees to the centerline of the car, then there will not be any huge pressure differentials in the air. What there will be is minor pressure differentials.
If there are minor "holes" in the air being turbulently attached to the car, a small vortex will help the air "fill the holes", just like a golf ball's dimples.
You and your strawman are of course welcome to disagree, but I again applaud the OP for DOING something and TRYING something where most of the rest of us are nitpicking his great efforts.
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