We had a discussion at Copper State on this subject.
For those people using single rads under the engine. Keep
the chin intake low on the cowl to benefit the most from the prop
slip stream on the ground and during climb. The prop slip
stream speed is much faster than the aircraft speed during climb 
and the pressure you get across the rad is proportional to the
square of  the air velocity. At 100 MPH you get up to
0.18 psi depending on the efficiency of your duct
and up to 0.26 psi at 120 MPH. 

So say you are climbing out at 100 MPH and your slipstream is 
120 MPH you have 144% better cooling with a low intake
close behind the prop. If it is too close to the spinner 
it will be in a lower velocity area caused by external diffusion 
where the air flow is being diverted around the more or less
blunt cowl and spinner.

This is another reason why I like to take air in under 
the the rad and let it exit the rad into the engine compartment.
Of course hot air rises and that is another good reason.

IMHO in this picture of an early Power Sport RV4
the intake is too far up under the spinner. It should
be lower to benefit more from the prop slip stream.

Take some measurements using a low cost water manometer made from
clear plastic tubing. 12 inches of water column is .45 psi
so one inch of water is .037 psi. This makes a water manometer
ideal for making measurements on these low pressure aerodynamic
systems. You will be amazed at what you will discover and you
will soon acquire a feel for what the air is likely to do in
any situation. The first thing you should check is the pressure
you are getting across your water or oil rads. The higher the
pressure difference the greater the airflow through the rad.

Paul Lamar
 
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