imho, on a blocky car, it is important that the air leaves all four sides at the rear edge of the vehicle at same time. to further define this picture, ideally the air on all four sides would have the same amount of energy, so pressures, speeds, and mass if taken all together would be equal for each surface at the back edge. simply put i wouldn't want air from say high prressure on top of the vehicle to come off the rear and plunge to a low pressure area coming from under the vehicle. so the air needs to leave the rear edge parallel to the streamlines of the vehicle. carrying this notion further fwd along the sides, the air needs to stay on the same side of the vehicle as it travels back along the car, on a square cornered car, i would put fins on the corners (they could be a on the 45) to keep the air from spilling over the sides. if the sides are angling in slightly as they come back, (11 to 15) degrees, the last couple inch should straighten out. Fins on these tapered sides have an added bonus of pressurizing the air between them slightly which causes it to speedup as it leaves. almost like squirting out the back. what do think of those wacky ideas?
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