<<I have 10:1s in my IO-320.  Got them from Lycon.  Steve Mehalek of Lycon
told me to NEVER (his emphasis) run any leaner than 100 ROP, 150 would be
better.  Q-Tip>>
<<the question was:  what about 2100 and 25" with 10:1 pistons???  tom
williams>>

I certainly can't call myself an expert, but I believe that the octane
requirement curve peaks at about peak EGT to 50 ROP.  The octane requirement
also goes down as the rpm goes up and goes up as the manifold pressure and
inlet temperature goes up.  And, of course, is higher with higher
compression and increasing spark advance.  With all these variables there is
probably no simple answer and the problem is that you may or may not hear
the detonation that will kill the engine.  As long as the octane requirement
is less than the octane rating of the fuel you have a detonation margin.
The trick is to run at the minimum detonation margin, which gives the best
power and lowest fuel burn; a little like trying to walk as close to the
cliff as possible blindfolded.  So how do you decide how close to walk?  You
could always take off the blindfold by using something like the PRISM
system.  Or use the experience of others to be the guide.  It's just a
guess, but I would think that operating 50-100 LOP would give about the same
detonation margin as running 100-150 ROP.  Max power is about 150 ROP and
max fuel economy is at about 50 LOP.  Carefully match the air/fuel ratio of
the cylinders by trial-and-error or by using GAMI injectors and you should
be in good shape.  It's not my engine, though.  Still didn't answer the
original question, did I?

Gary Casey