I was examining the guts of the PT6 gear box at Reedly
College yesterday and I got this idea based on the way the PT6
gives a torque read out on the instrument panel.
The ring gear in the PT6 has a helical spline on the outside
that reacts by moving slightly longitudinally to the input torque.
This moves a  piston in a cylinder filled with oil. The oil pressure 
is then proportional to torque. Very simple.

We can do the same thing with an annular piston around the input
shaft that will react to the thrust of the Ford helical sun gear.
Filling this cylinder with oil and connecting the oil line to a pressure gage
on the instrument panel will read out oil pressure and hence engine torque. 
Knowing torque and RPM you know HP.

HP is Torque times RPM divided 5252 as I recall.

Taking this one minor step further you could read out HP directly
if you had a one dollar single chip computer, an RPM input and a 
pressure sensor.

Knowing the HP is very useful as then you can judge the amount of drag your 
airplane has. If you go x speed with y HP you can use that as a base
line while you attempt to reduce the drag.

Paul Lamar
 
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