Understanding Subaru Air Mass Sensors

Understanding Subaru Air Mass Sensors

Improving engine power through mechanical upgrades such as free-flowing exhausts, intakes, turbochargers and intercoolers will boost the amount of airflow into the engine over stock. Any increase in airflow is immediately picked up by the air mass sensor. This causes the following things to occur:

Increased load forces the ECU to operate in areas of the fuel and ignition maps that it was not calibrated for, resulting in fuel mixtures that are too rich, eating up some of the power gains made by fitting the parts in the first place.

Reductions in exhaust back pressure enables more ignition advance to be safely run in the mid range than the active ignition control system is programmed to allow, thus compromising peak potential power.

Outside of the range of active timing control the ECU is effectively deaf and is programmed with a safe set of ignition timing values. These safe ignition values are for an unmodified engine. At high RPM and loads ignition timing needs to be retarded, especially after 6,000 RPM, or the engine can start to detonate.

Normally as engine load and RPM increases, both fuel and ignition mapping changes progressively in reaction to this. After modifications, very quickly the ECU can reach the end of its scaling so that in essence fuel and timing becomes fixed at the last set values. This leads to driveability and reliability issues.

The OE boost control is calibrated to suit a known set of exhaust and intake dynamics. More efficient parts can throw the boost control system to far out of sync for its closed loop boost control system to compensate for. Wild boost fluctuations, over boosting or even under boosting are all possible consequences.

Products that modify the way in which the engine management system works, in order to address the above issues are broken up into four broad groupings. They include re-flashing the OE ECU; fitting an interceptor type add-on controller; complete replacement ECU replacements; and finally plug-in chip upgrades. All of these upgrades have their place in the grand scheme of tuning things, each with their own pros and cons.

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