Panic asks, "7 psi of steam pressure will blow a head gasket - but 800 psi of combustion pressure won't?"
Dynamics of this do not quite work that way. Pockets of super heated steam gradually erode the weakest points. This is the head gasket joint, head gasket material itself. Effect is much like the old fashion way of installing finished upholstery. This is to dump your upholstery material into a steamer, this softens the upholstery material which is then quickly slipped over a seat frame. This material cools and shrinks for a snug fit.
Contrasting this, steam within a head repeatedly softens head gasket material, including copper, slowing eating away at the integrity of the gasket through repeated expansion and contraction beyond tolerance limits. In time, a leak develops. This is your common blown head gasket; coolant and steam come out of your exhaust pipe.
Another factor is you are forgetting a liquid, such as mixed water and coolant, is highly compressible. Most of the pressure from a steam pocket is simply absorbed by the coolant compressing. Should pressure become high enough, a radiator safety valve will open releasing both pressure and coolant. This is the need for an overflow tank, beyond normal expansion of coolant.
You write, "It's a closed system...." Not quite. Closed until pressure exceeds the safety valve rating in a radiator cap. Same principle as freeze plugs which pop out under pressure from a 10% expansion of the _water_ part of coolant freezing.
Compare this to the inner workings of a cylinder and a piston. We all know the four strokes. Difference here is pressure is deliberately created and there is no liquid to absorb this pressure, just steel pounding out horsepower and torque. Good quality head gaskets have a copper ring or a steel ring around each cylinder opening to reinforce this area. This prevents compression from blowing through. Cylinders, pistons and head gaskets are designed to handle this pressure.
Water passages are not designed for high steam pressure. However, it is this slow erosion of a head gasket around water jackets which is the problem; a head gasket slowly weakens.
Yet another factor is where a steam pocket exists, a head and a block are super heated further causing gasket degrading. Steam does not conduct away heat, not like a liquid. Where this steam pocket exists, is much like have no coolant; the metal becomes super hot. Exasperating this is a steam pocket will block coolant flow like vapor lock in a fuel line. This further adds to overheating in specific areas.
A comparison is hot water sitting in your water heater. Calm and quiet when heat is not applied although water may be circulating, a valve open somewhere. When the gas fires up, your water heater takes to gurgling; pockets of steam. Enough steam will cause your spigot or shower to blow out pops and bursts. Home water heaters have safety pressure valves just like a radiator cap, and for the same reason.
All of this circles back to why some heads have steam holes. A manufacturer, such as GM, invests a lot of money into design, foundry work, casting molds, machine work and such. Designing a new engine costs millions. To save money, GM will use an existing engine model and make modifications which only require machine work, maybe drilling, to allow a change, such as more cubic inches, more horsepower, whatever. There is always drawbacks to changes like this. Clearly, steam holes are added to a basic head and block in anticipation of overheating problems. What is really needed is a redesigned head and block but this is too costly.
A classic example of this, a mistake which is truly beyond understanding, is the original GM production V-8 engine, the 265. Production of this engine only lasted two years, 1955 - 1956. This short of a lifespan is unheard of. Primary reason for this is lack of a facility for an oil filter. In 1957, GM introduced its 283 engine which has the oil filter facility we all know so well; bad location, hard to change but at least on a vertical axis. GM could not easily add an oil filter facility to a 265 so no choice but to redesign this engine; the 283.
Panic, addition of steam holes to the 400 small block and to the 261 inline six is a quick fix, an afterthought. This opened up a wider market by offering more power, but overheating was an inherent problem caused by cutting corners or, in this case, drilling holes. The 400 block did not work out so well, became an unpopular engine because of overheating. The 261 inline six became very popular simply because overheating has never been a problem for the 235 nor the 261; heavy metal quality production.
An odd problem with the 235 / 261 engine is both are known for running too cold, hard to warm up. This is a benefit and a factor in why those engines seem to last forever. Mechanics behind this is both inline six blocks are thick and well made. However, the 235 does have a longer life expectancy than a 261 because of a slightly lower operating temperature for the 235, but when a well tended engine lasts for a century, who cares about a five or ten year difference in life expectancy?
We do care when a steam holed engine blows a head gasket every five years or so, as with the 400 small block. Fix is very simple, build a 383, a highly popular engine for a clear reason.
Billyray
Last edited by Billyray; 03/25/2010 10:28 AM.
