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Originally Posted by 67chevemall
Just wondering if your oil boils and burns when the rad goes off smoking like a fiend.
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I'll take a stab at it...
No, in
most circumstances your oil does not actually boil when you overheat. Assuming your antifreeze to water ratio is correct and considering the cooling system itself is under pressure, the boiling point of the coolant is
well below the boiling point of your average S.A.E. 10W-30 motor oil. I say "most circumstances" because it is
possible to boil oil inside an engine, but it's not very
probable with even the most exotic automotive engine.
Why? There are a surprising number of factors that prevent boiling, but the first real problems we encounter as we begin to approach the kinds of temperatures required to boil are viscosity and thermal breakdown. This is due, to a large extent, to the high polymer content in multi-viscosity oils. At high temperatures, these polymer additives burn, create nasty deposits, and even trigger a thermal failure of the oil. That's why the general recommendation is to select a multi-viscosity oil with the narrowest acceptable range. In other words, 10W-30 contains fewer polymers than 5W-40 and therefore has a higher tolerance to temperatures outside the norm which is generally considered preferrable.
Let's say, however, that you manage to get around polymer breakdown. Now you've got to contend with the oil's flash point. The flash point is the temp at which an oil gives off vapors that can be ignited. You know those fires they tell you about where grandma's whipping up some fried chicken and burns down the block? A likely culprit is cooking oil heated enough to exceeded the flash temperature which gave off vapors that were then ignited by an open flame.
Most motor oils have a flash point between 400-500 F. When you exceed the flash point of the oil in an engine, you will begin to vaporize the oil itself in the highest temperature regions which, not by coincidence, leaves things such as the piston rings to deal one-on-one with things such as the cylinder wall. This vaporization could be confused with boiling, but it's really quite different. Regardless, it's not likely you'll be maintaining a rotating mass capable of generating the extra 100-200 F required to actually
boil the oil in a pressurized crankcase once you've started mashing metal to metal.
I'm sure someone will correct me where I got my facts mixed up. Until then, I'm going to bed since I just realized how much all this typing makes me look like an arrogant ass.
