With thanks to Hedley Cox and Dai Gibberson.
What does this calibration factor mean?
These Heenan and Froude dynos were called “water brakes” and utilised hydraulic principles with the power developed by an engine on test being measured in the pull on a spring gauge of pounds (lbs).
So the dyno operator..at Veloce, Tommy Mutton, Frank Panes, Hedley Cox,Freddie Owen, Jack Passant etc…would record pull of the dyno in lbs at a given engine rpm and record the barometric pressure and temperature in the room, which was assumed to be the induction air temperature.
This was recorded in a small notebook and occasionally it would show the uncorrected bhp, that is, not corrected for the air temp. and barometric pressure of the day.
The formula was….