Brake mean effective pressure
From FlightSim
On engines such as the R-2800 (Convair, DC-6, and some others) and the R-3350 (DC-7, Constellation and some others), there is a torquemeter indicating system.
"This torquemeter indicating system utilizes the fixed gear contained in the planetary reduction gear assembly. As the reduction drive fixed gear moves forward on the diagonal splines on its outside diameter, in response to the torque applied to the propeller shaft by the crankshaft through the reduction drive gear, the forward thrust of the reduction drive fixed gear is counterbalanced by pressure oil operating on the gear through six torquemeter pistons. Each piston maintains contact with the fixed gear through a slipper bearing which seats on a ball end on the rear of each piston. Engine oil from the crankshaft oil transfer bearing is increased in pressure by the booster section of the front oil pump and maintained at 250-350 PSI by a relief valve. As the gear moves against the pistons, boosted oil pressure is admitted to the master piston which by its own movement meters oil to itself and to the five other pistons sufficient to just float the gear and pistons. Thus, the oil pressure against the pistons becomes directly proportional to the torque load exerted on the fixed gear by the reduction gearing driving the propeller. This oil pressure is then indicated in the cockpit through a magnesyn pressure and indicator calibrated in BMEP."
Above quoted from a DC-6 Manual, but almost identical to the Constellation Manual. I'm sure that's probably more than you or anyone wants to know, but those engines were operated almost exclusively using the BMEP Gauges.
As an example, if during run-up, one of the BMEP gauges was fluctuating, you were not to use Auto-feather during Take-off.
These gauges were also used almost exclusively during the Leaning Procedure while in Cruise flight.
skylab
See also the Wikipedia entry on mean effective pressure.
