To me, that is the environment ALL the time.

You have ~700 degrees around the combustion chamber with ~1000 degrees EGT's. (Don't quote me on the #'s)

Then, you have ~200 degrees of coolant above this area (normal temps).

So overheating is what, another ~100 degrees tops in the coolant area and maybe ~100-200 more around the combustion chamber? ~100-200 degrees of EGT?

I feel that overheating is kinda like splitting hairs when it comes to the integrity of the parts. Our jet engine parts are not scrapped due to overheating, just tested and reused if crack free.

Lets be realistic, if an engine runs out of coolant, it will start to detonate so badly that it will not pull itself around anymore. I know this, I have done it to an '87 Camaro and many a times in old beater trucks off-roading.

I highly doubt that a Porsche motor would even be subjected to this because the coolant warning light would of came on long in advance and the person with any brains would of stopped the engine in time.

Just my thoughts.

 

cooling system the temperature difference between various areas of the head (ignoring the surface of the combustion chamber and the exhaust valve seats and the alum. material immediately surrounding them), and block, are minimal. Coolant is say at 200 degs. F. And this is exit temperature. The heads are cross flow design so the cooler coolant incoming from its trip through the radiators flows to the hotter areas first and then to the cooler areas maybe even warming them up.

While valves (especially exhaust valves which can run a dull red color from the heat) are hot this relatively small amount of heat is transferred to the valve seats which have much more mass to transfer their heat load to. And there is considerable coolant flow (cool coolant as I mentioned above) flowing past these areas.

And all under pressure to avoid the fluid boiling.

In the event of a coolant loss pressure is lost as well. What coolant remains in engine, at least the hotter areas, flashes to steam. These pockets of steam prevent whatever coolant there still remains in the engine from flowing over the hotter parts.

While the surface of the combustion chamber may be quite a bit hotter than the coolant, the depth of the heat is minimal as the heat flows through the head material to the coolant passages and coolant just a fraction of an inch away.

However, with flow blocked by steam pockets, that heat extends way into the head material. It extends deep into areas that were never intended to experience that kind of heat load. Uneven heating can lead to cracks forming.

To add unsult to injury some cooler coolant may be forced over these hot spots. Thus these areas are subjected to sudden cooling.

None of the above is any good for the engine.

Additionally with coolant flow impeded and what superheated coolant there is may not even make it to the temperature sensor. The sensor can't register an overheating condition because it needs to be immersed in coolant and the water pump was never intended to pump vapor (steam).

As for detonation if this is detected the ECU will back the timing down. The engine will almost certainly continue to run and the driver may not even be aware of this timing retardation. If the ECU has to go too far, it will set the CEL engine light. Or if the combustion process is so affected by localized overheating the engine develops a misfire, the CEL will be lit. In fact, in the case of a misfire the the CEL will flash because misfires feed raw (unburned) gas to the converters and they can't handle the additional heat for very long.

(Word is, though I've never tested this and no one I know of has, but word is the ECU has a limp home mode that even in the event of say a loss of coolant it will alternate disabling cylinders by not injecting any gasoline to reduce the heat load the engine developes. However, I wonder how well this can work given the coolant temp. sensor may not reflect the temperature of the engine accurately. 'course, there's the oil temperature sensor and perhaps this is used to signal the ECU to enter this limp home mode, if it even really exists. Bosch documentation says it does... but.)

I repeat, one can rebuild any engine he wants to, but using an engine, a modern engine, a Boxster engine in this case, for a rebuild that has experienced an overheat condition is just asking for it.

Sincerely,

Macster.