The 911 happens to be an aerodynamic nightmare as a Sports car. It's the Icon of great sports cars, but with my eyes it's one of the worst sports car shapes ever. What gives the Porsche its look is its arch that extends all the way to the rear bumper, even in silhouette the 911 is recognizable.
Its one of the best lift profiles ever designed into a car!
As the air is being worked rushing down the back it creates a vacuum which draws air from the top of the boundary layer in a laminar flow through the entire arch, the act of which is the envy of most heavy lift aircraft. If you think the 911 is an low drag profile think again I've seen drag coefficient numbers (Cd.) as low as .33 the real numbers are .38 to .45 in the neighborhood of full size pickup trucks. The huge price tag isn't just lift but outright energy consumption just to move the car forward. The Porsche 911 is so bad that it is the only car to be more aerodynamic with an obstruction in the flow path a rear spoiler wing or the Whale Tail as its known.
Porsche knew this very early on, probably even before the 901/911 went into production. I’m not sure why they didn't change it until the 996 but you can see from various models that they were well aware of the problem.