Posted for "George Braly" <gwbraly@gami.com>:
 
 
 Mark,
 
  
 Nobody  with knowledge on this subject is saying that  shock cooling can
 not be created under extreme conditions.
  
 
 However,  if one first deals with the issue  of  "how hot is 'HOT' "  -
 - - and one appreciates that one cannot shock cool a cylinder that is
 not HOT to begin with,  then one can begin to put some rationality into
 the evaluation of the subject.
 
 
 
 15 years ago,  there were a lot of people and engine shops  that openly
 advocated routine operation of our engines with CHTs up around 420d F.
 Or  410 or 400.    But nobody thought continuous operation at 420d F was
 a "bad thing".
 
 
 
 I think some serious re-evaluation of that notion has taken place.
 
 
 
 There is an ever growing  overall  view that something at or under the
 380d F range for continuous operation is much more appropriate.    Is
 that "HOT"  in the context of engine operation ?    Probably not.  Is
 420d F "hot" in this context,  yes, it probably is.
 
 
 
 IF one has the CHTs under good control in cruise at 380d F or so,  and
 one needs to come down hill - -   then major changes in the throttle and
 RPM do not appear to be causing problems.
 
 
 
 My preference is  to first roll the RPM back to 2200 or 2100 RPM to
 begin to slow down,  and then,  not much later,  go ahead to reduce the
 MP substantially.   At that point further reductions in RPM to the
 bottom of the green for RPM and MP are really not a big deal.  I
 routinely make descents from 17,000 feet  with the RPM at 1800 and the
 MP at 15 or 16" .   I seldom touch the mixture control.
 
 
 
 Regards,  George