Air Density Online

Using Air/Fuel Ratio

On this site, we talk a lot about finding your ideal air/fuel ratio. It is the concept that our mechanical fuel injection jetting calculators are built on, and it is the concept used in electronic fuel injection in order to maintain a good fuel system setup. Understanding air/fuel ratio can be very helpful when setting up your fuel system. It can mean the difference between a successful run and a blown engine.

What is Air/Fuel Ratio

Air/fuel ratio (AFR) is the ratio of air to fuel in an engine cylinder needed to get the best combustion.

parts of air » 5 to 1 « parts of fuel

This ratio is the number of parts of air to the number of parts of fuel. For example, an AFR of 5 to 1 is 5 parts of air to 1 part of fuel (by weight, not by volume). The first number in the ratio - or the air part of the air/fuel ratio - is calculated to find the AFR. Keep in mind, the higher the first number is, the less fuel is indicated in your setup.

The amount of oxygen in the atmosphere fluctuates based on the weather, the elevation, and even the amount of pollution in the air. Having an AFR that is too heavy on the fuel can water down the combustion process, creating less heat, and result in less than ideal power. An AFR with too little fuel and too much air can generate too much heat which can break parts. An AFR with even less fuel may not burn very well, giving a cold indication on the spark plug. Differences between lean and really lean are difficult to detect, especially from a new setup when air/fuel ratio information is not established.

General Air/Fuel Ratio Guidelines

Knowing you need to find what the ideal air/fuel ratio is for your setup doesn't help you to know what that ideal number is. That number can change depending on what kind of fuel you're using, if you're using a high speed bypass jet, and what size fuel pump or other engine parts make up your setup.

To start, know your engine type - normally aspirated or forced induction - and the type of fuel you plan to use. Gasoline, ethanol, methanol, or nitromethane all have different characteristics that influence the air/fuel ratio.

There are a few general rules to shoot for:

  • normally aspirated methanol:
    AFR between 4.6 and 5.4
    (5.0 most common)
  • Roots supercharged high compression methanol wedge head engine:
    AFR between 3.6 and 4.4
    (4.0 most common)
  • Roots supercharged high compression methanol Hemi head engine:
    AFR between 3.0 and 3.6
    (3.4 most common)
  • Screw supercharged high compression methanol Hemi head engine pushing over 3000 horsepower:
    AFR between 2.8 and 3.2
    (3.0 most common)

These numbers can make a general starting off point. Fine-tuning from here is necessary depending on the unique characteristics of your engine.

Higher Engine Speeds

Many engine combinations require a change in air/fuel ratio at higher engine speeds. This would be from intake port limitations, cam timing limitations, supercharger limitations (if so equipped), and others. The lean-out in racing mechanical fuel injection is best done with a high speed bypass circuit. The amount of lean-out is often determined from engine testing as well as on a dynamometer. With a known combination, it can also be determined from appropriate calculations.

Because of this, a different air/fuel ratio might be necessary for idle and/or the start of the run and the shift point when you're engine speed is the highest. This means finding the right combination of fuel management that works for different points in your run.

For example: in our supercharged alcohol drag racing setup, a 0.040 inch diameter high speed bypass jet provided an air to fuel ratio change from 3.15 to 3.40. That produced the best performance for our combination with a seamless transition from the low end to the high end. A larger high speed bypass jet size did not work very well. The air/fuel ratio difference was too great. Either the low end was rich with a good high end or the low end was good with a lean high end. The optimum combination could not be achieved with a larger high speed bypass jet size.

Air/fuel ratio analysis was done for other combinations as follows:

  • engine: 14-71 blown alcohol big block Hemi: HS -- 0.040 inch dia. with a main bypass adjustment improved the air/fuel ratio map for a transition from 3.55 for the low end to 3.7 for the high end.
  • engine: normally aspirated 350 ci small block: HS -- 0.070 inch dia. >>> AFR lean out from 4.8 at the low end to 5.2 at the high end.
  • engine: normally aspirated 450 ci big block for Bonneville: HS -- 0.065 inch dia. >>> AFR lean out from 5.0 at the low end to 5.4 at the high end.

Bad Example

  • engine: 8-71 blown alcohol big block: HS -- 0.80 inch dia. >>> AFR leaned out to 4.9 at the high end and backfired the blower; jetting was changed for a 4.0 air/fuel ratio for a powerful high end without backfiring.

Other Considerations

There are other considerations to be made when determining the ideal AFR for your setup:

  • For a mechanical fuel injection engine, the engine or blower size determines the nozzle size.
  • The fuel pump size determines the size of the main bypass.
  • The transition between low end and high end air flow determines the high speed size.
  • Once an optimum air/fuel ratio is determined, it appears to remain stable at different altitudes and weather changes.
  • Jetting needs to be changed for those differences to maintain the optimum air/fuel ratio.
  • EFI does the same thing. It adjusts the fuel amount to maintain an optimum air/fuel ratio at different altitudes and weather.
  • Carburetors are adjusted for an optimum air/fuel ratio. They compensate some-what for minor changes in altitude or weather. However, jetting and transition circuits need adjustments for any significant change in the air density from altitude or weather differences.


Using Air/fuel ratio can be a good tool to have in your tuning arsenal. It may take a bit of effort to set up, but once you know, it can streamline the fuel system tuning process and help to ensure best power with few broken parts.

If you wish to learn more about how to determine your optimum air to fuel ratio for tuning, check out our books from These books feature extensive information that help in engine tuning with a lot of information regarding air/fuel ratio.

Our ProCalc jetting calculators are designed to do the calculations for you to determine jetting for optimum air/fuel ratios so that you can focus on your engine, driveline, or racer. If you would like us to set it up for you and assist you setting up a new combination, check out our ProTune Custom Jetting.

Related Articles:
Measuring the Atmosphere around a Racetrack
Historical Weather Data
Explaining the Effects of Weather on Mechanical Fuel Injection

Engineering Toolbox
National Weather Service Glossary

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By Jennifer on November 21st, 2017 in Info and Education, Main News


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