What Is Turbo Anti-Lag? The Tech Behind Turbocharging's Burbles and Pops

When your rally enthusiast car pals are dissecting the most recent race, they may mention “anti-lag” in their banter. You vaguely realize it’s associated with the epic, fiery exhaust pops these turbocharged terrors produce, but you don’t really know what specifically has been done to the cars to create all that sound and fury­. You may also wonder what all that loud fire might do to an engine’s internals. Well, we have answers.

What Is Anti-Lag?

In a nutshell, it’s any of several means of eliminating or reducing the delay between opening the throttle and a turbocharger delivering meaningful torque. In other words, an antidote to turbo lag. They were essential in some homologated racing series where restrictor plates greatly exaggerated turbo lag.

How Anti-Lag Works

There are several types of anti-lag systems (ALS) that can accomplish this, most of which involve making fuel and air available in the exhaust manifold, the burning and expansion of which serves to keep the turbocharger spooling during periods when the throttle is temporarily closed—as when shifting or braking to enter a curve. These generally result in an impressive display of fire from the tailpipe.

Is Anti-Lag Street Legal?

Except for the electric variety, almost certainly not. These systems inevitably dump loads of surplus air and fuel into the exhaust, meaning it would probably be dumb luck if they managed to burn at the stoichiometric air-fuel mixture required to make a three-way catalyst function and emit legally clean air. The production cars you hear burbling and popping on engine overrun do so under off-cycle conditions and most likely at legal air-fuel ratios—just with late ignition that lets some fuel burn in the exhaust pipe.

Can Anti-Lag Systems Damage an Engine?

Absolutely. Exhaust manifolds and turbochargers are not combustion chambers. Sure, they’re made to take heat, but they’re not all designed to endure prolonged operation at the insane heat levels involved in some of these systems. Some are therefore programmed to shut off above a certain coolant temperature. The turbine blades are also manufactured to extremely tight tolerances and designed to operate at a maximum speed—uncontrolled external combustion makes it difficult or impossible for the engine-control computer to monitor and control such things. In racing, teams don’t think twice about replacing turbos after a few races. Shade-tree tuners on tighter budgets should be mindful of how much they use an anti-lag system.

How Does Anti-Lag Affect Fuel Economy?

It torpedoes it. Prolonged use of an anti-lag system can as much as double fuel consumption. Remember, these systems are like keeping your foot on the gas even when you’re slowing down, not just when you’re charging forward. Speaking of which, obviously there’s no engine braking effect when you lift and an anti-lag system kicks in, so budget for more frequent brake pad replacement, as well. Hey, what price fiery pops? (And it’s only money.)

Anti-Lag FAQs

• Impact on turbocharger lifespan Anti-lag shortens turbocharger lifespan considerably, as the combustion that spins the turbocharger is largely happening uncontrolled in the exhaust system.

• Suitability for daily-driven cars The most suitable way to avoid turbo lag in a production car used on the road is via hybridization or at least electrification of the turbine (as on the new Porsche 911 GTS T-Hybrid).
• Does anti-lag add horsepower? Peak horsepower and torque are unlikely to change due to the addition of an anti-lag system; but rather, the power is more readily available when needed.
• Is anti-lag the same as 2-step? Two-step launch control systems that spool the turbo while holding a preferred launch rpm are sort of a subcategory of anti-lag system.
• Does anti-lag use more fuel? A lot. Fuel consumption can be as much as doubled.
• Is anti-lag legal? Depends on the system. Obviously the turbo-lag reduction systems that come from the factory (including most electric hybrid and e-turbo systems) are emissions legal. Scarcely any type of system like the ones popularized in WRC rally racing are street legal. If you see flames coming out the tailpipe, it’s way too late for a catalytic converter to clean up their combustion!

Electric Anti-Lag

We should also note that many of today’s hybrid vehicles with turbocharged engines use onboard electric power that directly fills in torque at low rpm, and/or mount an electric motor to the turbo to spool it up almost instantly when power is called for. In racing, sometimes, when electric power usage requirements allow it, even at high rpm, the motors spin the turbo so the wastegates can open wide to reduce backpressure. This can add 5 to 10 percent power. These are obviously the most eco-friendly anti-lag systems.

Benefits and Disadvantages of Anti-Lag 

• Benefits: No turbo lag when you return to the throttle after slowing or braking, or when completing a shift, and sometimes when launching at a dragstrip.

• Disadvantages: Potentially quite harmful to the turbocharger (see below). Increases fuel consumption (a lot), and results in greater brake wear.

Implementation Tip: Software-Defined Anti-Lag

The simplest anti-lag systems require no external plumbing or hardware. Simply holding the throttle-by-wire open to admit excess air, then ordering a bit of late fuel injection, and massively retarding the ignition (to maybe 45 degrees after top-dead-center) does the trick. This causes the air-fuel mixture to ignite as or after the exhaust valve opens. Now instead of pushing the piston down, the products of combustion expand in the exhaust. This keeps the turbo spooling and delights onlookers with pops and fire-belches. As soon as the driver drops the hammer again, fuel injection and spark timing return to normal and the car charges off with a full head of turbo boost. Some chip tuners use a similar approach for a launch-control mode. When holding the throttle at a launch rpm, the system retards the ignition and adds fuel to encourage late combustion that either spools up the turbo or launches at higher boost levels. Similar tuning can also be activated by the clutch, to keep the turbo spooling (and the exhaust belching fire) during shifts.

Implementation Tip: Secondary Air Injection/Inlet Bypass

Proper racing applications employ separate valves and plumbing to inject air directly into the exhaust manifolds in order to burn a rich mixture and keep the turbos spooled. Ferrari employed this approach in F1 in the 1980s, as did Mitsubishi and Toyota in their Lancer Evolution III and Celica GT-Four WRC cars. Brass tubes redirected air from the compressor bypass back into the exhaust manifold. Basically, when the throttle closes, the pressure left in the intercooler ductwork vents around the engine and directly into the exhaust where it ignites a rich mixture to keep the turbo spooling. An ECU controls the valves directing the flow. A run of 2,500 street-legal Celica GT-Fours were sold in Japan with this plumbing for Group A homologation purposes. That system was disabled for street use, but on later JDM models of the Mitsubishi Evo (IV-XI), the Secondary Air System (SAS) reportedly can be activated relatively easily. The most evolved anti-lag systems in WRC racing today use valves that meter compressed intake air directly into the exhaust manifold to precisely control the turbo speed to suit a map of ideal operating conditions. The Prodrive P2 uses such a system.

Implementation Tip: Turbo/Intercooler Intake Bypass—D-Valve

This system provides a short-circuit from the mass-air-flow sensor to the throttle body, bypassing the turbo and intercooler, allowing more direct airflow when the throttle re-opens. This is more of a quick-spooling feature than it is an anti-lag feature, but it can be combined with the above approaches to compound the effect. This is sometimes referred to as a Dan Culkin valve.