Question about %HP and mixture leaning

[Flying_Monkey]

I have been experimenting with mixture settings, %HP, fuel flow, LOP, etc with my PA32-300 and JPI EDM900 engine monitor. It has been hot out recently so DA has been pretty high. I have noticed that at altitude (6500ft for example), when I lean aggressively or to LOP territory, the %HP drops off. I expect to lose some power as I get to peak and past it in the lean direction. If I start Rich of peak at like 70-75% HP by the time I get to LOP the JPI indicates 60-65%. I have to download the latest flight data for the specifics. I do not touch the throttle or prop during this time. My question is, what parameters does the JPI use in the calculation of %HP? I thought it was MAP, RPM, and OAT. How does it know the %HP reduction as none of these change?

 

[Sandpiper]

 

[philiplane]

 

[crgreener]

 

[skitheo]

 

[Flying_Monkey]

 

[By9468840]

 

[Canuck]