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Anti Lag Strategy

 - On short strokes and V config engines, using two smaller turbos is better.
 - Some straight six engines will benefit from a sequential twin setup.
 - Fuel can be dumped into the exhaust during off-throttle periods.
 - Bigger turbos mean bigger turbo lag.
 - Smaller turbos will reduce turbo lag but will also reduce max possible boost.

Turbo lag is always a consideration when dealing with turbos. The bigger the turbo, the more extreme the turbo lag. Big power will always mean big turbos and more exhaust energy is required to turn bigger compressors.

There are various ways to overcome turbo lag. These methods are referred to as anti lag strategies. In its most basic form, an engine management system dumps excess fuel into the engine while the driver lifts from the throttle. At the same time, it retards the ignition which causes the fuel to pass straight through the engine and into the exhaust system. When the fuel hits the turbine wheel it ignites because the wheel is at close to 1000dec Celsius. The resulting burn causes a lot of black smoke, but it rapidly accelerates the turbine wheel so that it is close to full boost when the driver reapplies the throttle.

Another approach is to place a fuel injector onto the exhaust manifold and inject directly into the exhaust manifold.

For a brief time, Ferrari used a system which virtually eliminated turbo lag. The inlet and exhaust manifolds were linked via a bypass passage containing a valve which opened as the throttle was closed. Any time the driver lifted from the throttle, compressed air from the inlet manifold rushed into the turbo. With excess fuel available, and a rush of air into the red hot turbine, enormous flow was created which quickly accelerated the turbine. However, the turbine wasn’t up to dealing with the excessive forces and often failed under this pressure.

More sophisticated and fundamental strategies exist for minimising turbo lag. For certain engines, using two or more smaller turbos rather than a single larger turbo can improve engine response and lessen turbo lag. This is particularly effective on very short stroke engines or on v configuration blocks where the exhaust outlet exits on both sides of the block.

On longer stroke engines using a single turbo can be better because there is already decent torque available in low down rpm. In this case the use of split pulse turbine housings provides superior response and spool-up time. The use of two turbos in this instance only increases complexity and weight, without giving enough improvement to justify multiple turbos. In some cases, race teams go from the production two-turbo setup to a single turbo setup.

Towards the end of the turbo F1 era, manufacturers such as KKK, Garrett and IHI were able to supply variable geometry turbines which could survive in very hostile petrol engine racing conditions. Variable geometry turbines help performance right through the rev range. At low rpm they function like a small turbo, but as rpm increases, they act like a larger turbo, allowing large amounts of boost to be generated. Previously, this technology was limited to low temp diesel turbos. The use of silicon nitrate enabled these turbos to operate at 1170deg Celsius and at over 160000 rpm.

Anti Lag in Rallying.
 - Group A rallying greatly refined anti-lag techniques.
 - Fuel afterburn and EMU control were used.
 - Easy to cook the turbo using this technique.

During the Group B era in rallying(1980s), the same strategies as mentioned above were used to limit turbo lag, however, when Group B was banned and 34mm restrictors were made mandatory, turbo lag became a serious problem, which could not be dealt with by simple ignition retard. Overcoming the problem involved careful turbo matching, new turbo design and effective anti lag systems which did not compromise turbo reliability or overly affect fuel consumption.

Before the Group A and WRC era, turbos could only sustain temperatures of around 950deg, but the introduction of better alloys and ceramics, meant turbos could now operate above 1000degrees(up to 1250deg for short periods). This opened the possibility of using fuel afterburn to keep the turbo spinning.
There are a few different ways of doing this and it can sometimes depend as much on driver preference as anything else. Some drivers prefer instant response as soon as they get back on the throttle; others require a more progressive approach to anti lag, especially on forestry or gravel stages. This is one of the reasons forestry cars often require different maps to tarmac cars. The same car can require up to 5 different maps just to deal with the different antilag requirements of competitors who compete in the national forestry and national and international sealed surface events.

A simple antilag system for a rally or road car will instruct the EMU to increase fuelling by 15% when it senses a trailing throttle (less than ¼ open) at greater than 2000rpm. The EMU will than revert to 10deg before TDC and will cut spark at each cylinder by ¾. (MoTeC and Gems units automate some of these settings).

