Here is an excellent pic which shows just how far back the read aerofoil is mounted: how they needed, despite this, to mount an extra flow spoiler on the roof; and how apparently negative to good aerodynamics and CD the front air dam is.
The BMW engineers spent lots of time working on these aspects, at the time.
And that's now 35 years ago!
http://www.racingsportscars.com/etcc...-07-22-011.jpg