The Speed Density (SD) feature changes the way the engine load is calculated. When SD mode is activated, the ECU will disregard the MAF sensor reading and will instead calculate mass airflow based on engine speed, manifold pressure, variable valve event lift (VVEL) and air temperature. This feature can be useful in the following conditions:
Extra large MAF sensor intakes have been fitted causing low air speed and poor air speed stability
The MAF sensor reading suffers from air turbulence from the compressor wheel (turbo and supercharger models)
If an external blow off valve has been fitted (turbo models)
The MAF sensor reading hits 5 volts at high RPM
In addition, the Hybrid Speed Density feature offers a combined MAF and SD configuration, the transition from MAF to SD (or SD to MAF) can be calibrated against a range of activation thresholds like engine speed, mass air flow or manifold pressure. The Speed Density feature can be enabled in any of the map switch modes.