Excess Heat
Adding to the info from RT, if you add even more "cooling capacity" like I did when I plumbed in a Heatercraft cabin heating unit, you can really shut down the thermostat at idle speed.
Here is their URL:
http://www.heatercraft.com/Category.aspx?CategoryID=37
At cruising speed, about 2400 rpm, our M25XP 3 cylinder puts out an amazing amount of heat on the "low" fan setting, and we never use medium or high. Amp draw is a bit over 4, but the engine is running anyway for this device to provide heat.
This will not change your need for the right size engine heat exchanger, I admit. Our "later model" M25 has the 3" size, BTW.
In related news, sort of, one time many years ago I had the radiator fan switch die on my Honda civic on a hot August day in heavy traffic.... got home without overheating the engine by running the car heater on high with all windows open! Boy did I sweat!
No great lesson, except that engine heat can be shed in more than one way.
Cheers,
Loren