What does 'cheval vapeur' mean?

Answer 1

Well, literally it means "horse vapor." I'm sure you can see the possibilities from there. If it is an actual French idiom, I don't know.

Precision

Cheval Vapeur is a unit of power used in France to express the power of a vapor-propulsed engine compared to the power of a horse.

g is the accelaration, P is the power of traction of an horse and m is the mass

P = m.g.v

The European unit

m = 75 kg

v = 3,6 km.h-1 = 1 m.s-1

g = 9,80665 m.s-2

So

P = m.g.v = 735,49875 watts = 1 ch

The British unit: the Horsepower (symbol : hp)

m = 55 pounds

v = 10 foot/seconds

g = 9,80665 m.s-2P = m.g.v = 745,6998 watt = 1 hp