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Types of Anti lag

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Hey All,

I'm doing my best to fully get my head around antilag. I understand the theory (I think) but trying to understand the difference between normal ECU generated antilag and cold air antilag.

From what I understand ECU controlled antilag is simply using some type of butterfly bypass and feeding around -10degrees of timing and a bit more fuel into the engine.

I guess it works by the non-burnt fuel being tipped into the hot exhaust via the combustion cycle and igniting. Just a question, even though the timing is retarded, why does all the fuel not burn? Is this because the mixture is so rich?

When it comes to cold air antilag. From what I can work out it seems to be the opposite process to a waste gate where the valve opens at low boost. Where the anti lag valve opens and sends the compressed air directly from the turbo into the exhaust headers (extractors). The fuel is still having to come via the cylinder head so trying to understand the advantage. Is it something to do with the air being cooler and set directly to the exhaust that makes it more efficient and creates less "push"?

I guess since the injectors are still injecting into the ports, but under antilag condition with the cold air valve open the amount of air traveling through the cylinder head would be greatly reduced, does this mean that the fuel wont burn in the head and therefor there is no requirement for retarded ignition?

Maybe I have this all wrong, but nee to this tuning thing and trying to understand antilag. In my situation, I need as much torque a practically possible down low, even at the expense of top end HP and thus the interest in antilag.

With thanks

Bearing in mind I'm far from an expert, my thoughts.

Last paragraph first, "anti-lag" will make no difference to the vehicle's bottom end torque, top end power, or anthing of the sort. It is used to keep the turbo' spinning by keeping the exhaust gases flowing though it, so when the throttle is re-applied there isn't the lag as the impellor comes back up to speed.

The usual way of doing this is by holding the throttle partly open so there is air mass entering the engine, but running the ignition timing very late so there is effectively no pressure in the cylinder, which prevents engine torque being produced, and instead the fuel burning produces pressure in the exhaust manifold. As can be imagined, this is rather hard on the hot side of the system.

If instead one plumbed a bypass between the intake and exhaust that only opened on the over-run, it would mean no air-fuel would be needed to pass through the engine and so there would be an expectation that there would be a LOT less heating of the exhaust valve, head around the ports, and reduced heat into the manifold. I believe this is what is being referred to as a 'cold' setup? I don't know what fuel and timing stratagem is used, but your suggestion of a rich mixture ignited late would seem to make sense.

I'd be interested to know more, myself, so perhaps those who have worked with the latter can comment with their experiences of it?

From what I’m reading most of the R5 rally cars are now running this system and allows a far more aggressive anti lag with less push into the corners.

If you go to you tube and search the topic, HPA actually do a video on it, but trying to understand it a little more.

Have a look at below.




Also I do understand what your saying re torque. But if I can use more anti lag, surely that means I can get back on boost quicker and therefore get back into higher torque range quicker?

Ah, I misunderstood - yes, it will make it more responsive at lower rpm.

The R5's that I have seen the tuning for are using internal "cold" blowing techniques, where they are changing the cam overlap through the VVT system so that in a overrun state the inlet and exhaust valves are overlapping, allowing for un-combusted air to flow through the cylinder. This is combined with the DI system injecting on the exhaust stroke allowing for Fuel to mix with this air charge and then ignite in the exhaust manifold. They will also do fuel cut on the other cylinders to reduce the amount of torque created under braking.

So there's a lot to understand when we talk about anti-lag. The purpose of it, as you understand, is to keep your turbo spinning when you close the throttle, so when you open it again, your turbo is already spool up and voila; immediate torque.

The easiest way to do that is to actually not close the throttle all the way, keep the injection going and retard the ignition timing so the combustion event is happening later in the engine cycle to raise the pressure in the exhaust manifold. It's pretty easy to do with a DBW throttle and modern ECU. But this brings a problem, the engine is still producing torque because the air is still entering the combustion chamber, mixed with gas and combusting with the spark. That's why there's a "push" which can be hard to predict if the anti-lag system isn't well dialed-in. To reduce the engine torque when the anti-lag is active, you use ignition cut so there are less combustion events happening. To be honest, once the anti-lag setup is correct, it works pretty good.

I never played with a fresh air anti-lag valve, but as I understand it, the system works when the throttle is closed. So there is not much air entering the intake and the combustion chamber, so there is no "push" effect as the engine can't produce torque without oxygen even if you shoot fuel and you still have spark. As the air is sitting in the exhaust manifold, pumping fuel and retarding the timing will create a combustion event closer to the turbine, which will keep the turbo spooled up.

Depending on your valvetrain configuration, anti-lag can't be problematic as it can push the exhaust valve open. If you have shim on top of the valve, that shim could fall of as the valve opens and it's not in contact with the rocker or a bucket. Happened to me when I was running my SR20 with a VE head a couple of years ago, and no later than friday I had a customer on the dyno that changed to a shimless setup on his VE head because it happened to him as well.

Thank you, gentlemen, that makes 3 possible systems?

Frank, there's an additional potential risk as the piston is 'chasing' the closing exhaust valve and if it lags behind a little...

Haha Gord, you are right, at least in my case all I had to do is replace the shim, I was surprised it didn't even got caught in a valve spring winding or anything! But my customer engine did actually bend valves from contacting the piston.

There are also "Throttle kicker" system that opens the throttle if you have a cable throttle. Some people use the IACV (Idle air control valve) which is usually not enough air bypassing the throttle to do a good system.

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