The Society of Automotive Engineers maintains auto testing standards, among them a dynamometer testing standard known as "J1349". It's recently been updated to include provisions that prevent altering horsepower figures with creative uses of exhaust systems, fuels, and oils.
David Lancaster, chairman of the committee that developed the test rules, explained that new technologies open new loopholes in test procedures, which must be periodically plugged. Before 1971, for example, a manufacturer might connect the engine's exhaust system to a vacuum pump, gaining power by eliminating back pressure, the resistance to free flow caused by mufflers. Under the 2006 standard, a complete production exhaust system, or one of equal back pressure, must be used.
Full text: http://www.nytimes.com/2005/10/03/au...rssnyt&emc=rss