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The reason for Anti-lag is, as it's name suggests, to decrease or eliminate the lag induced by a turbocharger. It is mainly used in rally where all Group A or WRC cars are turbocharged. When you want big power out of a 1.8 / 2.0 litre engine you need a turbocharger, and the more power you want the bigger the turbocharger has to be. But a big turbocharger takes a long time to spin up and create boost pressure and that's where the ALS (Anti Lag System) comes in. Because every second a car is off boost a lot of time is lost on a rallystage. To get rid of the lag the needs to keep spinning at full speed at all times. This includes when the throttle is closed for a gearchange, when braking for a turn and when you're a the start line ready to take off. To keep the turbo spinning while the throttle is closed you need exhaust (lots of hot air basically) to keep turning the turbine wheel.
What you need for ALS:
A ECU capable of ALS.
Anti-lag valve - Replaces the original idle speed control valve.
Turbo with minimum 360thrust bearing.
EGT gauge - So you know when your turbo and manifold is going to melt.
Wiring to "arm" the anti-lag and switch it off - This simply involves wiring in a switchable earth to one of the ECU pins.
This goes for so-called "mild" anti-lag (see further down for info), for the real WRC stuff you need another (very expensive) turbo, exhaust manifold and a few other bits.
With the ALS installed - when you let go of the throttle, the valve that replaces the standard idle valve in a modified throttle body opens to let the air continue through. The amount of air this valve can flow is the only thing that affects anti-lag, the less the airflow, the less effect the anti-lag has. Then the ECU retards the ignition timing by 30 degrees so the fuel is ignited when the exhaust valves are open and most of the fuel is already in the exhaust manifold and turbo. That makes lots of nice and hot exhaust to spin the turbo and create boost pressure.
A small "mild" ALS valve can flow enough air to make 0psi boost pressure at idle/off throttle (rather than -25psi of vacuum), sometimes up to 7psi of boost, which makes the turbo hit full boost almost straight away when you press the throttle. The big WRC anti-lag valve can flow enough air to produce 22psi at idle (or off throttle), which makes the car have no lag at all. That will increase the driveability of the car a lot.
Unfortunately the WRC ALS puts a big strain on the turbo and exhaust, whereas with the 'mild' anti-lag setup is practically as reliable as not using an ALS at all. It will gradually burn out your silencer baffles, but all that does is make your exhaust louder.
Some people say that the fuel will cause "borewash" making the need for a rebuild all the time. But that is not the case. The amount of fuel used in anti-lag is not enough to cause any significant bore wash (unless you use £5 budget oil), most performance cars fuel on overrun, unless they have been mapped for good emissions, not mapped like that because of reliability reasons.
The only real thing you have to be wary of with anti-lag is that Exhaust Gas Temperatures (EGT) go sky high but there's no real worry as you could be on the overrun for well over 30 seconds on 'mild' anti-lag before the EGT will get dangerously high. If EGT gets too high it could melt the exhaust and/or turbo. The engine internals will not have any problems.
For full-on WRC anti-lag the specs is as with 'mild' anti-lag but with a bigger WRC ALS valve, and a anti-lag turbo with Maram 247 shaft and turbine wheel with 7° cut-back blades, Nimonic wastegate spindle and everything nicely put together by a reputable company. You will need that as the extra stop/start shocks and extra heat would eat a standard shaft in minutes. A stainless steel exhaust manifold may be handy too as it could stand more heat than standard one.
For road use you will not need more than 'mild' anti-lag, and for track use it might be best too as you're on track at sustained periods of time and the WRC anti-lag may be too severe for those long sessions.
The ALS system won't increase the outright power of your car but it will significantly increase the power delivery - much much more low down torque since the car gets on boost a lot earlier than without the ALS.
