I recalled basjoos weighing in on this question and found this nugget (which he posted at GS). Makes sense to me.
Quote:
You'd get better results if you blocked the upper rather than the lower grill opening, especially if your car doesn't have underbody panelling. In the absence of underbody panels under your engine compartment, all of your cooling exhaust air is going to end up under your car, adding to the drag turbulence down there. Some of the air that currently isn't entering your blocked lower grill is going to end up under your car anyway. But if you block your upper grill, all of that air is going to end up going over your car, where it creates less drag.
If you look at many sports cars (example: 1992 Camaro, and also on the 92-95 Civics), you will see that they place their grill opening very low on the front of the car just for those reasons mentioned above. On most cars, that upper grill intake is dictated by the automotive styling department, since many people don't think a car "looks right" unless it has a grill opening at the upper front of the hood. That look is a holdover from the cars of the 30's where the entire front the the car was designed around a large, tall grill.
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