[aerohead]

Originally Posted by some_other_dave

One note: The original "ducktail" spoiler, as used on the early-70s Porsche 911s, did in fact decrease drag as well as decreasing lift. The later "whale tail" (think late-70s 911 Turbo) was better at both.

This might be due specifically to the rather unique shape of the 911, but evidently extending the effective height of the rear of the car and making the end a sharp corner rather than a gradual slope helped both to improve stability (particularly when combined with a front spoiler) and to reduce drag.

-soD

Hucho's book has complete tables showing lift/drag/velocity relationships for the 911 rear spoilers.