I think that the "which opening" question may best be answered depending on where the factory coolers are located. Many cars do use the lower opening as the main source of intake air, so that is going to commonly be the best one to use. Be careful, cause some vehicles (like mine) use the side openings for oil and/or transmission coolers as well. I think that a lot of aero help could be done by completely boxing in the coolers as necessary to make sure that you are getting the most of the opening you have. By doing that, you may actually be able to further decrease the opening you have; doing the exact same thing that Nascar drivers do on the front of their race cars. Watch closely and you'll see the pit crew either adding or taking away strips of "200mph tape" to the front of the car. Those guys prove that a 2" strip makes a difference in aero, so every little bit helps. It just may be more prevelant at 200 mph
