I think it was last year when a discussion took place on this board in regards to how temperature, humidity, altitude and barometric pressures affect the performance of a race engine. Re-jetting of carburetors to richen or lean the mixture is sometimes required to get the best performance. Although a few of us tried to shed light on how it all comes together, there were several members that had their own opinions that ran counter to reality. Below I've copied and pasted information from a site that we recently purchased a weather station to be used for next season. I hope the write up sheds some light on the importance of proper jetting/engine performance/ET as the weather changes.
General Weather Station Information
The concept of weather stations evolved when racers began to realize that the performance of their vehicle was dependent on the atmospheric conditions they were racing in. The atmospheric condition I am referring to is the density or weight per cubic area of the air. This density measurement is important because it allows one to establish a relationship between the changes in atmosphere and the changes in performance.
Air density is a computation mainly dependent on the temperature, barometric pressure, and the humidity of a volume of air. Temperature is generally measured in degrees Fahrenheit, barometric pressure in inches of Mercury (inHg), and humidity in percent of Relative Humidity. One can relate to how these factors effect the density of the atmosphere by using a balloon to simulate the earth's atmosphere. When a balloon is filled with air and placed into a refrigerator it begins to shrink. The reason for this is due to the drop in temperature of the air inside the balloon. As the air cools it releases energy and slows down. Because the air molecules are not bouncing off each other as much they remain closer together and more of them will now fit in a smaller area. The opposite will occur if the balloon is heated. This same effect will occur if this balloon is placed in a pressurized chamber. If the pressure in the chamber is increased it pushes the walls of the balloon in causing the air inside to occupy less space. Again, the opposite will occur if a vacuum is created in the chamber. The effect of humidity is a little more complicated. A change of humidity in the atmosphere is caused by a change in the amount of water vapor mixed in with the common gases already present in the air. As more water vapor is put into the air is displaces these gases. The water vapor is also less dense (weights less) than the gases in the air. When we take air that is at a set temperature and pressure and start introducing increased amounts of humidity we begin to cause the overall density of the air to decrease. Therefore, the density of the air is the greatest when there is no humidity.
Changes in temperature, pressure, and humidity can have different amounts of effect on the associated change in air density. A change in temperature or pressure causes a proportional change in density. In other words, a 1% change in temperature causes a 1% change in density. Again, the effect of humidity is more complicated. This is because the effect of humidity on density is also dependent on the temperature. To further explain, a 50% increase of humidity when the air temperature is 70 degrees may cause a 1% decrease in total air density, but a 50% increase of humidify when the air temperature is 90 degrees may cause a 2% decrease in total air density. This effect is due to the fact that it takes lot more water to cause 50% relative humidity at a 90 degree temperature than it does at 70 degrees. One can relate this to a cup of hot water being able to hold more sugar than a cup of cold water. The humidity must also be considered in that it makes up some of the density of the air, but it has no value being there. These considerations are where the proprietary density altitude algorithms developed by ALTRONICS differ from a true density altitude measurement. Now we must examine how these factors effect performance.
The air in the earth's atmosphere is made of various gases and water vapor. Neglecting the effect of pollution there normally is 20.9% of oxygen, 75% of Nitrogen, Carbon and very small amounts of some other gases. Oxygen is the most important gas in the atmosphere as far as an internal combustion engine is concerned. This is due to the fact that the oxygen is used to burn the fuel placed in the chambers of the engine. When more oxygen can be placed in the chamber it allows one to also place more fuel along with it and therefore create more power. The air density relates to this because when the air density increases the amount of the combined gases and water now fit into a smaller area or more of it fit into the same area, this includes oxygen! A 800cfm carburetor takes in 800 cubic feet per minute of air. If the air is denser than there is more of it in 800 cubic feet and therefore more amount of oxygen will be taken into the engine in one minute.
The term commonly heard among racers is "density altitude". Hopefully, you now understand what density is. Density altitude is the density expressed if feet instead of grams per cubic centimeter. It's a lot easier to relate a change of density in a couple hundred feet rather than a change of 2.534 g/cm^3. The use of density altitude is taken from the U.S. standard atmosphere table. This table relates the density of an average day at sea level (59 degrees, 29.92 inHg) and how it changes at different elevations in the atmosphere. As one climbs in altitude the density falls off at a predetermined exponential rate. This is one way an aircraft can know how high it is.
Recently, oxygen sensors have been being used with weather stations. Remember from above that the % of oxygen in the atmosphere is normally 20.9%, well this is not always true. Sometimes the percentage can vary a few tenths of a percent. When this occurs the air at some same density can have a much different effect. When there is an increase in the percentage of oxygen it is like having much denser air and vice-versa with a decrease in oxygen. Until recently, a device capable of measuring the percentage of oxygen to the needed level was very expensive. Without knowing and incorporating the percentage of oxygen into density altitude there can be times when density altitude alone will not correlate to performance.
There are some important things to keep in mind when using a weather station to predict the performance of your car. It is important that your car is tuned with a rich air/fuel ratio in order for the performance to change accordingly to the changes in density altitude. If the engine is tuned on the lean side then there will not be any extra fuel to burn when the density increases (when more oxygen is available). Therefore, you may not see an increase in performance when the density altitude decreases. You may even see a decrease in performance because the engine will be running even leaner.
