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

 Gary,
 
 Please.... absolutely no trepidation required.  I screw up and get
 things wrong, all the time.  So feel free to jerk my chain when you
 think I have stepped off of the reservation.
 
 However,  in this case,  it is not my theory.   The 15d ATDC as optimal
 for location of peak combustion pressure in internal combustion engines
 is a published thermodynamic "proof".   I first read it about 7 years
 ago.  I have looked around,  but do not find it, handy, or I would give
 you a reference.
 
Frankly,  the math was over my head in the proof, at least for the
 amount of time I was willing to devote to reviewing the matter.    But I
 could sure as the devil appreciate  the benefits to be realized by
 further investigation.
 
 Real world testing shows that the 15d ATDC in the theory conforms to
 reality to a remarkably close tolerance.  For very high RPM engines,  it
 appears to be down at 13 (+-) degrees.   For some slower turning
 engines,  it may be around 16 degrees.     As it turns out,  at very
 high power settings,  the delta Hp across a  theta-pp varying from 13 to
 17 degrees is VERY small (holding MP, RPM, & F/A constant).  By
 contrast, the magnitude of the delta in the peak pressure is not
 insignificant at all.
 
 Your comments about turbulence as a function of RPM are spot on, as well
 as most of the balance of your comments.
 
 I'm not sure the RSA metering responds to  air density ^.5.   That is an
 interesting way of looking at the issue but I have not seen that before
 and I don't know why it would actually be set  up that way,  if they
 have done so.   Obviously,  by design,  the RSA system is responsive to
 dynamic pressure in the fuel metering servo as the first input, but I'm
 not sure how the internal fuel metering is scaled off of that input
 variable.   Which is why I was asking for some more information and
 flight data on that subject.
 
 Regards,  George