Various units of power:
In comparing measurements expressed in horsepower, it is important to consider not only which definition of horsepower is being used, but also the specifications for the conditions under which the engine is to be tested. For example, prior to 1971, the figures American automobile manufacturers gave for engine horsepower were taken from tests of engines running on test stands without mufflers or other impediments. Today the engine horsepower numbers are for the engine as installed in the car, which is roughly a third lower than the test stand figure.
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Last revised: 4 September 2013.