How long it takes for a 800 kg car going from 0 km / h at 90 km / h if we assume that the machine delivers a constant power of 40 HP during this period?

1hp = 745.66 W,

40 hp = 29826.5
v=90 km/h = 25 m/s

ΔKE =P•t
t=mv²/2•P = 800•25²/2•29826.5 = 8.4 s.