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

 Craig,
 
 Having done a full blown  FAA  type certificate block engine run one of these
engines  - - -   with internal combustion pressure sensors monitoring the
combustion pressures, 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.
 
 Regards,  George

"""
 I'm willing to bet at a 5 deg retard you are giving up power and fuel
economy.  Unless you are detonation sensitive, I would expect no more than a 2
deg retard on a 27 deg baseline for a 10 to 1 CR motor.  I would be happy with
the 3 deg shift.
"""