In more detail
One mechanical (imperial) horsepower is 550 foot-pounds per second, about 745.7 W. The metric horsepower (PS, CV or ch) is 735.5 W, about 1.4% smaller, and many official specifications now give kilowatts. For an engine, hp = torque (lb-ft) × rpm / 5,252; the constant is 33,000 / 2π, which is why torque and horsepower curves in lb-ft and hp always cross at 5,252 rpm. Multiply hp by 0.7457 for kW.
Why it matters
Manufacturers usually quote power at the crankshaft; a chassis dynamometer measures at the wheels and reads lower because of drivetrain losses, so the two figures are not directly comparable. When comparing cars across markets, check whether a number is hp, PS or kW. Acceleration depends on power-to-weight ratio, not on power alone.
Example
300 lb-ft at 5,000 rpm: 300 × 5,000 / 5,252 = 285.6 hp = 213 kW
Same engine in PS: 285.6 × 745.7 / 735.5 = 289.6 PS
Trap-speed estimate, 3,500 lb car at 100 mph: 3,500 × (100 / 234)³ = 273 hp