Originally Posted by MACDRIVE
You guys have to remember that the coolant needs to stay in the radiator in the path of on comming air for a certain length of time, to let go of its heat. If the coolant was constantly moving through the radiator without a thermostat, it wouldn't have enough time to cool off.
I'll dissagree with you on this in the strongest possible terms. Here'* why (I'll try to make this make sense, Thermodynamics wasn't my area of study)
Ok, first thing to remember: Heat energy can only flow from a high temp area to a low temp area. If both materials are the same temp, then no heat energy is transfered regardless of how long they remain in contact with each other.
The transfer of thermal energy from the coolant fluid to the aluminum is very efficient. Aluminum is a great conductor of heat energy. The transfer is instantaneous so long as the aluminum is cooler than the fluid. The transfer of heat from the aluminum to the surrounding air is poor. Air is a pretty good insulator.
Mr Science experiment for the doubters: Take a cheap aluminum funnel and toss it in the freezer. Takes a few minutes to get freezing cold because the transfer of energy from the funnel to the air in the freezer is poor. Now take the funnel, hold it in your hand, and pour boiling water through it. It will become too hot to handle in a mere second. The transfer of heat from the water to the aluminum funnel is that efficient.
The best analogy I can come up with is putting the family to work cleaning the house with little trash bags. Each person fills their bag with trash, then walks it to the curb and tosses it on the growing pile in the trash can. When the can is full, they can't put their trash bag in, and carry it back into the house like a lemming to try again. No mater how long they stand at the curb, staring at the full trash can, they still can't toss their little bag in. When the garbage truck comes along and empties out the trash can, they can resume dumping thier little bags of trash into it again. Ok, this is pretty silly.
That'* the way the coolant system works: Each droplet of coolant picks up energy from the engine till it is as hot as the metal it is in contact with. Then it imeadiately starts looking for a cooler material to transfer that energy to. If the aluminum of the radiator is cooler than the fluid, the energy get'* transfered. As soon as the aluminum is the same temp as the fluid, no energy is transfered and the droplet of coolant has no choice but to keep on moving. It doesn't matter how long it stays in the radiator...if it can't give up it'* energy to the aluminum, it just can't. And the speed of the flow through the radiator doesn't matter, cause it'* a contiunous bucket brigade of hot droplets trying to transfer their heat to the aluminum of the radiator. The only thing that matters is that the radiator can dissapate energy to the air faster than the engine can generate it. The coolant can be flowing at 10 gpm or 1000 gpm, it doesn't care. (Yes, the more time it has in contact, the more complete the transfer can be, but that'* not important because it'* a closed loop...there'* always another droplet of coolant in line behind it...so the net flow of thermal energy from the motor to the radiator is constant regardless of how fast the fluid is flowing.)
I hope this is making sense. I've heard this myth a few times, and at first blush and observation it does apear to be a truth, but what the observer is seeing and what was really happening were too different things. The no thermostat boil-over problem didn't begin in the radiator because of too fast a coolant flow not being able to dump it'* heat to the radiator'* aluminum. The real problem was hiding in the back of the block in the form of super heated steam created by zero coolant flow back there. When that steam finally filled the block, the super-heated steam bubbles went out the water neck, hit the cold radiator, where they expanded and popped the cap..too much thermal energy hit the radiator all at once..more than it could dissapate. All the while, the water pump is circulating the rest of the luke warm coolant like crazy...it just never came in contact with the hot parts of the block to transfer any heat energy from the block to the coolant and on to the radiator.
Same thing would happen if the water pump stopped moving coolant at all (like chucking a belt off the motor.) It would run along for another 30 seconds or so, then the temps would skyrocket as steam from the block finally hits the radiator.
If the system is circulating coolant properly, a boil over occurs only when the engines heat output outstrips the radiators ability to dissapate that heat. And it is a gradual problem...you can watch the engine temps slowly rise higher and higher, till steam starts forming everywhere and the cap on the radiator pops.