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5th July 2019
Anti lag – All Just Noise?

It kills turbochargers. It’s incredibly ASBO worthy. “Sounds like it’s broken, mate,” says everyone who hears it.  Lots of onlookers and more than likely all of your family probably think you’re a grade A wally for wanting it, so why has anti-lag remapping, secondary air injection and throttle bypassing become a thing for road cars? Is it even a thing for road cars? Should it really be a thing for road cars?  We delve inside the murky world of 2step rev limiters, hard ignition cuts and retarded ignition timing to find out why everyone wants to pop, bang and shoot flames like a demented sprout fetishist holding an AK47 at a WRC social gathering.   If you’re determined to blow-up petrol in your exhaust manifold, this is the blog for you.


So, first things first, what is anti-lag?  Well, in the most basic form it’s an ECU controlled and mappable way of preventing the turbocharger from stalling. The bigger a turbo the more rotational inertia it has, so the longer it takes to reach speed – exhaust gas flow dependant.  To keep the turbo spooled up, and add the borderline obscene fireworks from your exhaust, anti-lag systems ignite additional air and fuel that’s pumped into the exhaust housing as the ignition timing is retarded, as part of the ECU map. This causes additional fuel and air to ignite as it leaves the exhaust, leading to additional heat and pressure pulsing through the turbo and out of the exhaust as flames.  This in turn keeps the turbo spinning so when you get back on the loud pedal boost is instantly available – handy if you’re charging through a Welsh forest or a Scandinavian gravel road, but around the one-way system on the way to McDonald’s?

Dedicated anti-lag valves like this from Turbosmart make air bypassing even easier, but obviously involve considerably more plumbing

Is it only cool when race cars have it?     

Now, for fear of sounding like we should just put our slippers on, fire up our pipe and make a nice warm mug of cocco whilst staying at home, here’s what prompted our fairly recent social media outburst on the subject.  Recently a member of team ITG was walking along a very quiet (30mph) road close to his abode. Up the road came a Mk2 Focus ST that could only be described as travelling at warp factor 10. It sounded like a warzone was approaching, and for someone who likes the sound of a beefy engine, particularly the 2.5-litre 5-cylinder turbo in the Mk2 Focus, it seemed slightly out of place.  A sunny, mellow Friday evening, birds tweeting – you know, the usual kind of thing, pint of milk in one hand, perhaps a copy of Pacenotes (Rally Magazine) in the other, when out of the blue it sounded as if something from a WRC stage was hoving into view. Only thing was, it wasn’t particularly pleasant. It just sounded really offensive and if a petrolhead says that imagine how ‘everyday’ folk feel about it.

Perhaps most concerningly though, it’s an absolute turbo killer. In fact, it’s so intense that every WRC cars will undoubtedly receive a new turbo after each round of the championship, and that’s with high tech, dialable anti lag systems in place

You can expect to be seeing an awful lot more of this out of the engine bay if you opt for anti lag for the street. Most WRC teams replace their turbos each and every round to avoid issues

What sorts of systems are there?

Throttle bypass – The most basic setup where the throttle plate is always held slightly open and the ECU tune is adapted to retard the ignition timing and adjust the air to fuel ratio for a much richer mixture. Rather than firing the spark at top dead centre for optimum power, the spark goes off as late as possible on the downward stroke of the piston to keep the car running, so as the piston blows it forces the explosion out through the turbo and exhaust system.


Secondary air bypass (rally style)

These are dedicated systems with plumbing inserted before the throttle plate to draw air away from the combustion process and deliver it almost directly to the turbo or exhaust manifold. This additional air is monitored by the cars ECU which then adjusts the fuel mixture to cope with the additional demands of the incoming air. They’re far more complex than the throttle bypass systems and can be adjusted in modern WRC cars to alter the severity of the anti-lag, or even switch it off altogether with the twiddle of a knob.


Is Anti lag the same as a 2-step rev limiter and launch control?

Err, no, it’s not. Although the 2-step rev limiter might make a similar sort of noise and also retard the ignition timing to prevent revving, it is nothing like anti-lag.  First and foremost, 2-step rev limiters and launch control can work with naturally aspirated vehicles, so they are in no way connected to turbochargers. Both are used to maximise grip from the line by holding the engine at the optimum power to grip point in the rev range.  In many cases this is achieved by retarding the ignition timing, so although it may sound a little bit like anti-lag, believe us when we say, they’re totally different systems for totally different purposes.  Of course, if you have a turbocharged car with a 2-step rev limiter, it might also cause the turbo to spin a little, but nothing like a dedicated anti-lag system should.

Having said all that Volvo has recently instigated a system called PowerPulse on its V90 and S90 V5 production diesel engines which inject compressed air (at up to 12bar) into the exhaust manifold to speed up the smaller of the two turbos. If this isn’t a production friendly version of anti-lag we don’t know what is.

Anyway, that’s our two pence worth on the Anti-lag debate and how it all works. What do you think? Is it something that road cars really need? Or is it just a way to cause tinnitus and keep hearing aid manufacturers in business?  Over to you ITG-ers.

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5th July 2019
Battle of the Uber Hatches - Which Is Best?
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