Experienced tuners know that the stoichiometric point (or stoichiometric air-fuel ratio) of a fuel is the exact proportion of air and fuel at which a complete, balanced chemical reaction occurs, meaning all the fuel is burned using all the available oxygen in the air— no excess fuel or oxygen remains. It’s a theoretical value, but if you actually tuned to the stoichiometric air-fuel ratio under load, you would most likely create some ventilated blocks and pistons. BUT… it is still important to know.

Why is it important? Every fuel has a specific stoich point:

• Gasoline: ~14.7:1 → 14.7 parts of air for every 1 part of gasoline, by mass
• Diesel: ~14.5:1 (varies depending on the diesel fuel’s composition)
• Ethanol (E100): ~9:1
• Methanol: ~6.4:1
• Hydrogen: ~34.3:1
• Propane: ~15.5:1
• Natural gas (methane): ~17.2:1
• Nitromethane: ~1.7:1

Here you can see the stoichiometric point for gasoline in EFI Live as it adjusts for ethanol percentage. Most pump fuel is up to 10 percent ethanol, requiring a richer mixture than pure gasoline.

If you were to tune an engine running gasoline to the same air-fuel ratio with ethanol or methanol, you may have a similar experience as doing your power tuning to the stoichiometric point. This is why using the Lambda value is actually a much better way to talk about fueling. With Lambda, the stoichiometric point is 1.00 no matter what fuel you use. In fact, most afttermarket wideband sensors are actually Lambda sensors that convert the lambda reading into an air-fuel ratio reading.

Here’s a handy chart to show how stoich point lines up with various fuels using Lambda.

Once you know the theoretical point of complete combustion, you now have a proper measuring stick for optimizing the fueling no matter what fuel you use. Most tuning software has a place where you simply drop in the stoichiometric point for the particular fuel you’re running. And for those of you using race gas, it should be noted that each blend’s stoichiometric point is different from typical gasoline’s. The race fuel should state the stoichiometric point right on the fuel specs.

Where this really gets interesting is when you start mixing fuels like ethanol and gasoline (hello “E85”), methanol and gasoline (aka “meth injection”), propane and diesel, ethanol and methanol, or everyone’s favorite — gasoline and nitromethane.

For example, most pump fuel is E10 (90-percent gasoline, 10-percent ethanol). Gasoline has a 14.7:1 stoich point, while ethanol has a 9:1 stoich point. To calculate for E10 (10 percent ethanol) you would do the following:

14.7 x 0.9 + 9 x 0.1 = 14.13

We did a whole article on blending E85 with pump gas for different ethanol percentages, and how that affects octane, stoichiometric point, and more. That said, many ECUs will do a lot of the work for you. EFILive, for example, will allow you to completely change the fueling and spark based on ethanol percentage with a GM E38 or E67 ECU (commonly used with Gen-IV LS engines). Holley EFI has recently added this feature, and it’s been a hallmark of Haltech for some time because we all know Aussies love alcohol.

But what happens with methanol injection where it is difficult to predict the amount of fuel being added compared to the whatever is in the fuel tank? The safest method is not to rely on methanol injection for fueling, and only to use it for its cooling properties and knock resistance. We reached out to a few tuners and they all concurred with this method, setting your air-fuel ratio (or Lambda) before adding methanol injection. If you do something different, we’d love to hear from you. Tell us what you do and why at [email protected].