[brucepick]

[Pendragon - Yes, I've also considered adding an upper short spoiler to the scheme. To get the air following the template early.]

OK - I'm going to attempt to guess at what happens here. Like the kid in class "I know, I know!!" but guessing...

Those cars that have a truncated teardrop template design such as Prius and Insight (both gens, I think) have very low cd as a result of it. If the same or better cd could have been gotten with a notchback, even a modified notchback of some sort, I'm sure the designers would have gotne that route, and would have had a much easier time selling cars with a more conventional design.

So my guess is - whatever the reason, a body that follows the template actually does give lower cd than a notchback with carefully tailored curves and angles. Now, if we "fake" the template by adding a spoiler or false trunk lid that meets the template line - well, I'm not sure. But aerohead keeps pointing us to the template and I suspect he's right on that.

My guess is that the flow that comes off the trunk lid and parallel sides exits straight rearward and so does not bring top-bottom-sides air flows back together. But with a truncated teardrop or MAYBE even with a "faked" teardrop like we're considering, the airflow is being coaxed to an angled flow as per the template, so the streams are encouraged to rejoin each other with less turbulence.

Note the Civic coupe trunk is angled slightly downwards to the rear. I think they're trying to get some downward angle on that air, but it's just not very much. I do think that a bit of a down angle on a false trunk lid would be good. Just don't angle down more than the template shows, is my guess. On a typical spoiler, a down angle would catch some air and give lift. But a false lid I think can be angled down a bit, as long as there's no significant air flow passing below it.