Submerged duct (NACA) air inlets are an efficient low-drag design that can be used effectively on Landracing vehicles. Unfortunately, the principle is not well understood by the Landracing community for various reasons, including lack of technical information. As a result sometimes the optimal benefit of the NACA design is missed. An example is when the NACA duct is located at the leading surface of the vehicle, more like an air scoop.
The proper location of the NACA submerged duct is in the thin boundary layer parallel to the air stream, on the flat sides or top of the vehicle body.
There are a large number of declassified published studies from NASA [NACA] regarding submerged ducts on aircraft to provide low drag air inlet to the fuselage for both engine combustion air and other purposes, such as cooling. An early study, and most-cited, is NACA Advance Confidential Report No. 5120, An Experimental Investigation of NACA Submerged-Duct Entrances. Frick et al. 1945.
Examples of the correct application of the NACA submerged duct can be seen in the attached examples from the Honda Hawk streamliner and Project BUB which Woody and I collaborated on several years ago.