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Anti Lag is a system used to eliminate turbo lag. It was originally devised for rally cars to keep a turbo spinning at full boost even at low engine RPM. It means the driver has instant power between gear changes. When a driver takes their foot off the accelerator the engine rpm drops. This means the gas leaving the engine drops and the turbo slows down. It then takes a few seconds for the turbocharger to spool up again when the rally driver puts their foot back on the accelerator. The time it takes for the turbo to start producing boost is called turbo lag.

Ways to eliminate or minimize turbo lag
  • The throttle bypass / throttle solenoid system
  • Secondary air injection / inlet bypass
  • Turbo and intercooler bypass (D-valve)
  • Ignition Retard / Fuel Dump
World Rally Championship cars use anti-lag systems which feed air directly to the exhaust system. The system works by bypassing charge air directly to the exhaust manifold which acts as a combustor when fuel rich exhaust from the engine meets up with the fresh air from the bypass. This will provide a continuous combustion limited to the exhaust manifold which significantly reduces the heat and pressure loads on the engine and turbocharger. With the latest anti-lag systems the bypass valve can not only be opened or closed but it can actually control the flow of air to the exhaust manifold very accurately. The turbocharger is fitted with a turbo speed sensor and the engine management system has a map based on throttle position and car speed which is used to find a suitable turbocharger speed and boost pressure for every condition. When the engine alone can't provide enough exhaust energy to reach the turbo speed/boost demanded by the management system, the bypass valve opens and exhaust manifold combustion begins. This not only reduces turbo load, but it also allows boost to be produced at very low engine speeds where boost was previously limited by compressor surge or exhaust energy. With relatively high boost at low speeds, this makes the low end torque superior even to large naturally aspirated engines. This kind of system has reached such a refinement that it is even possible to use the system in a road car.

Increased in engine performance.
Increased compressor efficiency.
Immediate response to the accelerator pedal.
High pressure from the turbocharger even at low engine speeds.

Very high temperature that can exceed 1100 degrees celsius.
Short life of turbocharger and engine.
Short life of catalyst.
Engine braking is very much reduced during ALS operation

Ignition Retard / Fuel Dump
In most modern ECU systems, Anti-Lag can be programmed. The end result is similar but the method of action is a bit different to the versions described above which are far more common in high-level professional motorsports such as rally. When a car, ready for launch is being held at its launch RPM limit some ECUs (whether by switch or additional throttle) can be programmed to retard the ignition by quite a few degrees and add a lot more fuel. This causes the combustion event to happen much later, as the engine is driving the air/fuel mixture out of the cylinder, closer to the turbine, causing it to spool up either at an earlier RPM than it would normally – assuming the engine unloaded waiting for launch or make more boost at the launch RPM than it would without engaging this feature. Like other types of anti-lag, overuse of this type of anti-lag can cause damage to the turbine wheel, manifold and more due to the violent pressures created when the air/fuel mixture spontaneously combusts from the heat of the turbine housing or is ignited by a very retarded ignition. This form of "anti-lag" tends to work because the times it is active, the throttle is held at 100% allowing more air into the engine. Consequently, this type of anti-lag wouldn't work (well or at all) at part/closed throttle.

Secondary air injection / inlet bypass
This method working with a bypass valve that feeds air directly to in the exhaust pipe. Air is supplied from the turbocharger's Compressor Bypass Valve to each of the exhaust manifold tracts, in order to provide the necessary air for the combustion of the fuel. At the same time as the air supply, a rich fuel mixture is maintained at low engine speeds. The system was controlled by two pressure valves operated by the ECU.

The throttle bypass/throttle solenoid system
The throttle bypass/throttle solenoid system is combined with ignition retardation and slight fuel enrichment (mainly to provide cooling), typically ignition occurs at 35-45° ATDC. This late ignition causes very little expansion of the gas in the cylinder; hence the pressure and temperature will still be very high when the exhaust valve opens. At the same time, the amount of torque delivered to the crankshaft will be very small (just enough to keep the engine running). The higher exhaust pressure and temperature combined with the increased mass flow is enough to keep the turbocharger spinning at high speed thus reducing lag. When the throttle is opened up again the ignition and fuel injection goes back to normal operation. Since many engine components are exposed to very high temperatures during ALS operation and also high-pressure pulses, this kind of system is very hard on the engine, turbocharger and exhaust manifold. For the latter not only the high temperatures are a problem but also the uncontrolled turbo speeds which can quickly destroy the turbocharger. In most applications the ALS is automatically shut down when the coolant reaches a temperature of 110–115 °C to prevent overheating.

Turbo and intercooler bypass (D-valve)
A method by which a large one-way check valve is inserted just prior to the throttle body, enabling air to bypass the turbo, intercooler, and piping during periods where there is negative air pressure at the throttle body inlet. This results in more air combusting, which means more air driving the turbine side of the turbo. As soon as positive pressure is reached in the intercooler hosing, the valve closes.