Patterson's note of the chart referenced from altitude, as opposed to
manifold pressure, causes me to scratch my head, too.

May I ask why there is a limitation? I'm sure its not good to lug the
engine, and its impossible to lug an engine having a fixed-pitch prop.
But wouldn't the damage come from extra heat generated? So the temps
would change, but they could also be monitored. Yes, Lycoming knows
when their engine will overheat, but environment is not isolated from
it. Are rpm and manifold pressure so isolated to itself?

--Mike

Of course you can lug an engine with a fixed pitch prop. However, if you're
flying a certified airplane with the correct prop, you shouldn't have that
problem. In the experimental world, fast aircraft with cruise props (i.e.
RV's and the like) have real concerns with this, particularly during takeoff
and initial climb.

KB