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 when the altimeter is set to correctly read a known elevation. The altimeter setting is generally included in NWS reports. The altimeter setting is not the same as the sea level corrected barometric 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.

The barometric pressure 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 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 barometer reading, 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 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 actual air pressure is also called the station pressure or absolute pressure.

The vapor pressure is the contribution of water vapor pressure to the actual 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.