Now for the optimum part:

The absolute best possible performance would be a system that used just enough air Mdot to heat the cooling air to the same temperature as the water radiator exit temp ( water going back to the engine from the radiator). This condition is impossible to achieve in practice. So you try to get as close as you can. How close you get is called the heat exchanger effectiveness. The closer the air and water exit temps are, the better the effectiveness.

Monty:

I’ll respectfully disagree on this part.  It may apply if you have unlimited dynamic pressure to work with, because it doesn’t consider radiator thickness and air-side pressure drop.  We don’t have this case

Al