Why Air Density Is Important In Engine Tuning
This article highlights some of the basics about why air density is important in engine tuning. You can watch some helpful videos with more detail about this subject on our YouTube channel.
Some engine basics
The basic premise behind a vast majority of racing engines as well as cars on the road:
Take a fuel like gasoline (that produces a lot of energy) and put it in a controlled space such as a cylinder. When it is ignited, it creates a controlled explosion. This is the basis for an internal combustion engine. One of the goals of racing is to tweak the engine setup to make your engine the most efficient, reliable and powerful so you can go faster down the track. This is done by controlling how much oxygen and how much fuel are present in the cylinder to get the most power out of that controlled explosion.
If there is too much fuel mixed with the oxygen, it will essentially water down the mix, cooling it, and will not get enough heat to generate the maximum amount of power. If there is too little fuel mixed in with the engine, there will not be enough of the chemical that creates the explosion present to generate the maximum amount of power. In between, you may have enough to burn with the oxygen in the air so hot that engine damage may occur. Basically you want to control the ratio of air to fuel to get the most power without engine damage.
The how and why of racers tuning their engines to control this is long and varied. There are numerous books written on the subject. But understanding the basics of how the science of air/fuel ratio works in addition to understanding the nuances of your engine setup can take you a long way in accurately tuning your engine. Air/fuel ratio is so good that it is the basis for engine management in EFI engines throughout the world.
Some weather basics
To start tuning an engine, especially one without EFI, you need to know how much oxygen is in the air. This changes based on the altitude and weather at your location. We've talked about this before but this article will get into how that works.
If you take 1 cubic foot of atmosphere, inside of that is molecules that make up air (oxygen, nitrogen, carbon), moisture (H2O), and other molecules from pollution. To make it even more complicated, the amount of each of these items change based on how much pressure is present in the atmosphere (how much space is available for moisture and pollution), and how hot the atmosphere is (how fast the molecules are moving around).
note: This site is called Air Density Online because it's target is to provide the air density - how much oxygen is present - for racers to tune their engines.
Understanding air density
Knowing what the air density is helps to track how best to maintain your air/fuel ratio. The air density is the weight of air in a cubic foot of air volume compared to the amount of the weight of air in a cubic foot at standard conditions. The warmer it is, the faster molecules are moving around. This usually means less oxygen is present. The closer you are to sea level, the greater the atmospheric pressure will be and the more oxygen is present. If you get the air density from our tracks page, you can assume that fluctuations will affect your air/fuel ratio and will necessitate changes in your jetting, whether it is mechanical fuel injection, open loop EFI, or carburetors.
Many people use density altitude or adjusted altitude as a measurement of available oxygen. Adjusted altitude is the altitude the current air density would be at a similar altitude since air gets thinner at higher altitudes. Adjusted altitude is a comparison with sea level, or 0 ft, being 100% air density and any higher altitudes being a smaller percentage. This does not have to do with the actual altitude of your location. If the adjusted altitude is around 1000ft that means the air density is approximately 0.072 lbs/cubic foot (source).
Applying this to your engine setup
If you have records from your best or worst performance, see if you can figure out what the air density was. (note: you can compute air density with the temperature, humidity, and barometer here)
If your best racing performance was during a warm, sunny day at your home track, you will probably get away with similar jetting at your home track on a similarly warm and sunny day. If it cools down, you should recompute the local air density. Most likely, the air density will increase. In order to compensate for the increased amount of oxygen in the air, you should modify your jetting to allow more fuel to enter as well. On a simple level for mechanical fuel injection, you should decrease the size of your main bypass so less fuel is being diverted to your engine. If you are tracking individual cylinders and nozzles, you can increase their nozzle size as well. If you are using a jetting calculator, you should recalculate the recommended jetting so you can maintain your optimum air/fuel ratio when there is more (or less) oxygen in the air from air density changes.