"G.R. Patterson III" wrote in message
...

There's another chart in the manual that relates combinations of manifold
pressure
and rpm to horsepower (but not to altitude). It has a similar limit band,
but this
one makes sense to me. It basically says that you should never exceed a
manifold
pressure of 25.3 HG at 2200 rpm (for example). I don't know why that limit
band is on
the altitude/hp chart.

I think you're looking at Figure 3-6, right?

The chart on the left is a sea level power chart. The chart on the right is
a full-throttle power chart. Both show manifold pressure (MP) vs RPM as
coordinates. The difference is that the chart on the left has MP lines
vertical, the chart on the right has the MP lines sloping a little, because
at full throttle you get slightly more MP at lower RPMs. In either case,
there is a maximum MP that you can apply for a given RPM. Why the maximum
RPM at altitude is less than at sea level is left as an exercise for the
reader. ;-)

I'm hoping that it's something that only applies to CS props.
In that case, perhaps the chart simply means that it is impossible to
exceed a
manifold pressure of 25.3 HG at 2200 rpm above 6,000' ASL but below that
the pilot
needs to be careful?

It applies to the engine. But fixed pitch props are normally chosen so that
it is impossible to exceed the maximum (full throttle) manifold pressure for
the RPM achieved at any altitude. Part of the role of the line is to guide
the selection of prop. So if 29" of MP at sea level doesn't give you at
least 2400 RPM at all (flying) speeds, the prop pitch is too coarse.

Julian Scarfe