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

• MAF Airflow vs. Output Frequency: This table is used to calibrate the MAF sensor. The MAF sensor works by outputting a frequency signal that is proportional to the airflow through the meter. This table is used to lookup the Airflow (g/sec) value for a given frequency and ultimately calculate the Airmass (g/cyl) used for fuelling calculations. If you modify your MAF meter you will likely have to modify this table.
• MAF Airflow vs. Output Frequency (Low): This table is used to calibrate the MAF sensor in the low frequency range. The MAF sensor works by outputting a frequency signal that is proportional to the airflow through the meter. This table is used to lookup the Airflow (g/sec) value for a given frequency and ultimately calculate the Airmass (g/cyl) used for fuelling calculations. If you modify your MAF meter you will likely have to modify this table.
• MAF Airflow vs. Output Frequency (High): This table is used to calibrate the MAF sensor in the high frequency range. The MAF sensor works by outputting a frequency signal that is proportional to the airflow through the meter. This table is used to lookup the Airflow (g/sec) value for a given frequency and ultimately calculate the Airmass (g/cyl) used for fuelling calculations. If you modify your MAF meter you will likely have to modify this table.
• MAF Airflow vs. Voltage: This table is used to calibrate the MAF sensor. The MAF sensor works by outputting a voltage signal that is proportional to the airflow through the meter. This table is used to lookup the Airflow(g/sec) value for a given voltage.
• MAF Allowed Delta Airflow: These tables define the maximum positive and negative airflow (g/sec) change allowed every 12.5ms

Estimated Airflow

• Estimated Airflow Enable ECT: Above this coolant temperature, the estimated airflow tests will be enabled.
• Estimated Airflow vs. Total Fuel Qty vs. RPM: The estimated engine airflow in relation to Total Fuel Quantity and RPM.
• Estimated Airflow Mult vs. Baro: This value multiplies the Estimated Airflow in relation to barometric pressure.
• Estimated Airflow Mult vs. IAT: This value multiplies the Estimated Airflow in relation to intake temperature.
• Estimated Airflow Mult vs. ECT: This value multiplies the Estimated Airflow in relation to engine coolant temperature.
• Estimated Airflow Allowed Error vs. Fuel Rate vs. RPM: If the error between the MAF Airflow and the Estimated Airflow exceeds this value, the Estimated Airflow will be used.
• Estimated Airflow Error Counts vs. ECT: The number of failures required before the Estimated Airflow will be used.

MAF Correction

• MAF Correction: MAF Correction factor.

Main VE

• Primary VE vs. RPM vs. MAP: This table is used to determine airmass per cylinder in case of MAF failure (Speed Density Mode) and also to provide a base airflow value to check for MAF failure. It is also used to provide transient condition correction to the VCM main airmass calculations.
• Primary VE Multiplier: Used to correct the primary VE table under certain conditions. This table is enabled in HP Tuners "1 bar Speed Density Enhanced" package.
• Secondary VE vs. RPM vs. MAP: This table is used in some code variations when MAF failure is detected.

  
  

• VE - Idle: This table defines the volumetric efficiency and is used to determine airmass per cylinder when at idle.
• VE - High RPM: This table defines the volumetric efficiency and is used to determine airmass per cylinder during high RPM operation.
• VE - Low RPM Single Fire: This table defines the volumetric efficiency and is used to determine airmass per cylinder during low RPM operation in single fire fuel mode.
• VE - Low RPM Double Fire: This table defines the volumetric efficiency and is used to determine airmass per cylinder during low RPM operation in double fire fuel mode.
• VE - Low RPM Coastdown: This table defines the volumetric efficiency and is used to determine airmass per cylinder during low RPM operation during coastdown.
• VE Offset: This value is used to offset the entire VE table to enable VE values greater than 100%. The editor automatically includes the offset value in its display.

