Turbo lag has been the bane of performance enthusiasts’ lives since turbo chargers were invented, and while there are ways to reduce or even eliminate turbo lag, anti-lag measures are either not suitable for street use or they are very expensive, since they come attached to exotic supercars in the form of sophisticated launch control systems. Nonetheless, if you have ever wondered how anti-turbo lag systems (as opposed to launch control systems) work, this article will explain the underlying operating principles of these systems, but be aware that specifics vary between applications of any given anti-lag system. Let us start with this question-
Put simply, turbo lag is an expression of the laws of inertia, in the sense that since a turbo chargers’ rotating parts posses mass, these parts require a specific energy input either to get them spinning, or, to increase their rate of spin to the point where useful boost pressure is generated and maintained. As we know, the energy input derives from both the velocity and pressure of the engine’s exhaust gases, but under normal driving conditions this energy is not constant since its intensity varies with engine speed.
Therefore, in practice, turbo lag can be defined as the period it takes for the spin rate of the combined turbine/compressor in the turbo charger to increase from its current speed, which may be zero (or very low), to a speed where useful (or maximum) boost pressure is developed and maintained.
Note that this time period depends on factors like the mass of the combined turbine/compressor, the volume, velocity, and pressure of the exiting exhaust gases, the configuration and length of the exhaust manifold runners, the rate at which the engine speed is increasing, and the diameter and length of the exhaust system, among others, which brings us to-
Anti-lag systems come in two main “flavours”, and although both types of systems have specific advantages and disadvantages, both systems place extreme loads on turbo chargers and premature turbo charger failure is therefore a common, and inevitable feature of both systems. Let us look at the first option-
Throttle Bypass, aka Throttle Kick ALS (Anti-lag System)
Most anti-lag systems use either a standalone control unit (or sometimes a programmed function of the ECU) to alter the basic fuelling and ignition mapping in such a way that some fuel and air is combusted in the exhaust manifold while the throttle is closed.
As a practical matter, throttle bypass systems greatly enrich the air/fuel mixture, while the ignition timing is retarded to as much as 35, or even 45 degrees ATDC, which in combination, has two effects. The first is that retardation of the ignition timing causes some of the intake charge to escape through the exhaust valves into exhaust manifold where it combusts to provide the pressure required to drive the turbine. The other effect is that since much of the intake charge does not combust in the cylinders, combustion pressures and therefore, torque, is significantly reduced, which reduces the overall load on the engine.
In throttle bypass ALS systems, both the fuelling and ignition mapping revert to normal operation when the throttle is opened, but since the turbo is already spinning at a very high rate due to combustion taking place in the exhaust manifold, turbo lag is greatly reduced, if not eliminated altogether.
Secondary Air Injection, aka Inlet Bypass Systems
In these systems, some of the turbo chargers’ boost pressure is tapped off through a dedicated compressor bypass valve, and fed directly into the exhaust manifold to provide the air required for combustion to take place in the exhaust manifold. However, in these systems, the ignition timing retardation is not always as extreme as in throttle kick systems, and in some cases, ignition in the exhaust manifold is initiated by the heat of the turbo charger alone. Fuel required for combustion in the exhaust manifold on an inlet bypass ALS system enters the exhaust manifold through the exhaust valves as the result of the ignition timing retardation.
It must be noted that inlet bypass ALS systems are more adaptable to specific applications than simple throttle kick systems are, and as such, inlet bypass systems have reached a very high state of refinement and sophistication.
Sequential turbo charging
While sequential turbo charging is not a true ALS system, it does prevent and/or eliminate most, if not all of the problems and issues that come with true ALS systems.
In simple terms, sequential turbo charging involves fitting two turbo chargers in sequence, with the first being considerably smaller than the other is. In practice, turbo lag can be reduced, if not eliminated with such a system since the smaller turbo charger requires less energy to get up to speed. Instead of feeding the boost pressure from the smaller unit into the engine, its boost pressure is fed directly into the larger turbo chargers’ turbine along with most of the exhaust gas.
