Air fuel ratio is one of the other most misunderstood things when racing its seems as you walk around the paddock. "Lean is mean" or so they say is a hallmark saying of old drag racers reading their indexed plugs after all.
What are we after anyway? Power and power is mostly dictated by when you choose to fire the spark event. Uniform combustion and maximizing uniform and predictable cylinder pressures through the use of combustion is the how in how you make the most reliable power after all. To keep this timing of combustion predictable avoid knock we need to keep fueling in a happy range. I promise it really is that simple if you boil it down to a simple thought, but that's a topic for another day.
Like most things though in application its a bit different. fuel blends change over the course of a year you receive it at the pump or when racing in different classes with different fuels. When tuning cars it was always a challenge in turbo applications to know what fuel was in the car. Especially when running something like E85 which depending can measure up to E85 but could be E50 or the likes. This is only an issue though if you're tuning by AFR. For this reason, most experienced tuners will us the Lamdba scale instead.
So what is Lamda and what is the reason you care. Lambda scale (λ) is another scale for measuring the presence of air left over in the exhaust stream with 1.00 being the bar for a stoichiometric reaction, meaning you have no excess fuel or air available after a combustion event. So for pure gasoline 14.7 = 1 but the nice thing is if I wanted to change fuels the point for a stoichiometric reaction as far as the Lambda scale is concerned is still the same.
Here is a quick look at different stoichiometric values for various fuels. Why would I want to keep a spreadsheet of all these different values when I can just look at my wideband data and know > 1.00 Lean < 1.00 Rich.
Here is a visual representation of different values following the Lambda scale with the stoichiometric ratio high lighted as a benchmark.
Lambda is also helpful if you wanted to determine what air-fuel ratio to run for different fuels. It can be simply used as a multiplier. Let's say you have a datasheet for fuel with a stoichiometric ratio of 13.5 parts air to 1 part fuel and you were just running gasoline. You just wanted to figure out what the needed changes are to your fuel tables. The way to solve it will actually put you on the Lambda scale...
Say you're running 12.5:1 in whatever motor. We would simply take 12.5 / 14.7 (since the denominators cancel learning from jr high school) gives us .850 (Lambda, remember lambda only cares about the oxygen) to translate to the new fuel you just multiply 13.5 x .850 you solved for and you're back in the AFR scale to give you 11.47:1. With this information, you can now solve for how much more fuel you need to add to your table in a percentage.
I hope this at least gives you another thing to think about or at least more options when looking at fueling. In my opinion, it also simplifies things when tuning as most sensors are really only reading Lambda anyway and converting based on the fuel selected in the setting on the dyno.