Piston engine (G36) too powerful at low altitude, not powerful enough at altitude

Hi All I am trying to dial in the G36 performance figures as part of my work
on the G36 Improvement Project.
https://github.com/50North4West/MSFS-G36-Improvement-Mod The default version
of the aircraft and my own modification suffer from having too much power at
lower altitudes and not enough power at higher altitudes. As can be seen in
the graph below both my mod and the default loose airspeed at the same rate
more quickly as we climb than the actual IRL aircraft / POH.

I can tweak the performance in
the engine.cfg to get one or the other as close as I can but this then makes
the other even further off. To start with I wondered whether it was a manifold
pressure problem but this is pretty close to reality
What I wonder, is if it is
related to fuel burn? As you can
see in the graph above the fuel flow in the actual aircraft reduces after
peaking; then after 11000ft or so, the rate of drop, slows. However the sim
aircraft fall back at a fairly consistent rate. The lower altitude matches
reality somewhat so I wonder if there is some modelling in the piston engine
that is slightly off? Or, I am missing a crucial piece of the puzzle. Can
anyone point me to the right CFG entries to try and tweak? Many thanks Matt

Yeah ive noticed the new prop is way too powerful down low and slow, and too
weak high and fast. I can get 1500fpm climb in the DA40 but TAS up high is
like 10-15 knots slower

I figured it’s possible to improve high altitude performance by lowering
engine friction AND (logical “and”) decreasing mechanical efficiency at the
same time. Off course, that doesn’t fix the propeller behaviour, but you can
use it to offset the power loss due to the propeller. Another approach would
be to decrease Cd to a more realistic value, as many asobo airplanes have way
to high Cd to offset missing propeller drag. You will however get unrealistic
gliding performance due to way to low prop drag this way.