How do I pluralize "horsepower?"

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.

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.

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.