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Air Flow and Volumetric Efficiency

EFI Tuning Fundamentals

Discussion and questions related to the course EFI Tuning Fundamentals

Page 1

Air Flow and Volumetric Efficiency: How do you get the actual air flow? dyno ? the cfm value for the actual air flow?

The ECU/PCM calculates the airflow using MAF or MAP sensors along with other sensors such as TPS and IAT (Intake Air Temp). If you know the values these sensors are producing you can then use various calculations to find out how much airflow the engine is seeing.

Michael_AU yeah thanks for the reply bro. I saw the video, there was this "actual air flow" and theoretical air flow. If not mistaken, theoretical air flow is based on calculation on ideal condition right? so how do i read the actual air flow and compare with theoretical air flow?

So does the ECU read the actual or theoretical air flow? sorry im a bit confuse.

The ECU measure the REAL airflow and compare it to the theoretical airflow.

real/ theoretical =VE ;)

Measuring the actual airflow needs to be done using an accurately calibrated airflow meter. This is one of the reasons why most OEMs use mass airflow sensors instead of using the speed density system.

The thing to watch is that the accuracy of an airflow meter depends on the entire intake system. These are typically calibrated as an entire unit so if you change the airbox, air filter or the MAF sensor tube, this will affect the accuracy of the calibration.

As Ludo has said ECUs measure real airflow but as Andre has said some parts are calibrated to the rest of the intake system. You can't just change a MAF for a much bigger unit and expect an increase in horsepower based on increased airflow because the ECU, unless it has been told otherwise, is calibrated for the old MAF and the ECU will receive an output based on the calibration for the old MAF.

So if i am reading this correctly the ecu will tell us the actual airflow using the MAF. What if we are not running a MAF but we are using a MAP sensor. Will we be able to still get the correct actual airflow or will it be a guessing game?

When using a MAP the airflow is calculated by using the pressure in the manifold compared to the pressure outside the manifold, the TPS, and RPM.

@LostCause, if the ECU is using a MAP sensor and running on the speed density principle, the mass airflow is calculated rather than being measured. This is done using the ideal gas law.

Is it safe to plug in a generic OBD reader with generic OBD reader software and use the air flow presented by the ECU in a MAF equipped car (just for initial calculations)?

@renanmadruga provided the MAF and intake pipe are still factory, the scaling of the MAF sensor should be reasonably accurate so yes, this should give you a reasonable indication of true mass airflow.