Some of you may be familiar with my website Mulsanne's Corner. During my research for my IMSA GTP book I got to know fellow Industrial Designer Clark Lincoln. Mr. Lincoln was responsible for the famous GM Goodwrench livery on the Corvette GTPs. Clark Lincoln was also a designer for General Motors and worked in their Advanced Studio. Well turns out he was heavily involved with the HX3 Hybrid. He's graciously shared some of his wind tunnel test run notes and I've decided to share them with you all as well as they make for interesting, and very relevant reading:
The main conclusions they came to was the interrelationship/sensitivity between the slope angle of the trailing edge of the roof and that of the rear underfloor trailing edge ramp angle. The HX3 had essentially a flat underfloor. They found that if the roof trailing edge slope was effectively zero, sloping the underfloor/diffuser ramp actually INCREASED drag. But if an angled rear diffuser (15 degrees) was combined with a sloped roof trailing edge just the opposite occurred: drag REDUCTION.
More than likely the downwards "aimed" roof vorticies helped draw air through the rear diffuser, whereas without the top trailing edge slope the diffuser was creating a localized flow separation. Interesting stuff and quite a treat to take a peak into the development notes of a road car. Typically this type of information is considered verboten!