Warning
A lean air/fuel condition will damage the engine. Make sure that you monitor the air/fuel ratio at all times, and abort any dyno run if the air/fuel ratio becomes too lean.
Mass flow (AFM)
Mass flow uses the air flow meter (AFM or MAF) to measure the mass of air flowing into the engine. The fuel requirements are calculated as a ratio of fuel to air using a simple mass calculation, with no volumetric efficiency tables.
Since the fuel is determined by the measurement of air into the engine, it is critical that the AFM reads the air flowing into the engine correctly. Most after-market intakes will alter the reading from the AFM to some degree, and thus alter the amount of fuel.
The AFM will automatically compensate for increase airflow from headers, exhausts and even camshafts, but tuning becomes much more difficult if the engine modifications (typically race headers and large camshafts) result in a standing wave or reversion pulse reaching the AFM, which will causes it to read incorrectly.
Low load fuel
At part throttle / low load / closed loop the ECU uses the AFM reading to calculate the fuel. There are no volumetric efficiency adjustment tables for this calculation - if the closed loop fuel is not close to stoichiometry, then the AFM is not reading the airflow correctly - usually because of an after market intake. The AFM can be re-calibrated to measure the airflow more accurately.
AFM Calibration
This table contains the calibration from AFM voltage to air flow (in g/s). The AFM can either be calibrated using a flow bench, or by observing the fuel trim at different load levels while in closed loop. See Tuning AFM Flow
Some after market air intakes do not provide a consistent air flow over the AFM - if you remove and replace the intake, the airflow as measured by the ECU will change. In this case it is best to switch to using a MAP calibration.
High load fuel
At full throttle / high load / open loop, the ECU uses an additional table is used to enrichen the mixture from the stoichiometric calculation used from the AFM.
WOT compensation table
Note that the values in the WOT compensation tables should only be changed within certain limits. The ECU will not switch into open loop correctly if the WOT compensation table values are raised so that less fuel would be required in open loop than closed loop. Generally values should not be increased over the stock values to prevent this from occurring. As a guide, do not set the WOT air/fuel ratio above 12.50:1.
If the air/fuel ratio is not the same as that in the table then the cause is that the stoichiometry calculation using the AFM is incorrect - again the reason is usually that an after market intake is being used.