Performance Measurements - Basics

From James Watt and the steam engine to performance measurements in the racing industry; this series of three articles deals with all the questions that are essential when conducting powertrain performance measurements. This particular issue contains the history and fundamental physics of powertrain performance measurements.


James Watt (1736-1819) was an engineer from Scotland and you could say the first engine tuner. He improved the design of the steam engines and raised the efficiency. His aim was to replace horses in the coalmines. Horses were used to lift coal up to ground level. However, he had to convince the owners they would make more money if they went to his steam engines.


He made measurements on several horses and calculated the performance on an average horse. This was the performance it could manage during a whole day's work. Watt gave it the unit "horsepower". Later it has been named bhp (brake horsepower) and imp hp (imperial horsepower). Now he could tell the mine owners how many horses they could replace with one of his steam engines and also easily calculate the profits.

Imperial horsepower

  • One average horse managed to lift 550 lbs to a height of 10 feet in 10 seconds
  • Units [bhp], [imp hp]
  • Metric horsepower

  • Lift of 75 kg to a height of 10 meters in 10 seconds
  • Units [met hp], [PS], [ch], [CV]
  • Imp hp Met hp

    The original horsepower-unit was based on imperial units, i.e. pounds and feet. The rest of Europe wanted a definition based on metric units. It is nearly, but not exactly, the same. An imperial hp converted to metric hp shows a 1.5% higher number. Today all definitions are using SI-units, i.e. Newton, meters and seconds. The standard unit for power was named "Watt".

    ISO power - Watt

  • Lift of 1000 N (98.1 kg) to a height of 10 meters in 10 seconds
  • Units [kW]
  • Conversions

  • 1 kW = 1.34 imp hp
  • 1 kW = 1.36 met hp
  • 1 imp hp = 1.015 met hp
  • 1 lb-ft = 1.36 Nm
  • Watt


    The performance of an engine is normally described by the power output. If the engine is used over a wide range of speed, like in a car, it is equally important to know the torque output. The torque is actaully what you feel when driving a car. Power and torque are related. Maybe the most described formula in car magazines is the relation between "bhp" and "lb-ft". If you plot them using the same scales they will cross at 5252 rpm. If you plot with "kW" and "Nm" the crossing will be at 9549 rpm.

    Fundamental formulas

    If a scientist makes a new discovery it will not be accepted until somebody else can repeat the same test and achieve the same result. This is a simple way of convincing others, and yourself (!), the results were correct. One of the corner stones in measurements is calibration. If a tool cannot be calibrated it is not a measurement tool. The normal way when calibrating a load cell is to use a rod and dead weights. Today's standard of high quality in all aspects demands that the calibration is traceable. This means that the weights themselves must have been measured on a calibrated scale.

    Dead weight calibration

    If you need to be extremely sure about the results a good way is to calibrate before the tests and check the calibration afterwards. Then you can be sure of the accuracy of the equipment during the tests.

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