BMEP Indicator, single-engine, as used in DC-6 / C-118
Here is a Brake Mean Effective Pressure indicator as used in the Douglas DC-6 airliner and its USAF military transport version the C-118 Liftmaster, which had 4 of these indicators, one for each engine, installed in the center portion of the flight instrument panel between the pilot and co-pilot. BMEP is the average (mean) pressure which, if imposed on the pistons uniformly from the top to the bottom of each power stroke, would produce the measured (brake) power output. See photo for instructions on its use. Measures 3.25in diameter and ~3.25in deep.