Technical Information Lambda meters and Air/Fuel Ratios

What is a lambda meter?

A lambda meter measures the air/fuel ratio using an oxygen sensor in the exhaust gas.  It is important when tuning to measure and adjust the air/fuel ratio in order for the engine to run correctly.

What is not a lambda meter

Air/fuel displays which hook up to the stock oxygen sensor are not lambda meters.  These displays simply display the stock oxygen sensor voltage on a row LEDs.  There are three problems with this - the stock oxygen sensor lambda range is far to narrow to be of use when tuning, the stock oxygen sensor output is not linear and the display performs no compensation for the oxygen sensor temperature.

A stock oxygen sensor is designed to be used by the ECU to keep the mixture near stoichiometric, which is the optimum air/fuel ratio for the catalytic converter to work. The stock oxygen sensor range is usually only about lambda 0.95 - 1.04, while tuning we usually want a display that will be accurate from lambda 0.75 - 1.10

Lambda vs Air/Fuel ratio

The air/fuel mixture is expressed either as the ratio of air to fuel vapor or as a lambda value.  The lambda value is derived from the stoichiometric air/fuel ratio, which is the chemically correct ratio of air to fuel for complete combustion to take place.  The stoichiometric ratio is 14.7:1 when expressed as an air/fuel ratio, or 1 when expressed as a lambda value.  A richer mixture will have a lower air/fuel ratio and lower lambda value. e.g. an air/fuel ratio of 12.5:1 equals a lambda value of 0.85, and is a typical value for a naturally aspirated engine under full load.

Tuning mixture

The optimum mixture varies with engine type, engine load and speed, operating conditions and operator preference. This is a guide only, and .

In general it is best to tune the mixture close to stoichiometric up to moderate engine loads for engines using catalytic converters and running in closed loop. This will keep the long term mixture adjustment from altering the mixture at points in the fuel tables where the ECU is not running in closed loop.

From moderate load to atmospheric pressure the mixture can be made progressively richer for increased power, cooler running and less chance of damaging the engine.  Usually the air/fuel ratio which produces maximum power is from 12.5:1 - 13.5:1, but this varies with engine type.

Under boost the mixture should be made richer to prevent detonation, run cooler and reduce the chance of damaging the engine.  For more information see the tuning guide.