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The Bosch LSU4 wideband sensor is a low-cost sensor meant to provide reproducible lambda reading with a given fuel in a given application during a reasonable sensor lifetime (eg. upto 1000 hours in very good conditions).

Bosch specified a so-called Ip=>Lambda curve that gives a close enough lambda reading with typical gasoline in a typical application.

The lambda reading by this Ip=>Lambda curve deviates from real, depending on several factors:

Even if you know exact lambda, there is no predefined "best lambda" value. At high power, rich mixture (like lambda=0.77) is safer, but somewhat leaner (like lambda=0.82) at least upto 0.85 can give more power. But richer mixture allows higher ignition advance, that also allows more power. All this is engine (head, compression) specific, and depends on loadsite (RPM, manifold pressure) and even intake air temperature and humidity.

Since there is no absolute truth about lambda, reproducibility is most important, and tuning - though largely simplified by wideband lambda meters - still requires expertise and ears.

For a digital reading, the last digit can differ by +-0.01 : Therefore when comparing 2 digital lambda meters (even under same conditions), or one meter with different sensors, a difference of 0.02 does not necessarily indicate a problem.

AFR - Air to Fuel Ratio

Air to Fuel Ratio is an unfortunate, but widely used value, the mass ratio of air compared to fuel. Lambda=1.0 is AFR=14.7 for typical pump gas, although this is continuously decreasing as the alcohol (ethanol) content of gasoline climbs at most places (5..10% is not uncommon).

With high ethanol content, like the more and more popular E85 (with 85% ethanol and 15% gas), the AFR value is much lower.

It is recommended to use lambda value, because lambda=1.00 is much more natural value for stoich than AFR=14.7 , and at least the meaning is the same for different fuels