Throttle Mass Flow Outflow

Overview

The Outflow Function describes the relationship between the pressure ratio across the throttle and the amount of air that can flow through it.

As the pressure ratio changes, the relationship between pressure and airflow also changes. The Outflow Function models this behaviour and forms a critical part of the modelled throttle body fluid dynamics used in the TMF (Throttle Mass Flow) calculation.

See the TMF Setup help topic for more information on Throttle Mass Flow Setup.

See the TMF Tuning help topic for more information on Throttle Mass Flow Tuning.

The ECU calculates two Outflow values:

  • Outflow Measured - Directly calculated from the Throttle Pressure Ratio.
  • Outflow Reference - Generated by referencing the engine’s rpm, VE at an assumed pressure ratio of 1.000.

The Outflow Measured value can become unstable as the Throttle Pressure Ratio approaches 1.000. To maintain a stable and accurate TMF calculation, the ECU can blend the Outflow Reference value into the calculation in this region. The blend will start when the Outflow Reference becomes higher in value than Outflow Measured.

In summary, the TMF calculation blends between Outflow Measured and Outflow Reference under various conditions.

Outflow Reference Scaler

The TMF Outflow Reference Scaler scales the Outflow Reference value. The goal is for the ECU to smoothly transition to the Outflow Reference when the Outflow Measured value becomes unstable.

There are many contributing factors to the scaler value, but the single biggest one is the number of cylinders:

Typical values:
4 Cyl = 24.0 - 28.0
6 Cyl = 18.0 - 22.0
8 Cyl = 10.0 - 14.0

Outflow Measured Correction Table

The TMF Outflow Measured Correction Table allows the Outflow Measured value to be scaled using a percentage correction. The primary purpose of this correction is to apply a negative adjustment, reducing the Outflow Measured value and allowing the Outflow Reference value to be blended into the TMF calculation earlier.

Tuning Tip: Outflow Reference Tuning
  • The simplest way to set this value is to compare Throttle Air Mass to Speed Density (MAP) Air Mass at throttle pressure ratios close to 1.000. Adjust the Outflow Reference Scaler so that the Throttle Air Mass is the same as, or slightly lower than, the MAP Air Mass at a pressure ratio of 1.000.
  • When the throttle body size is well matched to the engine, this condition should typically occur at or near WOT.
  • MAP Air Mass MUST be valid and meeting lambda targets.

Tuning

For more information on tuning both the Outflow Measured and Outflow Reference, refer to Section 6 in TMF Tuning