Many manufacturers, including several manufacturers of exotic supercars use sequential forced induction along with sophisticated launch control systems and secondary rev limiters to achieve much the same result as true ALS systems, but without the cost and inherent unreliability of ALS systems, which brings us to-
One particularly exotic ALS system uses the rocket principle, in which a small jet engine-like device is incorporated into the exhaust manifold just before the actual turbo charger. This system, which has seen a practical application in some Subaru rally cars, employs a specially designed jet engine-like combustion chamber and flame tube that continually burns fuel to provide high-velocity drive pressure for the turbine, regardless of the engine speed.
While this system can deliver up to three bar of boost pressure at, or just above idling speed, it requires an incredibly complex control system that depends on sophisticated software and hugely expensive hardware to work for as long as the turbo charger remains in a serviceable condition.
Other developments in ALS systems include electrically driven sequential turbo charging, and one innovative scheme by Volvo that uses compressed air instead of combustion in the exhaust manifold to drive the turbo charger. In the Volvo system, a compressor is used to charge a reservoir, and when conditions allow, this charge of compressed air is injected into the exhaust manifold via ECU-controlled valves. According to some sources, this can accelerate the turbine to about 150 000 RPM in less than one third of a second, but there is no verifiable information available on if and when this system will enter production, or if this system will be available on production models, which begs the question why-
While there is no doubt that turbo lag can be eliminated, there are several reasons why no manufacturer of “normal” cars have adopted their use, some of which include the following-
Component reliability
Turbo chargers have not kept pace with other developments in internal combustion technology, and it would be fair to say that in terms of design and reliability, turbo chargers are essentially the same as they were when they were first invented.
Therefore, turbo chargers are not designed to cope with the extreme temperatures and rotational speeds that ALS systems generate. While some systems incorporate turbine speed sensors and control mechanisms based on the engine coolant temperature to deactivate the system under certain conditions, turbo chargers simply cannot cope with the extreme loads an ALS system places on them for extended periods.
Emission control
All anti-lag systems depend on very rich air/fuel mixtures, and combustion of that rich mixture to take place in the exhaust manifold to work which means that in practice, this out-of-cylinder combustion is rarely, if ever, complete.
In practice though, catalytic converters were never designed, or intended to cope with the huge additional hydrocarbon loads that come with this combustion. In fact, in some cases and under certain conditions, combustion propagates from the exhaust manifold, through the turbo charger, and deep into the exhaust system, which creates operating conditions that no catalytic converter core can withstand without melting for more than a few seconds.
Thus, in jurisdictions where the use of catalytic converters is a legal requirement, a vehicle running with any type of ALS system would never be able to meet emissions regulations.
Prohibitive cost
While most ALS systems are not prohibitively expensive to fit to a standard, road going vehicle, the need to replace worn out turbo chargers on a fairly regular basis most certainly is. In fact, the cumulative costs of fitting, maintaining, and repairing ALS-equipped forced induction systems is so high that only factory racing or rallying teams can afford it. For a racing or rally team, reliability (and overall cost) of the system is not a primary concern, and in most cases, the team management is usually overjoyed if the turbo chargers on their cars last for a single race.
Generally no, but if emission control is not an issue, and you (or your customer) are prepared for the associated costs of maintaining the system, it is certainly possible to fit an ALS system to a car that is intended for non-road use. Bear in mind though that doing so requires highly specialised and exert-level knowledge of forced induction, exhaust gas extraction, and ignition/fuel mapping, and that these parameters need to be calculated for the specific application; there is no one-size-fits-all approach that will work for all applications.
To illustrate the point that fitting anti-lag systems should not be attempted on a DIY basis, Mitsubishi is to this writer’s knowledge, the only manufacturer that has ever fitted a working version of a throttle bypass ALS system to a production vehicle, and that was for the Mitsubishi Evolution (generations 4-9), and then only for the Japanese domestic market.
The old adage that says, whatever you gain in one area you will lose in another, is as true with regard to forced induction and anti-lag systems as it is to anything else. Thus, while there are several companies, particularly in the USA, that manufacture and supply aftermarket ALS systems there is no guarantee that such a system will work as expected on any given application without extensive reprogramming, modification, or removal of existing emission control systems, which may or may not be legal in your jurisdiction.
