[Gaz64]

From an early book of mine as an apprentice:

Auto-mechanics, second edition, Herbert E. Ellinger, printed 1977.

Indicated horsepower = PLANK/33000.

P= Average indicated pressure
L= Length of stroke in feet
A= Area of cylinder cross section (square inches)
N= Number of power strokes per minute
K= Number of cylinders

Typical V8: 150 x .3125 x 9.62 x 2500 x 8 / 33000 = 273.29 hp


1 "horsepower" is determined from a horse could move 33000 lbs one foot in one minute.