Reversion

• Reversion VE Baro High: The Baro threshold for the Reversion VE High Altitude table.
• Reversion VE Baro Low: The Baro threshold for the Reversion VE Low Altitude table.
• Reversion Vacuum: Above this vacuum, reversion conditions can enable.
• Reversion Vacuum Hysterisis: Once reversion conditions are enabled vacuum must increase by this amount to disable.
• VE Reversion Vacuum: Below this vacuum, reversion conditions will be set.
• VE Reversion Baro Correction: Correction to the actual barometric pressure when calculating the reversion VE.
• Reversion RPM Thresholds: RPM thresholds to enable reversion condition.
• Reversion RPM Mult: RPM multiplier to enable reversion condition.
• VE Reversion Low Altitude: The VE lower limit during reversion conditions at low altitude reference.
• VE Reversion High Altitude: The VE lower limit during reversion conditions at High altitude reference.
• VE Reversion Low Altitude(DOD): The VE lower limit during reversion conditions at Low altitude reference.
• VE Reversion High Altitude(DOD): The VE upper limit during reversion conditions at High altitude reference.
• VE Reversion Low Altitude(Cams Parked): The VE lower limit during reversion conditions at Low altitude reference.
• VE Reversion High Altitude(Cams Parked): The VE upper limit during reversion conditions at High altitude reference.
• VE Reversion Low Altitude(DOD,Cams Parked): The VE lower limit during reversion conditions at Low altitude reference.
• VE Reversion High Altitude(DOD,Cams Parked): The VE upper limit during reversion conditions at High altitude reference.
• VE Reversion Baro Correction: Correction to the actual barometric pressure when calculating the reversion VE.

Cranking VE

• Cranking VE Percent vs. MAP vs. RPM: This table is used to determine airmass calculations during cranking. It is a percentage of ideal cylinder volume.
• Cranking VE Multiplier vs. Baro Offset: This table is used to correct the cranking airmass calculations against Barometric Pressure.
• Cal Temp: Temperature at which cranking VE tables were calibrated (square root of temperature in degrees Kelvin).
• Charge Temp vs. Airflow: Cylinder charge temperature estimate vs. engine coolant temp during cranking (square root of temperature in degrees Kelvin).

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

• EGR Airflow: Airflow through the EGR valve versus vacuum and percent valve opening.
• Filter Coefficient: Filter coefficient used to filter EGR airflow value.

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.

• Complex Temp Model: This chooses either the simple IAT or complex temperature estimation model.
• Charge Temp Bias vs. ECT: This table determines how much the charge temperature is biased towards the ECT or IAT. Higher numbers bias the resultant temperature towards ECT (generally higher bias at lower airflows. ie. the air has longer to be heated by the manifold at low airflows).
• Charge Temp Filter vs. Airflow: This table determines the rate at which the temperature moves to the new bias temperature. At low airflows the rate of temperature change is slower than at high airflows.

Intake Heater

• Intake Heater Timer vs. IAT: Intake heater timer contribution due to intake temperature.
• Intake Heater Timer vs. ECT: Intake heater timer contribution due to engine coolant temperature.
• Intake Heater Timer vs. Baro: Intake heater timer contribution due to barometric pressure.
• Intake Heater Timer Max vs. IGNV: Intake heater timer maximum while engine is running.
• Intake Heater Wait/Crank vs. IGNV: Intake heater timer maximum when the ignition is on but engine is not running.

Altitude Multipliers

• Multiplier High Altitude:
• Multiplier Medium Altitude:
• Multiplier Low Altitude:

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.

• Timer Base: The base, minimum or starting value of the timer.
• Timer Adder, TCS Off: This table adds to the timer dependant on airflow, in a standard calibration higher airflows increase the timer and lower airflows decrease it. This table is used when TCS is inactive.
• Timer Adder, TCS On: This table adds to the timer dependant on airflow, in a standard calibration higher airflows increase the timer and lower airflows decrease it. This table is used when TCS is active.
• Timer Max: This is the threshold, above which the BCV will open.