"I have an instrument rating and sometimes ATC will hold me at altitude too long, which requires a quick descent. Would like suggestions for descending safely while not shock cooling the engine." - Daniel T.
“If ATC is holding you beyond the point where you feel you should begin the descent, start reducing the power by about 2” of manifold pressure per minute. This will allow you to make a rapid descent when the time comes without exceeding airspeed limitations.
Plan the descent in an unpressurized airplane based on 500 to 700 feet per minute for passenger comfort.
Remember that you have the option of discussing the descent with ATC in a slam dunk situation. One option is to ask for delaying vectors so that you can get down with a reasonable vertical speed.
Reduce power gradually and don’t worry about shock cooling. But try to avoid going all the way to idle in consideration of vacuum pressure and the gyros that depend on this pressure.
You can extend the landing gear on retractable airplanes as well as flaps to increase drag, but be sure to respect the operating limitations regarding landing gear and flap extension speeds for your aircraft.”