An Anti-Lag System (ALS) is an engine management technique that minimizes turbo lag time. Between shifts, lag is partly dealt with by installing a bypass valve, which opens each time the throttle plate is closed and evacuates the pressurized air from the intake tract. This minimizes any backspin on the turbine, which helps it spool up quicker and helps prevent possible damage to the turbocharger from compressor surge.
In applications where larger turbochargers are used, significant amounts of lag are often present due to increased rotational inertia and bigger housings, and a bypass valve is not enough to reduce lag time. Additionally, rally cars are often fitted with an FIA-regulated turbo inlet restrictor, which substantially increases lag time. Anti-lag systems are usually employed in applications where torque and engine availability are critical performance factors.
When the driver lifts his foot off the throttle, the ECU retards the ignition timing up to 40° or more and adds fuel. The throttle plate is kept slightly open to maintain a constant air supply to the engine. The ignition process happens so late that the spark plug fires when the exhaust valve is opening, igniting the rich air/fuel mixture and blowing super-hot exhaust gasses directly onto the turbine. This allows the turbo to produce significant boost levels at engine idle speeds.