[basjoos]

An air dam is just an aerodynamic band-aid to divert air away from non-aerodynamic parts of the car (the rough underside). You can get a lower Cd by installing a smooth underpanel and allowing the air to flow straight and smoothly under the car rather than by installing an air dam and diverting the air around the sides of the car. The only reason to install an air dam would be to increase downforce (produced at the expense of increasing the drag) or if you didn't want to install smooth underbody panelling. None of the ultra-low Cd vehicles use air dams. They all have smooth undersides and, in many cases, have a lot of ground clearance to allow the air to pass under the vehicle with minimum disturbance. A low-flying aircraft fuselage suspended on wheels rather than a limpet scooting across the ground shoving everything out of the way.