GM > Engine > Airflow > General Airflow


Airflow and Airmass Calculations
The VCM generally uses two airflow related parameters for its calculations: Airflow (grams/sec) and Cylinder Airmass (grams/cyl). If a Mass Airflow Meter (MAF) is fitted, the airflow is measured directly by the MAF and converted to airmass (g/cyl) using RPM and the number of cylinders in the engine. The VCM also has the ability to calculate the incoming airflow and airmass by using speed density calculations involving the MAP, charge temperature and RPM. In most cases this is used as a backup mode by the PCM in case the MAF fails, however, some vehicles are not fitted with a MAF and use speed density mode all the time. On some vehicles the PCM also uses an older method of calculation that uses Throttle Position (TPS) as the "load" indicator/axis rather than MAP. This is known as Alpha-N however, the calculation remains very similar to speed density.

HP Tuners also offers a number of speed density mode enhancements that improve the VCM's ability to run in speed density mode and also provide additional parameters allowing for forced induction tuning (2bar/3bar) in speed density mode.


MAF Calibration


Estimated Airflow


MAF Correction


Main VE


Reversion


Cranking VE


EGR Airflow
Vehicles fitted with EGR systems, the PCM subtracts the EGR airflow from the speed density calculations.


Cylinder Charge Temperature
The VCM (depending on type) may use various methods to determine cylinder charge temperature. It either simply uses the IAT sensor value or it uses a complex estimation model based on a combination of IAT, ECT and airflow to account for aircharge heating from the intake manifold. The resultant temperature is used for speed density airmass calculations.


Intake Heater


Altitude Multipliers


Supercharger Boost Control
These values control the opening of the Boost Control Valve (BCV) on supercharged V6 engines. The BCV operates on a timer/credit based system where the timer starts at a base value and then is either increased or decreased cumulatively on each reference pulse dependant on airflow. If the timer reaches the Timer Max value the BCV will open and boost pressure will drop.