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Should I say that an engine can output 552 horsepower or 552 horsepowers? I've heard people use both.

If it is just "horsepower", what is the justification for the nonstandard pluralization?

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I've always understood it to be 552 (units of) horsepower, where units of is understood and rarely spoken. But Merriam-Webster and Wiktionary both list horsepower as the actual plural form.

Either way, horsepowers is definitely not standard.

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That was my first impression, but isn't this inconsistent with every other unit of measure? –  Larry Wang Sep 10 '10 at 18:27
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Yes, but that's language for you! :) –  Kosmonaut Sep 10 '10 at 18:32
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By the way, I am amused by the term horsepowers — it makes me think of unicorns. –  Kosmonaut Sep 10 '10 at 19:01
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Not every unit of measure needs to be plural. People say "A 60 Watt bulb" –  Mr. Shiny and New 安宇 Sep 10 '10 at 19:40
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@Mr. Shiny and New: "a 60 Watt bulb" is not a very good example, because 60 Watt is an adjective in that case. People also say "a 20 dollar note", "a five-year term" and "a 10 mile training plan", even though we do have dollars, years, and miles. english.stackexchange.com/questions/1366/… –  RegDwigнt Sep 10 '10 at 19:57
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Jeremy Clarkson from Top Gear sometimes says horsepowers when feigning technical ignorance. If you've heard it from him, he's just trying to be funny. He also uses carbon dioxides which is equally meaningless.

Horsepower is always correct.

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Yeah, I just started watching Top Gear, and that's where I heard it. Thanks for the info. –  Larry Wang Sep 10 '10 at 21:17
    
Thought as much. He's the only person I've heard say it. –  user774 Sep 10 '10 at 21:36
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Horsepowers is possible in some very limited contexts, but otherwise horsepower is standard. Here are two examples of horsepowers in the Corpus of Contemporary American English:

In general, Yamaha’s direct-injection motors have been considerably quieter and smoother-running than their competition’s, though Evinrude’s new E-TEC might challenge that. The motor lists for $12,750. Though the variety of horsepowers and induction systems makes an apples-to-apples comparison of new 2004 motors impossible, this one would probably qualify as “Editor’s Choice,” a motor likely to make a lot of folks with bass and walleye rigs in the popular 17- to 19-foot range very happy.
Outdoor Life, 2003

As of the model year 1994, all of the so-called “saltwater series” engines are limited to the 150-to 250-horsepower range, but in fact many (if not most) of the same materials and finish processes are also used in the lower horsepowers, too.
Field and Stream, 1994

In both these examples, horsepowers refers to the horsepower ratings of different models of motor. In the second example, the horsepower ratings of engines is used as a metonym for the engines themselves.

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Both of those examples somewhat abuse the term, in that they use a word with a precise definition when they should say "power outputs" or "power ratings". Since "horsepower" is a technical term, and both of those magazines are not technical, this is simply a misuse of the term. Bluntly, the phrase "some limited contexts" is "weasel words" and avoids directly saying that the writers simply misapply a technical term. This can happen in natural language to the point where it becomes accepted speech, so I realize the smallness of legality. After all, we understand what the writer is saying. –  Mark C Nov 24 '10 at 18:14
    
It’s metonymy: “a figure of speech used in rhetoric in which a thing or concept is not called by its own name, but by the name of something intimately associated with that thing or concept.” In this case, the name of the unit used to measure power ratings is used to stand for devices that have those power ratings. Surely you don’t mean to imply that use of metaphor in speech is somehow wrong? –  nohat Nov 24 '10 at 19:06
    
@Mark Well, I occassionally hear people say that you "shouldn't use the unit of measure to refer to the quantity being measured". Like, it's not "yardage", it's "length" or "distance". It's not "voltage", it's "electromotive force". Etc. Personally I disagree. Yes, it's true that people in another culture might measure EMF with some other units. But so what? Any rational person knows that. But using the same or related words for both the quantity being measured and the unit of measure eliminates any possible ambiguity. If you say "voltage", I know you mean the thing that is measured in volts. –  Jay Jun 25 '12 at 14:02
    
If you say "electromotive force", now I have to learn two words for what is essentially the same idea. –  Jay Jun 25 '12 at 14:03
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