Engine Tuner's Calculator - using 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.

There is also a special version of the Engine Tuner's Calculator for users of digital weather stations that uses the values of temperature, absolute pressure and relative humidity.

To use this calculator,  just enter the temperature, altimeter setting, relative humidity and altitude... 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 Altimeter Setting is the value in the Kollsman window of an altimeter when it is set to correctly read the elevation. The altimeter setting is generally included in NOAA weather data reports. (For more information about ambient air pressure measurements see the pressure measurement page.)

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.

The altimeter setting and dew point can often be gathered from a local weather service or the national weather service. Click here for NOAA weather data  including hourly listing of dew point and altimeter setting for worldwide locations, in both English and Metric units. 

If you are uncertain of the actual altitude,  a local airport may be able to help you estimate your altitude

Calculations:

The SAE J1349 JUN90 Relative Horsepower calculation shows how air density alters the power output of a properly tuned engine. For example, at 85 deg F, 30.14 in-Hg altimeter setting, 40% relative humidity and 5000 ft altitude, 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 Dyno Correction Factor, also calculated according to SAE J1349 JUN90, is simply the reciprocal of the relative horsepower value.

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.

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

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 ICAO 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.

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

The Actual Air pressure is also called the station pressure or absolute pressure. (For more information about ambient air pressure measurements see the pressure measurement page.)

The Vapor Pressure is the contribution of water vapor pressure to the actual air pressure. 

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 1998-2010, All Rights Reserved, Richard Shelquist

Last Updated:  20-Jul-2010