I used to be of the opinion (and I've heard it many times) that running without a thermostat causes a car to run hot because the coolant moves too quickly through the radiator to be properly cooled. This theory is backed up by evidence that I've seen many cars at work overheat without thermostats and run cooler with them.

Hell, I've probably given that advice on this board.

I've taken Thermodynamics I & II and Fluid Dynamics here at Auburn now, and from what I was learning, it seemed impossible, so I asked my instructor about the theory. He agrees that it cannot be true. Here's why:

Fluids all act alike, and the faster the fluid moves the more heat transfers. Think of it as wind chill. This is why a water pump with more volume cools better, not worse (which would seem to be the case with the old theory)

But it is a proven fact that on some cars, the lack of a thermostat causes overheat. Why?

Theory: The overheating stems from the fact that removal of the thermostat reduces the coolant pressure in the block (the pressure in the block is determined by the thermostat, the pressure in the cooling system outside of the block is determined by the radiator cap) and makes the coolant more prone to boiling because it is at a lower pressure, causing excessive "steam pockets" of air around the cylinders and combustion chambers. This aeration makes for poor heat transfer to the coolant.