I guess my point was that there isn’t enough information to draw the conclusion.  But if the exits are the same, and the pressure in front of the radiator is the same as that above the cooling fins; then the pressure drops should be about the same.  But of course it varies from one installation to another, rad thickness, fin density, etc.

In any case; I agree with your premise – I have seen no data that definitively supports the conclusion of lower pressure drop for cooling the air cooled engine.  Should one also infer then, that the flow rate required is also about the same – that the higher fin temp does not give a higher delta T to the cooling air?

Al

I have never seen it verified that the radiator has greater resistance, and if it did, wouldn't it depend on the fin density and other design variables. You could just go with a bigger lower density radiator to equal the pressure drop of the air-cooled engine.  Keep in mind that what moves the heat is not just deltaT, but the convection coefficient. So if you had a higher convection coeff. due to more turbulence in the radiator core (higher velocity) you could move the same heat with less deltaT. I am sure that there is some combination of fin density, flow and pressure drop where you could equal the heat transfer of the higher deltaT case. Now you have to figure out if the internal/external drag to do this is greater than the air cooled situation. Too many variables to have a definitive answer. I'm guessing the real difference is weight and complexity more than drag, except for the ultra optimized case, which none of us has the budget to attain or even measure if we did.

Monty