[robchalmers]

the two sierra 's you'v feature we're spoilers design as homologation specials for racing and to produce Downforce. The cosworth (white mono plane one) i know is a NACA profile across it section which is one towards the rear, from the glass to about 3/4 out there is nothing there so the flow is just the same as a normal hatch. The XR4i (black one) could well work on more of a lid extension basis, but a nascar wing is likely to create more 'churn' and therefore form drag.