On Sat, 06 Oct 2007 11:40:55 GMT, Matt Whiting
wrote in :

I'm not aware of any real data on the subject




http://www.lycoming.textron.com/supp...ces/SSP400.pdf
Descent

Plan ahead to make a smooth temperature transition between cruise
and descent. Start descent early and allow airspeed to increase
within aircraft limits. Maintain power as required and mixture
setting. Cylinder head temperature change rate should not exceed
50 degree F per minute to avoid rapid shock cooling.




http://www.lycoming.com/support/tips...Operations.pdf
And finally, power-off letdowns 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 50° F. 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
ensure 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.



http://whitts.alioth.net/Pageb31%20E...ystems.htm#SC_
Shock Cooling

An aircraft engine spends much more time developing near full
power than does an automobile engine. The wear on an aircraft
engine is made shorter through negligent operation, non-operation,
corrosion, and the shocking effect of hot and cold cycles. Shock
heating cycles the metals of an engine just as much as does shock
cooling.

Heat shock can be reduced by starting the engine at idle leaning
to reduce oil dilution by excess fuel and then allowing the oil
pressure to rise before aggressive leaning. The start of an engine
its most damaging cycle of operation.

A sudden reduction of engine power after a period of increased
power causes a rapid reduction of engine heat being generated.
This heat change inside the cylinders is reflected in the heat
released by the cooling fins and increased cooling airflow through
the engine plenum. The result is called shock cooling. Lycoming
says that shock cooling results in worn piston grooves, broken
rings, warped exhaust valves, bent pushrods, and plug fouling.
Recommended cooling rate is no greater than 50-degrees per minute.

Shock cooling occurs when the pilot reduces power to off and goes
into a descent. The effect of this is to suddenly reduce the
internal heat of the engine and greatly increasing the cooling
effect of the air over the cooling fins of the engine. This may be
a damaging shock to the bimetallic cylinder blocks. The principal
effects of shock cooling are cylinder-head cracking, valve seat to
valve seat, plug to plug. Anywhere inside the engine where tool
marks, sharp edges and other stress points are liable to damage.
Any engine operation that makes it possible for the valve guide to
shrink faster than the valve will cause sticking. Valves stick
open and the pushrod bends. A raised valve hits the piston dome,
breaks and is redistributed throughout the engine and turbo if
any. This situation often occurs when poor navigational planning
causes the pilot to arrive over his destination at several
thousand feet too high. Never make descents that will shock cool
the engine. It may not fail on your but it will on some pilot down
the road.

To prevent all these bad things from happening to your engine keep
some power on the engine, re-lean during altitude changes to keep
the EGT near cruise values. If you have CHT on all cylinders
maintain a controlled (slow) decrease rate. Use of factory CHT on
one cylinder is a very poor second. Regardless, always reduce
power in increments so that engine temperature changes will be
gradual. Retard the throttle during descents. Do not enter a
descent that will both give a throttle reduction and an increase
in engine cooling air. Use landing gear and flaps to keep the
speed down. control the thermal changes of the engine to limit
temperature and cooling related damage.

When on the ground, take advantage of any cooling wind, lean the
mixture, open cowl flaps on the ground and during climb. All
engines should be run for at least two or three minutes on the
ground after a long flight to allow the oil to carry heat away
from the engine. In hot weather or with a turbo engine allow more
time. Before killing the engine run it up to 1200 and lean to but
not into roughness for 20 seconds. This will clean the plugs from
any residue of lead or carbon.