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Engine Tuner's Calculator

- using absolute pressure and dew point -

To use the calculator, just click the type of units that you will be entering, then enter the  temperature, absolute pressure and dew point... then click the calculate button.

Engine Tuner's Calculator

Air Temperature deg F deg C
Absolute Pressure inches Hg hPa
Dew Point deg F deg C

Relative Horsepower %  
Dyno Correction Factor  
Air Density lb/ft3 kg/m3
Density Altitude feet meters
Relative Density %  
Virtual Temperature deg F deg C
Vapor Pressure inches Hg hPa

 Copyright 1998-2017,  Richard Shelquist

The atmospheric pressure, temperature and humidity all affect the density of the air. On a hot day, or at high altitude, or on a moist day (high dew point temperature), 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. For tweaking the fuel/air mixture, or predicting engine power, the air density is the most important consideration.


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). Easily measured by most smartphones and portable weather stations.

Note:  For more information about ambient air pressure measurements see the pressure measurement page.

Dew Point temperature is used in this calculator, rather than relative humidity, because the dew point is essentially constant for a given air mass. That is, the dew point changes rather slowly and is not significantly affected by temperature. On the other hand, the relative humidity changes greatly during the day as the air temperature changes.

Note: The absolute pressure and dew point can often be gathered from a local airport, local weather service or the national weather service.

Calculated Values:

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, 58 deg F dew point, 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.


For in-depth technical details, see my web page on Dyno Correction Factor and Relative Horsepower and also see the page describing Air Density and Density Altitude.

A useful source for simple atmospheric theory, explanations and calculations is the USA Today Weather web site. Also, there are several weather conversion calculators available from El Paso NWS.

Copyright 1998-2017, All Rights Reserved, Richard Shelquist

Last Updated: 15-Jun-2017

----- Shelquist Engineering -- Richard Shelquist -- Colorado, USA -----