Air/Fuel Ratios

AEM wide band guage

Air_Element GAs

When it comes to doing anything performance-related to your car, you have to tune off of several different outputs to create the safest power. The biggest thing you will hear about is the AFR (Air/Fuel Ratio). This is a series of numbers that can determine how the engine is performing. The perfect ratio, or stoichiometric, is 14.7 parts of air to 1 part of fuel. This is the perfect ratio, but it isn’t what we tune to because other ratios create more torque. Stoichiometric means that the burn is theoretically perfect. No oxygen is remaining when the burn is completed. When the burn is stoichiometric there is no o2 and gasoline left, like I said it is a theoretically perfect burn that leaves no traces of the original ingredients in the exhaust. An AFR of 12.2 is best because even though the hotter a gas is the more it expands, there isn’t enough of the ingredients to make a higher pressure. For example, if you filled a balloon with stoic gas (14.7) products vs another balloon with an AFR of 12.2, the balloon with 12.2 actually fills the balloon up more than the stoic because there is more left over gasoline to create higher cylinder pressures.

  • 6.0 AFR – Rich Burn Limit
  • 9.0 AFR – Black Smoke / Low Power
  • 11.5 AFR – Best Rich Torque at Wide Open Throttle
  • 12.2 AFR – Safe Best Torque at Wide Open Throttle
  • 13.3 AFR – Best Lean Torque
  • 14.6 AFR – Stoichiometric AFR (Stoich)
  • 15.5 AFR – Lean Cruise

The smaller the number the “richer” the burn is because there is more more fuel in the mixture. The bigger the number the “leaner” the burn is because there is less fuel. A rich burn is cold, it cant burn as much of the mixture due to the amount of the fuel in the ignition charge. It is safer to have a rich mixture rather then a lean mixture. When you have less fuel in the burn it becomes a violent explosion that creates a lot of heat. Running lean is what destroys engines. Once you reach stoichiometric and start to go past it, another reaction begins to occur. As the burn gets hotter the 02 will begin combining with itself and the nitrogen in the air to create O3 and NOx. That is why in the chart above there is a Safe Best Torque number. It is the perfect balance of fuel and air to create optimal SAFE power at wide open throttle with good cylinder pressures.

Each vehicle uses sensors to run; one of these sensors measures oxygen. Your everyday vehicle’s oxygen sensor is a narrow band which only allows a narrow range of operation with a 0-1 volt range. It’s only function is to sense a stoic scenario. It can sense when it has gone lean or rich, and that tells the ECU to adjust for that setting to bring it back to stoic.

When you add on a Wide band, it obviously has a wider range of operation, and functions using of a 0-5 volt range. This can sense the exact ratio to a tenth of a degree and is therefore much more accurate.

However, stoic is different for every fuel. Some fuels may need 14.7 lbs of air some may need 9 lbs of air for a complete burn. Lambda is used to show the air fuel ratio of any fuel. Lambda 1.0 is always the perfect ratio for any fuel in use. A lot of tuners have the lambda converted to the gasoline 14.7:1 because of the common knowledge of that ratio. For those individuals running E85, as an example, their gauge would still read 12.2(.83 lambda) under WOT and full boost even though stoic for E85 is 9:1 which would mean there gauge would read 7.4 (.83 lambda) if set up to read E85.

3 thoughts on “Air/Fuel Ratios

  1. Well put and extremely informative! I love how it explained why the meter itself needs to be calibrated to the fuel that you’re using.

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