[Phase]

Originally Posted by aerohead

Yeah, if you can find the sweet-spot, where the inward curve is just right, you'll pick up a smidgeon of pressure recovery.
You'll have the 'smaller' wake, AND it will be of higher base pressure.
Is there a reason for 'saw-toothing' the fins?
The pressure at the trailing edge of the 'point' is going to be greater than at the shorter inclined locations, a setup for shear-induced vorticity, something you want to avoid like the plague.
--------------------------------------------------------------------------------------
The SAAB 9.3 was tested with and without a rear diffuser, and it had no effect at all on drag, as the car had no complete belly pan ahead of it. If you haven't already paneled in the underbody, you might run into the same situation. Don't know. It's a $4,000 answer.
--------------------------------------------------------------------------------------
Also, you have a 'cavity', but a 'box-cavity' is an entirely different animal.
--------------------------------------------------------------------------------------
The tearing edges will help reduce vorticity, and the wake will kind of 'look' like an imaginary boat-tail, but it has no facility for pressure recovery, as with an actual boat-tail, tapered cavity, or box-cavity.
And measuring from the IONIQ blueprint, there's no chance of any 'phantom' tail affect until you've got 140-inches of boat-tail behind the car.

The pointy shape is just an extension of the originally OEM hard separation edges that Hyundai already tested in the wind tunnel. I didn’t want to alter the rear side shapes because I have no idea how it would affect up wash or down wash or wake shape. I cut the inside shape to add a more stylized sporty look. In all, it’s just an extension of the original sides from Hyundai. The poster board tests with the same shape and duct tape already gave me a 4 extra mpg boost from testing before.