<<what about 2100 and 25" with 10 to 1 pistons in a parallel valve IO-360
???
tom williams>>

Well, that's a different story - I would be worried about the detonation
margin with that set-up.  I heard that Lycoming revised the approved timing
of 8.7 compression spread-valve IO-360's from 25 to 20 because of the
detonation margin with that engine (but for 90 octane or 100?  I'm not
sure).  If you use fixed-timing mags would it make sense to retard the
timing a little?  Actually, I would think that if you run either full rich
or LOP you are probably okay.  But that's just a guess without knowing the
actual detonation margin.  Interesting question, though, as I plan to run a
IO-540 parallel valve with 10:1 compression.

Gary Casey