A turbocharger control system that maintains turbo speed during throttle lift, eliminating turbo lag during reapplication of throttle.
Anti-lag is a turbocharger control system that maintains turbo speed during throttle lift (when the driver lifts the throttle pedal). Without anti-lag, the turbo spins down during throttle lift and must spool back up when throttle reapplies, creating turbo lag that delays power delivery. Anti-lag systems prevent the spool-down by various mechanisms, providing instant boost when throttle reapplies.
Two main anti-lag mechanisms exist. Mechanical anti-lag uses a bypass valve that recirculates air through the turbo during throttle lift, keeping the wheel spinning. The system is simpler but less effective. Electronic/fuel anti-lag deliberately enriches the air-fuel mixture during throttle lift, causing exhaust combustion in the manifold (the famous "popping" or "banging" sound) that maintains exhaust gas flow to the turbo. The fuel-burn anti-lag is more effective but harder on the engine.
Anti-lag is essential in rally racing, where the throttle is lifted briefly during corners but boost is needed immediately for corner exit. Most WRC rally cars use sophisticated anti-lag systems. In street cars, anti-lag is more controversial; the systems can damage exhaust components from the high temperatures generated during operation, and aftermarket installations are typically race-only. The popping sound has become associated with rally and aggressive tuner culture.