An anti-lag system (ALS) or misfiring system is a system used on turbocharged engines to minimize turbo lag on performance cars. It works by arranging for fuel and air to be in the exhaust duct after the engine, and before the turbocharger. This ignites in the hot ducting and the combustion process that occurs there keeps the turbocharger spinning when the engine is not delivering enough exhaust gas. The system is characterized by a popping combustion sounds when the throttle is released.
ALS was first used in the early days of turbocharged cars in Formula One racing circa mid to late 1980s, until fuel restrictions made its use unsuitable. Later it became a common feature in rally cars because of the increased turbo lag from the mandated restrictors at the intake manifold inlet. Due to the pressure drop across the restriction, the pressure ratio for a given boost level is much higher and the turbocharger must spin much faster to produce the same boost as when the engine operates without restriction. This increases turbo lag significantly compared to unrestricted turbochargers.
An ALS requires an air bypass, generally done in one of two ways. The first method is to use a throttle air bypass; this may be an external bypass valve or a solenoid valve which opens the throttle 12-20 degrees. This allows air to bypass the closed throttle and to reach the engine. The second method is to use a bypass valve which feeds charge air directly to the exhaust manifold.