Dyno Correction Factor Calculator
Air density is affected by the temperature, pressure and humidity of the air. On a hot day, or at high altitude, or on a moist day the air is less dense which means that there is less oxygen available for combustion which, in turn, means that there is also less engine horsepower and torque.
This calculator is specifically intended to help evaluate the data from a chassis dyno run when you would like to know how what the readings would have been if they were taken on an SAE standard dyno.
Drag racers and engine tuners... take a look at the features in the Engine Tuner's Calculator. The engine tuner's calculator now includes relative horsepower, air density, density altitude, virtual temperature, absolute pressure, vapor pressure, relative humidity and dyno correction factor!
Originally, all of the major US auto manufacturers were in or around Detroit Michigan, and the dyno reading taken in Detroit were considered to be the standard. However, as the auto industry spread both across the country and around the globe, the auto manufacturers needed a way to correlate the horsepower/torque data taken at those "non-standard" locations with the data taken at the "standard" location. Therefore, the SAE created J1349 in order to convert (or "correct") the dyno data taken in, for example, California or in Tokyo to be as if the data had been taken at standard conditions in Detroit.
For example, at 85 deg F, 24.71 in-Hg absolute pressure and 0.121 in-Hg vapor pressure, the engine only produces about 81.1% of the SAE rated power so that the required dyno correction factor is 1.233. Therefore, when dyno testing under these conditions, it is necessary to multiply the measured torque and horsepower values by the correction factor of 1.233 to determine the SAE corrected readings. The dyno correction factor takes into account all of the effects of temperature, altitude, atmospheric pressure and humidity to arrive at corrected horsepower and torque values.
The air temperature should ideally be the temperature of the air that is going into your engine.
The absolute pressure is the actual atmospheric pressure, also called station pressure. This is not the barometric pressure or altimeter setting as is typically reported on the local weather report.
The vapor pressure is the partial pressure of the water vapor in the air.
For these calculations, the standard reference conditions are: Air temp 77 deg F (25 deg C), 29.235 Inches- Hg (990 mb) altitude-corrected barometric pressure, 0 ft ( 0 m) altitude, 0% relative humidity.
Copyright 1998-2012, All Rights Reserved, Richard Shelquist, Shelquist Engineering.
Last Updated: 3-Apr-2012