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Anti-Lag System: What It Is & How It Works (Benefits & Risks)

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Last Updated on: 7th September 2023, 12:40 am

Anti-lag is designed to reduce a phenomenon called turbo lag, where there is a delay between power output to the wheels and throttle input, but what is anti-lag and how does it work?

The anti-lag system (ALS) is a system and technique designed to decrease turbo lag on turbocharged engines which helps to reduce time to boost, improve throttle response, and improve performance and in tuner and racing vehicles.

Anti-lag systems can work in a variety of ways. The main method of anti-lag is slightly delaying the ignition timing to increase the pressure on the turbocharger’s charging side.

This is accomplished by allowing a tiny quantity of fuel/air mixture to escape through the exhaust valves and burn in the exhaust manifold instead of in the combustion chamber.

Most anti-lag systems work while off the throttle and others work while on throttle (rolling anti-lag), in some cases, both can be used.

Anti-lag is different from two-step but they can work via a similar mechanism.

In this article, I’ll explain the various types of anti-lag and how they work, along with the benefits and risks of these systems.

What Is Turbo Lag & Anti-Lag?

As mentioned above, anti-lag is a system designed to reduce the delay between throttle input and turbo boost pressure, known as turbo lag.

Turbo lag is caused by various factors within the design of the engine, such as the following.

  • A large turbo
  • Large diameter charge piping
  • Large diameter exhaust piping

You will also feel turbo lag when trying to accelerate at low rpms before the engine reaches high enough rpms to spool the turbo, this will differ based on the size of the turbocharger.

Changing gears also drops boost pressure almost instantly, an anti-lag system could maintain boost pressure throughout the gear change.

Some anti-lag systems are capable of producing turbo boost pressure even while at low rpms that usually cannot spool the turbo.

Some competitive motorsports such as Formula One, rally, and circuit events use anti-lag.

Anti-lag was initially utilised in Formula One racing in the early days of turbocharged vehicles, during the mid to late 1980s, until fuel limitations rendered its usage impractical.

Because of the increased turbo lag caused by the required restrictors at the intake manifold inlet, it later became a popular feature in rally cars.

Due to this restriction, the pressure ratio for a given boost level is considerably greater due to the pressure drop across the restriction, and the turbocharger must spin much quicker to generate the same boost as if the engine ran without the restriction.

When compared to unrestricted turbochargers, this substantially increases turbo lag.

Some anti-lag systems require an air bypass, which may be accomplished by one of the following methods.

  1. To use a throttle air bypass (external or solenoid valve) that opens the throttle 12-20 degrees. This allows air to enter the engine with a closed throttle.
  2. To employ a bypass valve that redirects charge air into the exhaust manifold.

Though, there are multiple variations and types of anti-lag which work in different ways.

Types of Car Anti-Lag Systems

Many types of anti-lag exist, some require the usage of an air bypass, some require excess fuel usage, some are even electrical motor systems or complex NOS configurations.

The various types of anti-lag are listed below.

  1. Ignition retard & fuel pump
  2. Throttle bypass (throttle kick)
  3. Secondary air injection (inlet bypass)
  4. Turbo & intercooler bypass (D-valve)
  5. NOS injection
  6. MGU-H & electric motor systems

These are the various types of anti-lag systems that exist, some vehicles may use a modified version, a combination, or a hybrid of various of these systems.

Below is each anti-lag system explained in more detail and how they work.

1. Ignition Retard & Fuel Pump

Many programmable ECUs/ECU software include an “anti-lag” function for spooling turbos off the line or between shifts.

These are used for launching and drag racing, and have a different outcome than the variants mentioned above.

When a vehicle is being kept at its launch RPM limit, certain ECUs may be set to delay the ignition and inject more fuel.

So, because the engine is pushing the air/fuel combination closer and closer to the turbine, the engine spools up sooner than usual, or produces greater boost at the launch RPM than it would ordinarily.

A clutch input may activate this “fuel dump and ignition retard” anti-lag technique, making it function between gear shifts.

