How do you define horsepower? Ask a car enthusiast and most of the time you'll get a blank look, a shrug of the shoulders and maybe a guess along the lines of "What a horse can do!".
That answer begs the question: What horse? A thoroghbred race horse that can carry the small weight of a jockey with a lot of speed, or a working horse that can pull heavy loads albeit slowly? Obviously there is a more precise answer. Car manufacturers, despite their reputation for being creative regarding the horsepower ratings of their products for marketing reasons, require a more stable definition.