These notchback and aeroback cars are the toughest to figure.
Even if the spoiler corrects the flow on top,it does nothing for the sides,which are still part of the separation line.And it is the aggregate pressure at the separation line which determines the base pressure of the wake and subsequent pressure drag.Which is what we're going after.
An improper spoiler could affect the vortex-induced downwash which helps keep the flow attached as much as it is,as the flow see's the longer 'step' in front of it.An up-kick to the spoiler complicates matters even more.
With a wind tunnel we could just read the force measurements and know immediately if our mod was heading in the right direction.Smoke is good too!