The pressures produced when the air/fuel combination spontaneously combusts from the heat of the turbine housing or is ignited by a highly delayed ignition event (after the exhaust stroke starts) may possibly cause popping/flames.

This kind of “anti-lag” works because the throttle is kept at full open, enabling more air into the engine. This anti-lag system wouldn’t function well, or at all, at partly or closed throttle.

ignition retard fuel dump anti-lag

2. Throttle Bypass (Throttle Kick)

The throttle bypass anti-lag method is coupled with ignition retardation and modest fuel enrichment which is mostly for cooling.

Because of the late ignition, the gas in the combustion chamber expands very little, and the pressure and temperature remain extremely high when the exhaust valve opens.

At the same time, the torque supplied to the crankshaft will be negligible, just enough to keep the engine running.

The greater flow, coupled with higher exhaust pressure and temperature, is sufficient to keep the turbocharger spinning at high speeds, minimising turbo lag.

When the throttle is re-opened, the ignition and fuel injection return to normal functioning.

Because numerous components of the engine are subjected to very high temperatures as well as high pressure pulses during this type of anti-lag, it is extremely taxing on the engine, turbocharger, and exhaust manifold.

Not only are high temperatures a concern, but so are uncontrolled turbo spool speeds, which may cause damage and accelerated wear to the turbocharger.

To avoid overheating and damage, this type of anti-lag is typically switched off when the coolant reaches a temperature of 110-115 °C.

3. Secondary Air Injection (Inlet Bypass)

This anti-lag system uses brass tubes which carries air from the turbocharger’s compressor bypass valve (CBV) / blow off valve (BOV) to each of the exhaust manifold tracts, supplying the required air for fuel burning in the exhaust manifold.

The system is managed by two pressure valves that are controlled by the ECU, but this can be different in certain vehicle setups.

secondary air injection anti lag

4. Turbo & Intercooler Bypass (D-Valve)

The D-valve or anti-lag valve is another method of anti-lag, it directs air flow away from the intake directly into the exhaust manifold (pre-turbo) to help it spool while off-throttle.

The Dan Culkin valve is another name for this anti-lag system.

When there is negative air pressure at the throttle body, a one-way check valve is placed before the throttle body, allowing air to skip the turbo, intercooler, and pipework.

More air is combusting as a consequence, which implies more air is pushing the turbine side of the turbo. The D-valve shuts once positive pressure is detected in the charge piping.

To ensure appropriate A/F ratios when utilised in a MAF setup, the D-valve should pull air via the MAF. In some vehicle ECU setups, this is not needed.

anti lag D-valve

5. NOS Injection

As an anti-lag method, NOS (nitrous oxide) injection works wonderfully. It is sometimes set by RPM, and other times by boost pressure.

It improves acceleration off boost at low rpms and also spools the turbo extremely fast owing to enhanced combustion and higher exhaust volume; after the turbo is spooled, the nitrous progressively turns off and the turbocharger takes over.

Even if you are outside of the usual boost rpm range, the boost will not decrease since the fact that it is now producing boost lower in the rpm range means that it is forcing more air than it normally would at that rpm into the exhaust.

Unlike some other kinds of anti-lag systems, it will not harm your engine if properly installed and set.

NOS injection anti lag

6. MGU-H & Electric Motor Systems

Modern Formula One engines are turbocharged six-cylinder V configurations with an extra hybrid system. The hybrid system is made up of two motor generator units, one kinetic and the other generator heat.

The MGU-H is used to reduce turbo lag by acting as an electric motor that pushes the turbine to spin when the driver is off the throttle, thus essentially eliminating turbo lag.

This is one of the most effective antilag techniques since the MGU-H collects heat energy when on the throttle and converts it to electrical energy, which is then stored in a battery.

It is therefore very efficient. Some other vehicles, including standard OEM vehicles such as Audi, include a variant of an electric motor to help reduce turbo lag.

