Paul, this would have no use in a tractor but in a pusher it might do some good..
Put a can on the prop hub coming back a bit into the cowl, put impeller fins on it. Some ducting around the impeller takes care of the flow.
Why not resort to an active air system and create a bigger pressure differential?
...Chrissi

That would be an axial flow air pump.

We measured about a half inch to one inch H2O reduction in cowl pressure using
just the prop bolts and the spinner backing plate as a centrifugal air pump. My guess
is a real impeller might get you a couple of inches.

However, as centrifugal and axial flow air pumps go the RPM is not very high.
It would help if the spinner backing plate were 14 inches in diameter.

BMW in WW II geared up the co-axial cooling fan for their radial engines.

Don't forget however all this stuff sucks HP and reduces the MPG.

It is best to carefully work on the scoops, the ducting and
cowl flaps. 

I still like the boat tail and vertical cowl flaps idea
for pushers. It is nearly impossible to prevent separation
behind a P51 scoop by curving the body up to meet the prop.
Too short of a distance. Klause tried that with a NACA duct no less
and went back to his boat tail. Might as well re shape it into
an inverted  vertical fin or boat tail and put the cowl flaps
back there. Could work as a speed brake in the bargain.

Paul Lamar
 
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