Posted for Lorn H Olsen <lorn@dynacomm.ws>:

 Jim,
 
 Back in the 70's, when I sold 421s, we were told that a sustained  throttle
reduction would cost between 20 & 50 hours of engine life. I  don't know if
the problem was shock cooling or cavitation.
 
 Have we also determined that letting the prop drive the engine  (cavitation)
is OK? When I hear the backfiring or whatever sound it  is, I give the engine
more gas.
 
 Lorn