What is Anti Lag? Why so important in rally?

Anti lag firing on our WRX STi rally car

What is Anti Lag?

Anti Lag is a system used to eliminate turbo lag. It was originally devised for rally cars to keep a turbo spinning at full boost even at low engine RPM. It means the driver has instant power out of corners and between gear changes.

The history of turbos in rally

Let’s go back and delve into the history before we can answer what is anti lag. As most of us know turbo charging didn’t really come into rallying until the early 1980s with the Audi Quattro. The introduction of four-wheel drive rally cars allowed more power to be put to the ground than with 2wd rally cars.

A turbo charger is a very ingenious piece of kit that allows a smaller engine to produce the power and torque of a larger engine. The turbo does this by compressing air and forcing it into the engine (boost). Great for saving space and weight, however turbos do have some inherent draw backs. Without anti lag, the  biggest problem is turbo lag.

How a turbo charged engine works. Air forced in creating boost.

Turbo Lag

To produce maximum boost, exhaust gases need to leave the engine with great force to keep a turbo charger spinning. When a rally driver takes their foot off the accelerator the engine rpm drops. This means the gas leaving the engine drops and the turbo slows down. It then takes a few seconds for the turbocharger to spool up again when the rally driver puts their foot back on the accelerator. The time it takes for the turbo to start producing boost is called turbo lag.

Why is anti lag needed?

In a rally scenario we are always racing the clock. Turbo lag can cause loss of time. It might only be split seconds here and there however that can add up. Say over a 20 kilometer special stage you could lose one to two seconds per kilometer. By the time you’ve got to the end of a stage that could be 20 seconds of lost time. The solution was to find a way to eliminate turbo lag hence the name anti lag.

Old school anti lag

There were a few primitive and semi effective methods of anti lag in the early days of turbo rally cars. Some were technical solutions and some were driving techniques. The most common was for the rally driver to keep their foot down to keep the revs and therefore the boost up!

No parts required for anti lag

Believe it or not there’s no bolt on anti lag “kit” as such. The current form of the system was developed around the mid 90’s. It came about due to the advances in engine electronics and tuning. Generally speaking anti lag is programmed into the rally car’s engine management system and that tune is known as a map.

The right conditions

A specific set of conditions needs to be met for the anti-lag to work. Let’s say you’re roaring along a special stage and you’re approaching a corner. You start down shifting and braking which means you’re taking your foot off the accelerator. The engine management senses that the throttle is closed and the engine is revving above 4,000 RPM so it runs the anti lag part of the map.

How Anti Lag works

 What anti lag does is fire the injectors and the spark plugs on the exhaust stroke of each cylinder. The very rich fuel mixture is ignited however instead of producing power it actually exits the engine into the exhaust manifold creating a pressure wave. The explosive nature of that somewhat external combustion keeps the turbocharger spinning at high rpm to maintain maximum boost.

Anti lag firing on a Subaru WRX rally car during a special stage of the 2019 Kirup Stages Rally

The sights and sounds

The sensation of anti lag that we hear and see when watching a rally car is the back firing and the flames from the exhaust. From the rally driver’s point of view when you lift off the accelerator and then put your foot down again there’s no lag. You have maximum boost all the time and the throttle response is instantaneous.

Its not all fireworks and fun!

 Now we know how it works and the advantages, let’s look at some disadvantages. Anti lag is actually quite brutal. All of that backfiring can cause extra wear and damage to the rally car engine and turbo. Turbochargers are manufactured to extremely fine tolerances and not designed for explosive pressure waves.

Hot enough to melt steel

Anti lag also produces intense heat. It’s not uncommon to have the turbo glowing bright red under the bonnet. Therefore rally car engine; cooling, lubrication and induction (intercooler) systems all have to deal with that extra heat. Up-rated engine coolers are on the menu beside frequent oil and spark plug changes.

Terrible miles to the gallon

Anti lag uses a lot more fuel as well. Obviously the engine is using fuel when you’ve got your foot on the accelerator however it’s also using fuel when you’ve got your foot off the accelerator too. Depending on the rally car and how it’s tuned, sometimes the fuel consumption can be doubled.

Anti lag causes intense heat. Hot enough to make the turbo and dump pipe glow red.

Brakes and drive train

There are a couple of other disadvantages that you probably wouldn’t think of. It’s actually to do with the brakes and the transmission. Anti lag doesn’t produce any engine braking or vacuum. In other words when you lift your foot off the accelerator the rally car doesn’t slow down.

When the anti lag is firing it keeps on pushing you forward. You have to work the brakes much harder therefore generating more heat, wear and strain on the drive train. There’s also no vacuum assist so generally rally car brake boosters are deleted creating a hard brake pedal.

Is it worth it?

All that trouble to gain a few seconds. Is it worth it? To take advantage of anti-lag you do have to cop some disadvantages too. At the pointy end however, every second counts. Rallies are often won and lost over a mere handful of seconds. Let’s not forget Anti lag also sounds and looks awesome!

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