Not that it's gospel but....

http://www.lycoming.textron.com/supp...enCooling.html

"And finally, power-off let downs should be avoided. This is especially
applicable to cold weather operations when shock cooling of the cylinder
heads is likely. It is recommended that cylinder head temperature change not
exceed 50oF. per minute. Plan ahead, reduce power gradually and maintain
some power throughout the descent. Also keep the fuel/air mixture leaned out
during the descent. If an exhaust gas temperature gage is installed with a
normally aspirated engine, keep it peaked to insure the greatest possible
engine heat for the power setting selected; for a turbocharged installation,
lean to peak during descent unless otherwise specified in the Pilot's
Operating Handbook, or under conditions where the limiting Turbine Inlet
Temperature would be exceeded."


And from AvWeb....

http://www.avweb.com/news/maint/182883-1.html

"My own gut tells me that shock cooling-while bound to induce dimensional
changes in the engine-is not a great contributor to cylinder cracking. We
know it induces dimensional changes, because (for example) valve sticking
has been induced in some engines by sudden power reductions. (A Lycoming
Flyer article once stated: "Engineering tests have demonstrated that valves
will stick when a large amount of very cold air is directed over an engine
which has been quickly throttled back after operating at normal running
temperatures." See 101 Ways to Extend the Life of Your Engine, page 96.) But
it's a big jump to go from that to saying you can make a cylinder head crack
just by pulling the throttle back too quickly."


Given that cylinders are 1k each, I'll keep the temps up on decent when
possible. I mean how hard is it to pull back to 2,000 RPM instead of
idle??? Yeah, it's probably a waste of gas but I'm burning the cheap stuff
anyway.


Just my $.02



"Dennis O'Connor" wrote in message
...
I shock heated my engines again on Sunday... This is ~ 5,800 times on the
left engine and 6,400 times on the right engine that the metal has been
taken from ambient temperature (a balmy 28f degrees sunday) to some
1200-1300 degrees F internally in ~10 milliseconds... I watched with
incredulous eyes as neither engine had a jug fly off, nor seized up, nor
started a death rattle, nor immediately dropped compressions into the
40's...

Then upon landing I reduced the throttle and watched as the EGT took some
5,000 milliseconds to to begin to decrease and the CHT's barely dropped
until I was half way back to the hangar on the taxiway... I obviously
shock
cooled it into oblivion...