[aerohead]

When this topic came up it triggered something we talked about back in 5/16/2009, when Ernie Rogers shared his thread,'Describing my Beetle wing ( VW New Beetle)',here at EcoModder.
Julian's photograph of the 911,undergoing flow visualization with tufts,on the surface,does imply fully-attached flow,over nearly the entire aft-body of the Porsche. And it is, however, with a very important caveat, which Hucho addresses on page 114,Figure 4.10, Transverse velocity vector diagrams and drag coefficients for a fastback car with different rear end slope angles,after S.R.Ahmed,1984.
Ahmed presents a car model tested as a squareback,then the same vehicle as a a fastback,of varying rear slope angle.
As a squareback,with zero slope,the car registered Cd 0.272. As a fastback,with 15-degrees down-slope, the car registered its lowest drag,at Cd 0.23. When the same model's down-slope was increased to 30-degrees it registered its highest drag,at Cd 0.33.
Hucho comments, ' The C-pillar vortices are so strong that flow remains attached over almost all of the inclination. [T]he vortex strength is significantly higher on a fastback than a notchback,and decay of the vortex strength behind the vehicle is less pronounced. Beyond 30-degrees the vortex bursts and flow pattern changes to the squareback regime.
Elsewhere it's been mentioned that the attached,counter-rotating, longitudinal vortices allow for an attached downwash which gives the illusion of what would otherwise pass for 'attached flow' of the tufts,while not indicating the high-drag vortices which would only be visible on a grid screen or with smoke.
W.A.Mair and Rolf Buchheim et al research revealed that a turbulent boundary layer ( TBL ) cannot exist in an aft-body inclination steeper than 22-23 degrees. The trigger for separation.
The Porsche 911/912 rear slope is measured @ 26-degrees, indicating a compromised TBL. C-pillar tufts indicate cross-talk from the roof flow to the body side flow,an indication of the presence of attached vortices.
At 26-degrees,even though the tufts are apparently following the body contour,it would be impossible for the inviscid flow to be imparting any kinetic energy into the TBL.
Basically speaking,the flow is lost by the time it reaches the cooling air inlet, and any opportunity to decelerate and gain pressure over this region is lost to direct air heating, presenting a lower-than-attainable pressure 'OVER' the tail of the Porsche, allowing rear lift.
Both the duckbill and whaletail spoilers reach up and away from the low static pressure regime,into more energetic air,attempting to gain flow re-attachment, spoiling the lift.
By 1989,aero modifications for the 911 Carrera 4 cut drag from Cd 0.39,to Cd 0.32 without any significant 'visual' difference in the car.
After 1994,when the wasserboxer engines came onboard,Porsche lofted the rear end of the 911, softening the rear angle to 21.5-degrees.
In 1996,to win Le Mans, Porsche chose an 11.5-degree back slope foe their 911 GT1 racer, aft-body of 48% of total body length, and Verjungungsverhaltnisse of 2.209,achieving 210-mph on 600-horsepower. This car is a 'template' match.
The 2010 Porsche 918 Spyder, Cd 0.29,is also a 'template' match,incorporating only a 14.5-degree back slope, and is 'double-ended',with as much aft-body,as forebody.
Volkswagen also chose the 'template' for it's 2010 Lamborghini Sesto Elemento.
The center of the rear spoiler on the 2010 VW Audi R8 GT,runs right through the 'template.'
The Cd 0.29, 2010 Porsche Panamera is not far from it.
Even the VW Bugatti Veyron's 'low' spoiler flirts with it.
VW's 2011 Cd 0.186 XL1 is right on it.No spoiler.
VW's Lamborghini Aventador LP 700-4 goes beyond it a smidge (and no spoiler).
2011 Audi A8L, Cd 0.27 is dead on.
2011's Porsche 911 GTR RS 4.0 reaches above with it's rear wing for really clean air @ 193-mph.
VW's 2012 Audi A7 sticks it's pop-up spoiler right up to the 'template.'
VW's 2013 Lamborghini EGOISTA looks familiar.
A comparison of aft-bodies between the 1964 911/912 and 2014 911 bear very little resemblance.
VW chose the 'template' for it's 2017 Audi R8 V10 PLUS.
And today's Cd 0.25,Porsche Taycan Turbo S stays close to the profile.No spoiler.
I'm going to suggest that the beloved 911 was TBL flawed from the beginning,and after generations of Band-Aids added as palliatives, Volkswagen AG slowly distanced itself from the original 911 architecture,without alienating customers,and we'll probably never see aft-body contours like that again.And it wasn't so much lift due to an 'airfoil' shape that was responsible for instability,as it was low pressure caused by a corrupted boundary layer acting 'over' the rear body. Restore the 22-23-degree contour,and throw away the spoilers.