Horsepower has an interesting backstory. If you're a keen petrolhead, however, I'd advise you not to read it, as I have this week. Enjoy your blissful ignorance if you can.
There are only three things you really need to know about it, in any case. First, that celebrated Scot James Watt didn't actually invent it, he just formalised it. Second, that it's more of an idea than a measurable product of an engine anyway - a bit of mathematics. And third (perhaps only academically), that it has actually been misapplied to automotive engines all along. It's simply the wrong kind of measure for any motor that sits within and propels a moving vehicle.
Not that the last bit matters much, because imagining an automotive world without horsepower is almost impossible. Who gets excited by the idea of a new supercar with 883 kilowatts of peak power? How much horsepower has it got? 1184 imperial ponies? Yes, please. Wax on about whacking great torque figures and even add in engine speed ranges if you like (any expression of engine power is only torque multiplied by engine speed and then 'interpreted'), but horsepower is what matters, isn't it? What sells. It's the only real, universal performance car X factor.
Which is lucky, because otherwise why on earth would you choose to measure the output of a modern car's engine based on a pretty arbitrary and disputed estimation of the amount of work that a brewery draught horse could get through in the late 18th century? How has an entirely notional figure, dreamed up to help sell static steam engines to early industrialists, become the closest thing to a global currency for comparing modern cars?
For Watt all those years ago, horsepower was a sales tool - "translational marketing". Before they would sign their bankers' drafts, mining entrepreneurs needed to know not only how much coal they could load into their railway hoppers but also how quickly that coal could be moved from the bottom of the mineshaft to the top of it. Horsepower gave them an idea of how many horses they could put out to pasture for each steam engine they bought-or else, unless they had some very large stables indeed, just how transformative this early form of mechanisation could be for their profit margins.
But here's the dirty truth about horsepower: Watt lowballed his maths. As the story goes, when he observed those brewery draught horses in 1782, his sums were calculated to represent not what a particular horse could achieve when working intensively but what the average horse could sustain for a full working day. He was selling static steam engines, remember, not the kind of engines fitted into moving vehicles, on which load might have been considered a more dynamic (ie ebbing and flowing) factor. He sold engines that just grunted away constantly all day long. That's why, as has been proven since, one fit horse can usually produce a good 15 horsepower when it's really working up a sweat.
I'm not sure exactly how long a working day was in an 18th-century brewery. But imagine loading up, say, a Land Rover Defender Octa with whatever ballast might be needed to stress its engine to maximum load (think a trailer with several dumpy bags of sand on it). Now drive that Defender flat out on an oval, in third or fourth gear and at a steady 6000rpm for eight, 10 or even 12 hours straight (you will have to imagine some clever air-to-ground refuelling system too).
