Originally Posted by sony2000
In quoting me you dropped the word "overall". This term is to encompass efficiency related to
1. maximum range on available fuel.
2. a level of RPM that causes above normal wear on the engine.
3. time to destination.
4. maximum torque.
5. hull speed.
An over propped engine ends up operating at a higher manifold pressure, resulting in a more frequent, destruction of head gaskets.
I am confused here. You talk about manifold pressure. Now this is relevent to a superchrged or turbocharged engine but NOT a normally aspirated diesel as there is no form of restriction to the inlet manifold opening; it runs wide open all the time.
Also I do not see how time to destination figures into this. I wonder if you are using data from an aircraft fitted with supercharged petrol engine fitted with a device to control mixture strength and boost pressure plus a variable pitch prop.
Finally in my experience almost all cylinder head gasket failures can be traced to engine overheating from coolant failure, some are corrosion related.