These cars had wicked stability problems before the spoilers were fitted.
Over the entire top surface of the car, a pressure differential of 0.1 psi will create several hundred pounds of lift.
When Porsche replaced the 908 with the 917, the tail was about a foot higher, and still had stability problems without a spoiler.
About the Jaguar C-Type long-tail:
"Low and rounded at the nose, low and pointed at the tail, it not only looked like an airplane wing (in cross-section) but acted like one. The cars wanted to fly. In fact, their rear ends lifted so much that load on the rear tires was reduced by a good quarter, and the drivers came back to the pits ashen-faced to report evil instability. The older body had shown a tendency to lift a bit, but nothing like this."
