Here's a drawing.
There is no air dam.
A = the plastic wrap around nose;
B = the bumper
C = air flow from front of car to radiator
A is 6 inches above the ground, and B is 8 inches above the ground. There are 4 inches of space between the bumper bottom and the nose, which tapers to 0 inches at the sides (in front of the wheels), resulting in 100 square inches of space along the bottom length between the nose and the bumper. This is plenty of space, but there is no direct force to the radiator from in front of the car.
1) Should the large opening be plenty to provide good airflow to the radiator?
2) Should I close off the sides of the opening, so it's just open right in front below of the radiator?
3) Might the 4" wide opening create drag, even though it is perpendicular to the airflow?