.You are absolutely correct about pressure being directly related to heat dissipation. Lets take a good example from a friend with a '69 Mustang that was always overheating...
He brought it over with a new Edelbrock waterpump, thermostat, etc, etc, etc. We changed it all out and guess what? As soon as we let the engine warm up it promptly overheated and started spilling coolant. Hhhmmm What could this be? Well, you probably guessed it. He had NO thermostat installed... but this was not really the problem! We checked his radiator cap and it would start venting under 2.3psi of pressure (normal is 7-14psi).
Knowing that a low-pressure liquid has a lower boiling point, I sent the fellow out for a new radiator cap (the old one was over 30 years old!) and guess what? Even without a new thermostat, it DID NOT boil over and his temp stayed dead even at 150 degrees. True, it took forever for the tempurature to come up, but when it did, it stayed steady.
Next on the list was to let the car cool to ambient tempurature (about 65 degrees). About two beers and 45 minutes did it. We then changed the thermostat with a 190 degree unit. Well, guess what happened? His temp gauge quickly warmed up to 205 degrees (manifold water temp) and fell like a rock to 190. It went up and down a few times as the thermostat cycled but eventually settled on 190 and stayed there.
This poor guy has invested over $400 in electric fans, chromed hi-po water pumps, new belts and two (yes two) radiator flushings to discover that his $2.50 radiator cap couldn't hold in the hot fluid below 3psi.
It would seem that any fluid given its "free flow" would conduct heat the best but that is not the case. Lets take an example:
One cup of water, placed on a stove on High measure the temperature at boiling... Roughly 230 degrees right? (depending on atmospheric pressure, but we'll leave that out right now).
Take that same cup of water and put some kind of cap on it that lets go at roughly 7psi and the boiling point will increase to roughly 300+ degrees.
* If the fluid is under pressure it will conduct more heat (heat soak) because it is DENSER (under pressure). This same law applies to metals in the regard that aluminum will melt at a lower tempurature than steel (less dense plus other factors, but the concept is the same).
I welcome argument on my next point: The thermostat in any engine is simply there to retain a preset heat threshold within the engine until it exceeds that tempurature setpoint (190 degrees in the above example). Once that threshhold is exceeded (rising temp), then the thermostat opens to let a given amount of cooler fluid in until the thermostat closes again (because its low temp threshhold has been violated). Once the engine (and radiator coolant) is above the thermostat opening threshhold then it will stay open allowing full flow (to the hydrodynamic stall speed of the thermostat, but we won't get into that!).
All engines need a very narrow heat range to operate efficiently (roughly 180-240 degrees) and the purpose of the thermostat is to keep the block coolant within that narrow range. Remember that we are talking about a thermodynamic (chemical reaction to heat) engine here.
Why do engines overheat?
1. Too much heat production. We are outside of the coolants ability to transfer the heat away (did we lose an oil pan?)
2. Too LOW a pressure in the cooling system lowering the point of the antifreeze/water mixtures' boiling point. (my friends problem with the worn-out radiator cap)
3. No coolant flow (or clogged flow). The coolant will attempt to "sink" as much heat as possible before it reaches its boiling (superheat/flash) point. Coolant in the block superheats while the radiator is bone-cold.
* * *
Now for the answers to your specific questions.
"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. "
The ONLY reason anyone would remove a thermostat "to make the car run cooler" is that they already had overheating problems to begin with. This is one of the worst things you can do to your engine. By not maintaining coolant pressure (applied against the "closed" thermostat), they were in effect LOWERING the pressure within the cooling system, causing the boiling point of the coolant to become lower (boil-over). I have myself experienced a situation with an '85 Ford LTD (no thermostat) in below-freezing temps to actually STOP RUNNING because I ended up hyper-cooling the coolant to the point where the pistons seized in thier bores. This happened because as I was running 70 mph in ice-cold weather, it was cooling the engine BELOW the temp it was when I started it! (I had a temp gauge and watched it fall when the engine started to miss)
There are only THREE reasons why a car WITH a thermostat will overheat. No (or little) coolant flow, a blocked thermostat or other restriction, or inadequate pressure to raise the boiling point of the coolant. We all know scale builds up in our cooling systems but the effect is the same, to plug the passages or plug our thermostat.
If it were possible (don't have me over THAT day) to pressurise our cooling systems to 3000psi then, according to my calculations, it would take roughly 7.3 hours running full throttle on a stock 302 V8 engine at 72 degrees outside temp to cause the tempurature of the coolant around the cylinder walls to exceed its boiling point (assuming 50/50 glycol/water at 72 degrees). WITHOUT A RADIATOR AND ASSUMING NO COOLANT FLOW. We'll not get into caloric heat soak but suffice it to say that the higher the pressure (density) of the coolant, the better a heat sink it is. Of course at 3000psi the coolant would be a better conductor of heat than the cast iron block!
Your educated guess:
"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."
Your REALLY close! You forgot the third part of the equation, low pressure in the secondary (radiator & hoses) section. If the block is allowed to come up to (its) normal temp and the thermostat opens to normal atmosphere (bad cap? Open cap) then you have a case of SuperHeat. If a fluid, under pressure, is heated above its normal boiling point and suddenly released to normal atmospheric pressure, then you have a case of SuperHeat (thermodynamic superexpansion by exposure to negative pressure differental for the academic types 8^).
"Aeration" has nothing to do with this (Atmospheric air combining with coolant), with exceptions to be noted shortly. The coolant will be superheated (its under pressure remember?) before it exits the block through the thermostat. Once the thermostat opens then the coolant will suddenly release its pressure, then equalise the radiator, etc and start its journey back into the block. If the radiator/hose/etc is unable to maintain the block pressure then WHOA! Superheat! (allowed to flash at atmospheric pressure).
One exception to this is a drained coolant system that is refilled (my '98 Explorer does this). I drain, flush and re-fill it and it overheats almost instantly. I have to allow the engine to come to absolute ambient temp and refill the coolant (about 2 quarts) to allow for the superheated air trapped in the intake manifold to escape (through the radiator). After that, it runs fine.
Of course, this is just my opinion, and I *could* be wrong .Comments welcome,
Tracy
'76 Maverick "Pegasus"
This is kind of a controversial topic, but why not open a can of worms 
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.
So you still want the thermostat (or a restriction). But this theory makes a whole lot more sense to me.
