Bob had mentioned the surface area thing as well, and this is a linear
relationship, but what I'm refering to is turbulence which has an
exponential relationship to the effective heat transfer capability of
that air mass. As you may know, in clean air, a film of heated air
clings to the interface between the metal surface and the air mass as
it passes by, this impedes heat transfer. A car radiator, while being
small and available, is also exceedingly draggy, which is why its not
an optimal choice choice for a fast airplane. The auto radiator is
designed for different conditions mainly:
1) High disipation at low air flow speeds
2) Clean air entering front surface (Reynolds number less than 10,000)
2) Drag not an issue

An aircraft/cowl-surface scenario doesn't have the condition of high
power output and low airflow and thustly should not besigned for this
condition. Even on the climb out, while the IAS may be low, the prop
wash is turbulent and higher in velocity than the speed of the vehicle
itself.

Regards