PPRuNe Forums - Naca ducts

I'm struggling to reconcile something:
We know that a NACA duct is designed to reduce air flow without inducing drag, but what is the benefit of employing a NACA duct at the (piston, non-turbo)engine air intake? I understood that a scoop with internal stabilisation of the air flow is better (from automotive engineering studies done 30 years ago!).
If it can be shown that a NACA duct is indeed beneficial in this application how does one go about calculating the optimum size duct?
I've spent quite some time googling this but can't seem to find an answer.
Any comments appreciated....