Posted for Gary Casey <glcasey@adelphia.net>:

 With great trepidation I'll disagree with George, if ever so  slightly.
 According to my thermo books the Otto cycle is defined as  one with equal and
constant volumetric-ratio compression and  expansion AND a constant-volume
combustion.  The Diesel cycle is  defined as one with constant-pressure
combustion.  In the real world  neither work exactly like the definition.  So
neglecting heat  transfer and combustion time the thetaPP should be at TDC in
order to  satisfy Herr Otto.  Heat transfer between a gas and a solid can be
 extremely high under high pressure and turbulence, so that alone  works in
the favor of burning as much of the fuel as possible ATC.   Then in order to
get the fuel burned in a reasonable time the timing  has to be advanced so
that the efficiency is optimized while NOT  neglecting heat transfer and burn
time so it turns out that for a  reasonable range of engine speeds and loads
the theoretically correct  thetaPP does, in fact, turn out to be about 16ATC
(back in agreement  with George).
 
 There were a few other questions:
 
 Regarding the need to change ignition timing with speed it turns out  that in
normal operating speeds (like above 2000 rpm) most engine  will exhibit
turbulence and swirl rates roughly proportional to rpm,  increasing the burn
rate in proportion and therefore reducing or  eliminating the need for timing
changes with rpm.
 
 How much does a Lycoming lean out as the altitude is reduced?  From  about
15,000 feet down to 1600 feet (pattern altitude?) the air  density changes to
150% of what it was and the RSA system is affected  by the square root of air
density, meaning that if the original A/F  was about 15.5 (50 LOP??) it would
be 23% higher or at 19:1.  That is  an air/fuel ratio that is theoretically
combustible, but is probably  at the lean edge.  The engine will likely accept
throttle, but it is  possible that it would quit or at least run roughly.  A
Continental  system without altitude compensation would change the full 50%
and  would more than likely quit at low altitude.
 
 I certainly agree with Brent in that with a turbocharged engine most  of the
operation will be done at constant manifold pressure, negating  any advantage
from most EI systems.  If I had a turbocharged engine I  would likely keep 2
magnetos.  The lightning strike problem is real  and I don't know any way to
protect an EI system absolutely.  I'll  stick with one magneto as it is very
well protected against lightning  strikes.
 
 Why would the EI system mentioned by "configured" differently for
 turbocharged engines?  It's a mystery to me, maybe it's because most  turbo
engines happen to be configured to run closer to the detonation  limit?
 
 I think it was Brent that was right in clarifying the one-spark  operation
correctly by saying that the timing stays the same, and  even the flame
velocity stays the same, but the time for combustion  increases, resulting in
a later thetaPP, making the result similar to  retarding the ignition.  The
detonation margin is significantly  REDUCED when running on one ignition as
the end gases are compressed  and heated for a longer period of time.
 
 Gary