[ricola]

Couple of aero basics for you:

To get downforce the rear exits would have to be channelled to point upwards, the downforce is a result of a reaction force.

The air over the top of the car would be detached from the surface way before the engine cover, typically at the top part of the rear window.

Downforce is mostly a result of STATIC pressure and the difference of this between top and bottom surfaces of a body. Target is to get higher pressure at the top surface which will result in overall DOWNforce. According to Bernoulli's equation: faster fluid flow results in lower static pressure. Using this, you want to get the air under the car flowing as fast as possible to get the static pressure as low as possible giving any increase in pressure over the body a chance to create the downforce.