Posted for "George Braly" <gwbraly@gami.com>:

"""
 "I would not be comfortable with the timing set at 25d
 BTDC for a 10:1 CR experimental version of the 8.7:1 Lycoming engine. George"
 
 George,  is 25 deg BTDC over advanced?  Or is the engine not structurally
capable of carrying the added torque?  What is best timing (that is timing to
produce peak torque)?
"""


 Craig,
 
 Sorry for the delay in responding.

 Yes.
 
 Yes.
 
 At high MP,  Max Brake Torque on these big bore engines happens with peak
combustion pressures at around 15 to 16d  ATDC.
 
 There is a broad range of spark advance for which the torque is within  + - 1
to 2 % of  MBT.
 
 However,  that range of spark advance covers an operating spread of peak
cylinder pressures that ranges from benign to  RTTSPB  (ready to turn spark
plugs ballistic ).
 
 So, to pull a  Willy Clinton on you,  the answer to your question all depends
on the meaning of the word  “best”.
 
 If  “best” means  “the difference between 600 and 605 ft-lbs of torque,  I
may not want to be part of the discussion!

 If best means what is the best possible combination of  engine operating
parameters for  very high continuous torque and reasonable engine durability -
- then we have a lot to talk about.
 
 The recent data that came out of the trial in Texas suggests that the six
cylinder 300+ Hp Lycoming crankshafts are rather marginal in design.  From
data we have developed at our test stand,  the crankshaft torsional issues get
to be rather excited if one tries to push the peak cylinder pressures to the
15-16d ATDC range - - even if that would make more torque.
 
 I put those two pieces of information together and reach the conclusion that
if there are ways to operate the engine at high power but minimize the peak
torsionals,  then that method ought to be used.
 
 I’m not sure I answered your question.  If not, let me try again.
 
 Regards,  George