[aerohead]

At:
forums.pelicanparts.com/porsche-911-technical-forum/740377-anyone-have-911-cfd-analysis.html
there are some additional wind tunnel tuft study images of the 911 Carrera, shot from a higher vantage point.
There is clearly separation behind the backwards facing step at the top of the recessed backlight, where vents are positioned. An inverted pyramidal area of disturbed tufts extend halfway down the glass, illustrating the reversed flow. This area is sloped at 26-degrees to the horizon, three degrees steeper than what can support attached flow.
The angling of the C-Pillar tufts also suggest high pressure body side flow directed towards the low pressure, fast-moving flow above the roof, rolling up into the tornadic vortices as these two regions' attempt at pressure equalization fails at the intersection.
Further down the rear surface, the tuft orientation is 'corrected', presumably,now caught in the vortex-induced fastback downwash, strongest of all body forms.
According to Hucho, page 281,in his Second Edition, by simply lofting the 911's rear contour ,up to the thickness of the flow separation,you'd kill both the lift and drag, and better than with the spoiler.(page 175). This WOULD, however, alter the 'look' of the car, and potential for a ' risk to sales.' ( page 188).
It would take until after 1994,and the end of the air-cooled 911s, before Porsche would begin this lofting of the rear body contour. Parked side-by-side, the aft-bodies of the early and current 911s would show little resemblance. A good thing!