Watch out for naca ducting for cooling. they have a

reputation for not working.  Having said that, look at

an article in this months Sport Aviation. I forgot the

plane type, but using the naca duct on an IO-540.  It

completely failed in several positions until he added

difusers (sp) in front. Now works fine. There are

pictures. Interesting!

 

Not diffusers, Jim.  Vortex generators (vgs).  It is on an E-racer.  Claims the vgs doubled the air flow.

 

Al