"Charles Vincent" wrote I would guess because the fan
is a liability to flow at speeds over
thirty or forty mph and because a twelve inch 1/8 hp fan has little to
offer over a 54 inch 40hp fan.

But that big fan is trying to blow air into a couple 3 inch round holes, and
then diffuse that speed of airflow to many times the square inches of
radiator. What you end up with is no airflow past the radiator, and
overheating.

Instead, you put a fan right next to the radiator pulling air directly past
the radiator at a decent speed, with no diffusion losses.

It works. That's why cars have them. That's why water cooled airplanes
overheat if they tai for more than a few minutes.
--
Jim in NC