Originally Posted by Volumex
So to paraphrase, they are great for air-breathing engines but don't use them for vents.
The original goal of this NACA AR 5I20 investigation and report was to find the suitability of low drag inlets for jet engines, so it is no surprise that the summary specifically mentions this. For example, see figure 38, second to last page where the inlets are shown on the fuselage sides just ahead of the wings. In this position they do not recover much if any ram pressure and you never see them employed in this manner. However, numerous smaller NACA vents are used on the nacelles of pod mounted jet engines for various purposes. They are also used as general purpose low drag inlets.
An earlier post mentioned they should be placed in an area where the fuselage cross section is increasing. A more general statement would be they should be placed in a flow field with a positive pressure gradient. If placed in a flow field with a negative pressure gradient they will act as an exit vent (and not very efficiently). There are lots of examples of poor placement like this on race cars and some aircraft.
To the OP's question, the NACA inlet is not drag free, but I don't think you could measure the difference in speed of your aircraft between one NACA inlet and two.