MGU-H electric motor anti lag system

Benefits of Anti-Lag

Although some types of anti-lag systems can result in engine damage and potential engine failure, turbo failure, etc, there are some major benefits associated with anti-lag.

Some of the major benefits of anti-lag are as follows.

  1. Increased throttle response
  2. Reduced turbo spool latency
  3. Improved acceleration

1. Increased Throttle Response

Due to the enhanced spooling of the turbo, boost pressures come on sooner resulting in increased throttle response.

This improved throttle response is very useful when consistently on and off the throttle, such as in circuit events, rally racing, drifting, etc.

2. Reduced Turbo Spool Latency

Due to faster turbo spooling, it can reduce or essentially eliminate the time needed for the turbo to spool, meaning your vehicle will have full power as soon as your on throttle.

This is especially useful when requiring full power throughout and with milliseconds making the difference, such as Formula One and drag racing.

3. Improved Acceleration

Anti-lag can improve acceleration, this is useful for almost every occasion in motorsport, such as when accelerating from a corner.

It also allows predictable power delivery instead of the power surging all at once.

Risks of Anti-Lag

Some types of anti-lag has massive risks and effects on engine, exhaust and turbo lifespan.

This extra stress caused by anti-lag is especially prevalent on systems which use ignition retardation and fuel dumping.

NOS injection can cause detrimental effects if not properly setup.

However, if properly setup NOS injection, along with MGU-H, electrical motor systems, and bypass valves, can be very effective and almost risk-free.

Though, some of these anti-lag systems does come with its risks, such as those listed below.

  1. Risk of engine damage
  2. Risk of turbo damage
  3. Risk of exhaust manifold damage

1. Risk of Engine Damage

Due to the violent pressure and shock waves created by some types of anti-lag, it can increase the risk of serious engine damage, such as bent connecting rods, over heating, snapped valves, cylinder and head cracks, etc.

There are many components that can be damaged as a result of anti-lag systems.

2. Risk of Turbocharger Damage

Anti-lag can also cause a decrease in lifespan of the turbocharger.

Some anti-lag systems, especially ignition retardation and fuel dumping, can cause powerful shockwaves, rapid pressure variations, and heat spikes.

These characteristics of anti-lag can increase the risk of turbocharger damage, especially on the hot side of the turbo (the turbine side).

It can cause cracking, warping, fin damage or completely breaking off, bearing failure, gasket failures, and other failures.

The lifespan of the turbo is likely reduced due to the usage of certain anti-lag systems.

3. Risk of Exhaust Manifold Damage

Again, due to the massive pressure and temperature variations, and powerful shockwaves, it can cause a risk of exhaust manifold damage and failure.

It could possible even cause cracking and warping of the manifold, exhaust leaks, gasket failures, and more.

Other Drawbacks

There are also some other drawbacks of anti-lag systems, such as the likelihood of being illegal due to harmful emissions, flames coming from the exhaust, and very loud pop and bang sounds, probably exceeding noise laws.

It may also be irritating to you and other drivers to hear popping and banging every time you come off the throttle.

Two-Step vs Anti-Lag

Although many people mix up the words, anti-lag and a two-step are not the same thing; though, in some cases, they do work in a similar way.

Anti-lag is a system designed to reduce turbo lag and increase boost pressure between brief off throttle situations, such as gear shifts and when cornering, for example.

Two-step is an additional rev-limiter which helps build turbo boost pressure, mostly used as a type of launch control.

There are usually two rev limits, as follows.

  1. A lower than redline launch rev limit designed to improve acceleration from a standstill.
  2. A high rev limit (redline) designed to prevent engine damage from too high rpms.

A two-step rev limiter holds the launch rev-limit and keeps the turbocharger spooled. Two-step is only utilised during launches, while anti-lag is employed while moving.

  • Andy Lewin

    Andy Lewin is a senior mechanic, ASE qualified master technician, and an experienced automotive engineer.He's passionate about serving the automotive community with the highest-quality and trustworthy information on all things automotive. He loves to write about car repairs, maintenance, car modifications and tuning, faults, and much more.

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