Hardtop models are usually stylishly and aerodynamically slippery, but with a little more weight thrown in for the structural integrity supports..... Where as the post door Sedans are usually square and light by nature but with quite a few hot spots as far as un-aerodynamic chambers or dead corners both on the main body and/or underneath. Creating aluminum foils/dams that divert streams of air away or around these pockets will aid in air management and keep the weight down. There's a few air management issues to be dealt with underneath most cars.
As far as my front grille area, I've foiled it with aluminum to completely direct all incoming air through the radiator. Here's my aluminum grille cross support sitting next to the hollowed stock support in the very beginning leading up to my home made foils. Stock one weighed 3-3/4 lbs, mine....1-1/2 lbs. Saved 2-1/4 lbs and no $$ other than some extra aluminum lying around.
