A negative horsepower run measures parasitic loss in the vehicle.

• A. Not Applicable
• B. Not Sure
• C. True
• D. False

Answer: (C)

Parasitic losses are the power the engine must supply to run things that don’t help move the vehicle—like the alternator, power steering pump, water pump, oil pump, and internal friction. A negative horsepower run is used to quantify those losses because it highlights when the engine’s output is being drawn away by these auxiliary loads. In testing, if the measurement shows a negative value, that amount of horsepower is being spent on parasitic components rather than going to the wheels, so the negative figure directly represents the parasitic loss.