What anti-lag does
When you close the throttle, exhaust flow collapses and the turbine loses speed. When you get back on the pedal, the turbo has to spool up all over again, and that pause is lag. Anti-lag keeps exhaust energy up while the throttle is shut, typically through a gear change, so the turbine is still near speed when you need it.
How much boost anti-lag can hold depends mainly on how much extra air you let into the engine while the throttle is closed. The fuel and timing side is covered in anti-lag enrichment and anti-lag ignition advance/retard. This page covers when anti-lag switches on and off.
Getting extra air in
There are three ways to feed the engine air with the throttle closed:
- Drive-by-wire throttle: the easiest. The ECU simply holds the throttle open during anti-lag, with no mechanical changes needed.
- Cable throttle with an opened throttle stop: set the throttle stop so the throttle rests slightly open and passes the air you want, then recalibrate the TPS so that resting position reads 0%. The engine would now idle too fast, so enable and set up rotational idle to bring idle speed back down.
- Idle control or a bypass valve: route air around the throttle through the idle system or a dedicated intake air bypass valve. See idle valve bypass.
How anti-lag arms and activates
Anti-lag works in two steps, based on Engine Demand (TPS on a cable throttle, pedal position on DBW):
- Arm: you push past the arming engine demand. That proves the engine was already making real airflow and boost worth preserving.
- Activate: you lift below the activating engine demand, and anti-lag takes over.
From then on the safety limits below decide whether it's allowed to keep running.
Anti-lag settings
| Setting | What it does | Default | Our advice |
|---|---|---|---|
| Arming Engine Demand | Demand you must exceed to arm anti-lag | 50 | High enough that cruising never arms it |
| Activating Engine Demand | Demand you must drop below to activate it | 1.0 | Just above fully closed, so a real lift triggers it |
| Min RPM | Anti-lag turns off below this RPM | 3000 | Stops it running at low speed and in the pits |
| Max Time | Longest it can run before all conditions must be met again | 5.00 | Keep it short. Long anti-lag events cook exhaust parts. |
| Max Coolant Temp | Turns off above this coolant temperature | 110 C (230 F) | Protects an engine that's already running hot |
| Min Coolant Temp | Turns off below this coolant temperature | 60 C (140 F) | Avoids heavy wear on a cold engine |
| Max EGT | Turns off above this exhaust gas temperature (needs an EGT sensor) | 1000 C (1832 F) | Set to what your turbo and manifold can tolerate |
| Max Turbo Speed | Turns off above this turbo shaft speed (needs a turbo speed sensor) | None listed | Use the turbo maker's limit with some margin |
Two more settings control extra hardware where it's fitted: Intake Air Valve Frequency sets the drive for an intake air bypass valve, and Exhaust Air Valve Frequency does the same for an exhaust anti-lag valve.
Anti-lag works by burning fuel and air late, often in the exhaust manifold. It raises exhaust temperatures, stresses the turbo and wears valves and exhaust hardware. It's a motorsport tool. We always fit an EGT sensor and use Max EGT and Max Time on any car that runs it.
A worked example
Take the default values. You accelerate hard past 50 engine demand, which arms anti-lag. You lift for a shift and demand drops under 1.0, so anti-lag activates. As long as RPM stays over 3000, coolant sits between 60 and 110 C (140 and 230 F) and EGT stays under the limit, it keeps working. If you stay off the throttle longer than the Max Time setting (5.00 by default), anti-lag ends and you'll have to arm it again with another hard press.
Troubleshooting
| Symptom | Usually |
|---|---|
| Anti-lag never activates | Never armed (arming demand too high), coolant outside the window, or RPM below Min RPM |
| Activates but makes little boost | Not enough extra air; open the throttle stop, DBW position or bypass valve further |
| Cuts out mid-corner | Max Time too short, or EGT limit reached |
| Idle too high after opening the throttle stop | Rotational idle not set up |
Anti-lag FAQ
Can I run anti-lag on a street car?
We don't recommend it. It's loud, hard on the turbo and exhaust, and shortens component life. Most street cars are better served by good boost control and launch control.
Do I need drive-by-wire for anti-lag?
No, but it makes it far simpler. With a cable throttle you'll open the throttle stop and use rotational idle, or add a bypass valve. See anti-lag wiring for the outputs.
