If I may, let me add a little science to the discussion of egt and it 's effects on pistons and coolant temperatures.
I'm not taking sides or discounting anyone, just describing the mechanisms behind it. I'm going to try and keep the language plain english, so some of the examples i use may not meet strict engineering accuracy.*
What many average Joes don't quite get a grip on is the dynamics inside a combustion chamber in a compression style engine, be it spark or compression ignited.
The thing relevant to this discussion is a property known as "boundary layer". *In a nutshell, this is an insulating layer of "air" that keeps the actual combustion process off the metal in the engine. This is why egt's in excess of the melting point can be sustained in the engine.*
What kind of happens is the ignition of the fuel source creates a wave of air in front it. It burns some of the air in the cylinder and displaces some (ie: the wave). *Since it's in a closed cylinder, it expands in all directions trying to fill the volume of the container. This wave reaches the limits of the container first and "piles up". *The combustion hits this layer and since it was not mixed with the fuel it insulates. Air not mixed with fuel does not burn. This layer insulates the cylinder components the same way a double pane window or fiberglass batting insulates your house.
An interesting fact is that the boundary layer is thicker at the piston crown and the head chamber than the walls. This is also important in understanding the cooling system impact. The crown is (within limits) protected from temperatures above aluminum melting point by the layer, as are the heads. Things like piston oil cooling jets or ceramic coatings further improve heat handling capabilities. Bit this boundary layer effect is how components can survive temperatures seeming over the temperature limits of the parent materials themselves.*
However, with respect to the cooling system it is important to note that the boundary layers at the cylinder walls are as much as half the thickness of the crown/head. There's various reasons for this that i won't bother going in to here. But what is important to us is that this means more heat is tranfered to the walls than the crown/heads due to lessened "insulation" properties.*
Fortunately, with the cylinders surrounded by the water jackets it can transfer the heat away relatively quickly. The harder you work the engine, the more heat is tranfered away by the cooling system and this is evident on the water temp guage. Run it too hot (more and more fuel for longer periods) and the cooling system starts to become insufficient. Then things start to repond to the thermal load. *Lots of things happen, bit the main impact to joe driver is piston to wall clearance eventually reduces to the point of scuffing. Push it too hard too long and failure is going to happen.
You can melt pistons from combustion only, but it is highly dependent on the insulating properties of the boundry layer. Get too hot and more heat makes it across the layer, same as a window begns to get colder on the inside payne as the temperature continues to drop outside.
Like anything, run it too hot and you can eventually exceed the ability of the physics inside the engine to protect the parts and they fail. *"Melted" pistons are more a function of improperly mixed fuel that makes it to the crown in liquid form vice atomized in the injection event. This is far more likely to be caused by a malfunctioning *injector than anything else although there are other ways for liquid fuel to make it to the crown. Cracked crowns are often a function of timing errors (ie
ver advanced) or life cycle fatigue. Cracked crowns cited as overheating are often misdiagnosed faults or are caused by scuffing/siezing events and the resulting forces.*
Now, outside of the cylinder there is no boundary layer so you have to rely on the properties of the materials themselves or whatever cooling methods that are engineered into the component. That may very well be your limiting factor for egt vice the combustion chamber.
So, keeping egt's lower is a good idea but maybe not for the reasons that people have commonly thought.
*
Heres an abstract from a technical paper if you want to give it a once over and come to hour own conclusion: *
http://www.sciencedirect.com/science/article/pii/0017931084901698
Remember: theres always more to it all than an abstract from one little paper....
