Anti-lag Systems Explained

Anti-lag has become a great option for eliminating turbo lag on any turbocharged car in all forms of motor sports. We here at Brewed Motorsports uses ALS in every last one of our  turbocharged engines. Drag racer applications use anti lag to make power when they launch the car, rally cars use another form of anti-lag to keep the turbo spooled while lifting off the throttle and now even drift cars are using a “rally style” anti-lag option.  Most aftermarket ECU’s as well as factory ECU tuning options are now available with a simple drag style anti lag.

Turbo Lag Explained

Turbo lag is a phenomenon in which throttle response is negatively affected due to the inertia of spinning components within a turbocharger. There is a delay between the depression of the accelerator pedal and the extra boost of power gained from the turbocharger. This delay exists in all turbochargers, but the bigger the turbocharger, the more turbo lag it will have.

Turbocharger system

The lag is inherent in the design of turbochargers because they rely on exhaust gases to spin the turbine wheel. This turbine, however, has mass. As a result, you must wait for the engine to rev and exhaust gases to build before the turbine begins to spin. Because of such characteristics of turbocharged engines, street or race cars that rely on very large turbochargers to make power must either suffer from very poor throttle response or search for alternative solutions. When building a car for drag racing this can be addressed with simply a combination of launch control and boost controllers which allow the engine to freely rev at an RPM high enough to spin the turbo before launching. Race cars that continually brake and accelerate, like rally cars, use other components such as anti-lag systems.

Turbo Lag Solution

An anti-lag system operates by intentionally combusting fuel and air within the turbine housing of a turbocharger. The system controls the addition of extra fuel into the housing and relies on the hot temperatures of the surrounding components to ignite the mixture. This combustion provides the force necessary to keep the turbine spinning even when the engine is not creating significant exhaust flows. As a result, the turbocharger is always spinning and creating a useful amount of boost, regardless of the amount of exhaust flow.

The downside to this, as you might imagine, is that the combustion forces within the turbine housing apply a large amount of stress on turbocharger components and greatly reduce the life span of these expensive parts. World Rally Championship (WRC) cars, for instance, generally replace their turbochargers after every race due to the destructive forces of anti-lag systems. These systems are easily recognizable by their backfiring sounds, somewhat similar to that of a gunshot as can be witnessed in the video below.

There a few different methods of adding extra fuel and air into the turbine housing resulting in some variation in designs. Most anti-lag systems rely on a computer (sometimes a standalone unit but often a function of the ECU) which modifies fuel and ignition maps while the throttle is closed. Making the air fuel-mixture richer (more fuel) and retarding the ignition (less spark) means some extra fuel will be pushed through the engine without combusting. When this unburned mixture reaches the turbocharger it will then combust, and result in the effects discussed above.

In addition to the extra fuel required to create explosions within the turbocharger, some systems use different methods of piping air into the exhaust system. This can be done using a combination of pipes, valves, and the inherent vacuums created in some intake systems.

Drag Style

Drag style anti lag systems are one of the simplest ALS , mostly commonly used to build boost on the line just before the race begins. By setting up a 2 step rev limiter to hold the desired RPM for launching the car (2 step rev limiter is usually activated by either the clutch switch and or a desired speed)  While a 2 step rev limiter may build a small amount of boost this is not a true anti lag system however it is VERY close. The simplest and one of the most effective drag style anti lag systems is simply a 2 step rev limiter that retards the ignition timing and adjusts the fuel just before it reaches it set rev limit.  This results in the combustion cycle happening while the exhaust valves are open causing a sudden increase in egt’s as well as pressure in the exhaust manifold pre turbo, effectively spinning the turbo much faster and creating the desired boost level. The down side to this is all the extra heat and pressure puts extra stress on everything in its path, (exhaust valves, exhaust manifold, turbo, even the exhaust system post turbo). With proper tuning of the ALS can however be very reliable. Using a High quality wastegate will also ensure for better control of boost while the antilag system is active.

Rally Style

Rally style ALS are slightly more complicated, there are also a few different techniques used to get the desired results. I will only be touching on the secondary injection style anti lag as this has proven to work VERY well for our Brewed projects as well as 90% of WRC cars (Toyota, Subaru and Mitsubishi just to name a few). By introducing oxygen and fuel (in the proper mixture) directly into the exhaust manifold pre turbo you can produce combustion similar to the drag style ALS without moving the oxygen through the engine (the throttle plate can be closed or at a very low angle such as lifting off the throttle while cornering)  This is the perfect way to lower the amount of stress on the engine/exhaust valves when compared to the drag style ALS as well as provide instant response when reapplying the throttle after lifting.

Anti-lag systems tremendously boost performance of turbocharged vehicles, but because of the destructively high pressures and temperatures these systems create, you might want to consider another approach if you’re on a tight budget.


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