Engine Tuner's Calculator
- using absolute pressure and relative humidity -
Air Temperature should ideally be the temperature of the air that is going into the intake of the engine.
Absolute pressure is the actual ambient air pressure (also called station pressure).
Note: For more information about ambient air pressure measurements see the pressure measurement page.
Relative Humidity is the ratio of the partial pressure of water vapor to the saturated vapor pressure at a given temperature.
Note: The calculator which uses Dew Point is often more accurate because the dew point is fairly constant for a given air mass and changes very little until another air mass arrives, while the relative humidity varies greatly as the ambient temperature changes.
Note: The absolute pressure and relative humidity can often be gathered from a local airport, local weather service or the national weather service. Click here for NOAA weather data for many worldwide locations, in both English and Metric units.
Relative Horsepower shows how air density alters the power output of a properly tuned engine. For example, at 85 deg F, 25.09 in-hg absolute pressure, 40% relative humidity, the engine only produces about 81.1% of the rated horsepower.
Note: The relative horsepower calculations are made in accordance with SAE J1349. The standard reference conditions for SAE J1349 are: Air temp 77 deg F (25 deg C), 29.235 Inches- Hg (990 mb) actual pressure and 0% relative humidity.
Note: Section 5.1 of SAE J1349 AUG2004 makes it clear that the equations are not intended to provide accurate corrections over an extremely wide range, but rather that the intended range of air temperatures is 15 to 35 deg C (59 to 95 deg F), and the intended range of dry air pressures is 900 to 1050 mb (26.58 to 31.01 inches-Hg). Values outside of this range may produce inaccurate results for SAE Relative Horsepower and Dyno Correction Factor, but all other calculator results (such as Density Altitude, Air Density, etc) will still be correct.
Dyno Correction Factor, also calculated according to SAE J1349 JUN90, is simply the reciprocal of the relative horsepower value.
Air Density is the actual mass of a given volume of air. This is a key parameter for engine tuning.
Density Altitude is the altitude in dry air that would have the same density as the input conditions.
Note: The ICAO standard conditions for zero density altitude are zero meters altitude, 15 deg C (59 deg F) air temp, 1013.25 mb (29.921 in-Hg) pressure and zero % relative humidity.
Relative Air Density is the ratio of the calculated air density to the air density at sea level using the International Civil Aviation Organization (ICAO) standard reference conditions.
Note: The ICAO standard sea level air density is 1.225 kg/m3.
Virtual Temperature is the temperature of dry air which would have the same density as the input conditions.
Vapor Pressure is the contribution of water vapor pressure to the total absolute air pressure.
Last Updated: 1-Mar-2015