Engine Tuner's Calculator -

using Absolute Pressure and Relative Humidity

Features:  The actual air density, relative air density (RAD), density altitude and virtual temperature are all calculated, in addition to the SAE J1349 relative horsepower.

To use this calculator,  just enter the temperature, absolute pressure (also called station pressure or actual pressure), and relative humididty... ...then click the calculate button.  

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, 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, the air density is the most important consideration.

Inputs:

The Air Temperature should ideally be the temperature of the air that is going into the intake of the engine.

The Absolute Pressure (also called actual pressure or station pressure) is the ambient air pressure.

Relative Humidity is a measure of how much moisture is in the air compared to the amount of moisture that the air could hold at saturation. Relative humidity is a function of temperature and therefore changes as the temperature changes, even if the amount of moisture in the air remains constant.
 

Calculations:

The SAE J1349 relative horsepower calculation shows how air density alters the power output of a properly tuned engine. For example, with an air temperature of 85 deg F, an absolute pressure of 25.09 in-Hg,  and 40% relative humidity, the engine only produces about 81.1% of the rated horsepower. 

For the SAE J1349 relative horsepower calculations, the standard reference conditions are:  Air temp 77 deg F (25 deg C),   29.235 Inches- Hg (990 mb) actual pressure and 0% relative humidity.

The air density is the actual weight of a given volume of air. This is a key parameter for engine tuning.

The relative air density is the ratio of the calculated air density to the air density at sea level using the ICAO standard reference conditions.

The density altitude is the altitude in dry air that would have the same density as the input conditions. The ICAO standard conditions for zero density altitude are 0 meters altitude, 15 deg C (59 deg F) air temp, 1013.25 mb (29.921 in-Hg) pressure and 0 % relative humidity.

The virtual temperature is the temperature of dry air that would have the same density as the input conditions.

The vapor pressure is the contribution of water vapor pressure to the absolute air pressure.

The dyno correction factor shown above is the reciprocal of the relative horsepower number. The dyno correction factor, the actual air pressure and the vapor pressure are included for comparisons to DynoJet chassis dyno test data.
 

Resources:

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 1998-2007, All Rights Reserved, Richard Shelquist, Shelquist Engineering.

Last Updated: 18-Sep-2007