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Relative Horsepower Calculator Relative Humidity Factors

This calculator requires the use of Javascript enabled and capable browsers. This Relative Horsepower Calculator determines the relative torque and horsepower available at any temperature, barometric pressure, humidity, and altitude. The calculations are derived from public domain information, SAE J1349 Revision, June of 1990. The calculator shows you how much the air conditions on any given day will alter the horsepower of a normally aspirated internal combustion engine. This calculator uses relative humidity which requires some caution because the relative humidity changes radically during the day as the air temperature changes. To use this calculator, just enter the temperature, barometric pressure, humidity and altitude and click on the calculate button. Air density is affected by the temperature and humidity of the air. On a hot day, or at high altitude, or on a moist day, the air is less dense. A reduction in air density reduces the amount of oxygen available for combustion and therefore reduces the engine horsepower and torque. This calculator shows you how much the air conditions on any given day will alter the horsepower of a normally aspirated internal combustion engine. For example, at 85 degrees F, 30.14 Inches Hg (mercury) barometer reading, 40% relative humidity and 5000 feet altitude, the engine only produces about 81.1% of the rated horsepower. The dynamometer correction factor shown above is the reciprocal of the relative horsepower number. The dynamometer correction factor, the actual air pressure and the vapor pressure are included for comparisons to DynoJet chassis dynamometer test data. The air temperature should ideally be the temperature of the air that is going into your engine. The barometric pressure is the corrected to sea level atmospheric pressure that is typically reported on the local news or may be available from a local airport (this is not the same as absolute pressure, actual air pressure or station pressure). The humidity and barometric pressure can generally be gathered from a local weather report or the national weather service. The humidity inside a closed building may be markedly different from the outdoor readings. The altitude can be found on topographical maps or by calling a local airport. For these calculations, the standard conditions are air temperature 77 degrees F, 29.235 Inches Hg Barometer, 0 feet altitude, 0% relative humidity.

Enter Current Air Temperature (OAT) Degrees F
Enter Ambient Barometric Pressure Inches Hg
Enter Ambient Relative Humidity %
Enter Physical Or Pressure Altitude Feet


Calculated Relative Horsepower To Rated %
Calculated Dynamometer Correction Factor  
Calculated Air Pressure Inches Hg
Calculated Vapor Pressure Inches Hg
Updated 8.15.11


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