Mohit_C - taken from Sir Stanley Hooker's book.
A=K*B/C(D-E/F)
A= Charge Weight (pounds) Comprises 93% Air 7% Petrol. For every pound/min of charge consumed 10.5 horsepower is produced. From this to find the rated power of the engine you have to subtract the horsepower consumed in overcoming frictional losses and power needed to drive ancillaries (eg supercharger).
B= Revolutions per Minute
C= Charge Temperature (degrees Kelvin)
D= Boost Pressure (inches Hg)
E= Ambient Air Pressure (inches Hg)
F= Compression Ratio
K= Constant applicable for the particular engine in question. Is related some how to the bore, stroke, number of cylinders (ie capacity) of the engine. Unable to deduce (too dumb) how the number is derived exactly, but the number for the RR Merlin is .422