From my understanding and I think others will back me on this, the air "Piles Up" in the area in front of the grill and ends up going around it, in this case, it will head up over the hood. A small amount of turbulence may be created, but it isn't that significant. If the air moves through the radiator & engine compartment, it creates a fair amount of drag.
So to look at it in basic terms, you have a roughly 1.5 sq. foot area on the front of the car, and lets say it is blocked off smooth for our Zero starting point.
Smooth - 0 Drag
Open - .03 Cd Hit
Blocked from Behind - .002 Cd hit
So the blocking of air in the front of a car is very effective, even if done in a way that looks "rough". It is still 90-95% as effective as letting it find its way through the engine compartment. Roughly speaking. And as always, there's a chance I could be really wrong.
