Broomstick, you've probably got a point there - I was looking at a Gypsy Queen today (I think!), actually two of them in a Devon. They clearly have a great deal more mass, and possibly less efficient cooling, than your average small Lycoming or Continental.
These later engines with less mass and more efficient (relatively!) cooling will alter temperature at a faster rate, potentially leading to greater thermal stress and the cracking etc that others have noted. Just the same sort of thing where you can heat up a glass, then dunk it in cold water to make it crack - but if you heat it up slowly to the same temperature, then cool it slowly it won't destroy itself. Somewhere in between these two extremes will be a level of change that is acceptable for the glass, or engine, in terms of both thermal stress and practical use. There are also a number of other factors involved with engines, including changes in clearances due to temperature etc.
I'll admit that, like Chuck, I've been made more aware of this with respect to the P&W engines but from a first-principles perspective it's a no-brainer and almost any engine could be affected in some way.
As Pilots we have the ability to make or break the engine, particularly over time. Amongst other things smooth changes in throttle level and being aware of the induced temperature changes and resultant stresses involved will assist in ensuring a longer-life engine as well as improving the life chances of the Pilot & Pax. It doesn't really cost anything, and in the long run could save money, so why not?
Last edited by First_Principal; 11th Dec 2007 at 07:35.
Reason: add a word for clarity.