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

"""
    Can you please explain the math there?
    How do you get more HP at 5,000 with the same MP as sea level?
    Is that because there is less pressure at the exhaust?
"""

 Yes.
 
 The exhaust back pressure is lower.
 
 There is a whole set of curves for this published in one of TCM's engine
manuals.
 
 Generally,  for the same MP and RPM,  the power at 10,000 feet will be
somewhat higher than it is at sea level.
 
 Purely a back pressure issue, which results in the cylinder volumetric
efficiency improving slightly at altitude.