[aerohead]

Al printed off a blueprint of the 6 for me, and I photo-enlarged it up to my contour scale and threw it on the light table.
* Without the upper and lower rear spoilers, the roofline is a very close match to Wolf Hucho's 1978, basic, 1/2-body roofline, on display in the Deutsches Museum in Munich. It goes as steep as 29-degrees slope, and produces Cd 0.15.
* If you create a vertical line, originating at the rear face of the 6's rear tires, and project it upwards, it will intersect the backlight where there's a probability for flow separation.
* The upper spoiler's tearing edge lofts the separation point upwards, with a tangent angle relaxed to 15.5-degrees off horizontal. This re-accelerates the flow, mitigates the adverse pressure gradient, allows for re-attachment, and some pressure recovery before the flow reaches the wake. This tearing edge 'contour' is almost a dead match to Wolfgang Klemperer's 1922, basic, 1/2-body, tested at the Zeppelin Werke.
* From the Hyundai wind tunnel smoke-flow imaging, the lower spoiler is almost completely embedded within the turbulence behind the upper spoiler, catching only a little edge flow.
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If you're tracking parametrics, you'll notice that the Ioniq 6's aggregate dimensions are very similar to those of other cars of similar Cd.