At the same time, a suitably modified EGR valve opens in response to an EMU command linking the EGR to the inlet vacuum reservoir. Air which is under pressure from the compressor side of the turbo is forced into the exhaust manifold. Here, close to the turbine, the burst of air causes the unburned fuel to ignite.
This combustion creates heat and pressure which spools the turbo to produce over 20 psi boosts with the throttle closed. As the throttle is reopened at the exit of the corner, boost quickly drops to about 8psi. Once past ¼ throttle, the EMU discontinues antilag operations and normal operation resumes.

Turbo Lag on Road Cars.
 - Great improvements in the past five years.
 - Turbo lag almost eliminated on latest high compression, low boost cars.
 - Turbo sizing and good gear matching have helped.

Turbo lag is becoming less and less of a problem on road cars because of better electronic control improved turbo design, but it is still a factor which must be considered on road car applications and it is an essential consideration of any competition engine setup. And tuning beyond the stock setup leaves way for turbo lag to become a serious problem. In an effort to reach maximum top end horsepower, the lower down power and turbo lag can suffer and become even worse than stock. No improvements in top end power should be applied if it grossly affects low down response. On average a turbo car must have 30% better overall power over a naturally aspirated car just to keep a level footing on a twisty road or a race track because of the sacrifices that are made lower down the rev range. Add to this the fact that a turbo car requires more driver skill to keep it in the power zone and the need for an effective anti lag strategy becomes clearer still.

There are four main areas that have to be addressed to combat lag:

- engine hardware choice
- fine tuning fuel and spark (incl. antilag fuel and spark strategy)
- gearbox ratios and final drive ratio
- turbo sizing and technology choices

Launch Control.
 - Spark cut to one cylinder at a time.
 - Boost is built up at the line by passing unburnt fuel to the turbine.
 - Very easy on the turbo.
 - Not so easy on the drive train.

A good launch strategy is essential to overcome turbo lag at the start line. A rev limit set to anything from 4000 to maximum revs is employed through a wheel mounted switch or through the clutch pedal. When the throttle is floored and the clutch is fully depressed, the launch limit is in effect and the engine will not pass the set rev limit. Spark is cut to one cylinder at a time to hold the engine revs. With spark alternatively being held from the plugs, unburned fuel is passed into the exhaust along with the air from the cylinder. Once in the exhaust manifold, the hot air/fuel mix ignites and drives the turbo turbine. Because spark isn’t retarded using this method, the exhaust gases are kept reasonably cool. Therefore, afterburn combustion is slow and incomplete, so turbo temps don’t reach dangerous levels.

Soft Rev Limit Control.
 - A more sympathetic alternative to anti-lag.

 - Launch control on the move.

Sometimes you can leave the launch control on during hard driving and use it instead of the much more severe anti-lag (Motec and Gems) system employed by grpA and WRC rally cars.

Each time the clutch is depressed at engine rpm higher than the set rev limit, the turbo runs in afterburn mode. The turbo is able to maintain its speed during gear change. Along with the afterburn, the throttle is kept open and air is free to course into the engine from the compressor (rather than hitting a closed throttle plate), which allows the compressor to coast more freely. This system is so effective that a bigger/heavier turbo can actually be an advantage because of the flywheel effect keeping the compressor turning.

Along with the benefits to acceleration, there is also a benefit in that the drive never has to lift off the throttle during up or down shifts. This means you don’t have to worry about heel and toeing into and out of the corner. The rev limiter will hold the revs at safe levels.

Proper Engine Tuning.
 - Good fuel and ignition mapping is essential.
 - Turbo matching is important.
 - All setup needs to be done on the dyno with diagnostic and monitoring equip.

There is no point spending thousands of euros on components without setting up the fuelling and spark to take advantage and to optimize the install. Dynoing the engine management system to get the fuelling and spark right throughout the entire rev range at all engine loads is essential.

Turbo lag can be as much a product of poor tuning as incorrect turbo sizing, wrong cams etc.
A replacement EMU or a piggyback ECU that woks in series with the existing EMU is essential when carrying out any mods to your car. The engine must be supplied with the additional air and fuel as well as having the capability to use MAF or MAP or throttle position sensors to meter AFR.

Turbo Technology.

Lots of options exist to reduce lag by using various turbo technologies. In the early 90s manufacturers tried to use small sequential setups to combat turbo lag. Taking Porsches lead in the 70s, Toyota and Mazda went the sequential route with the Supra and RX7. Subaru also tried in the Legacy B4. However, sequential setups eventually fell by the wayside and tuners now offer many packages which convert these cars from twin sequential to single big turbos. Another area investigated was in the use of ceramic turbine wheels to cut inertia (the GTR also uses very light carbon composite compressor impellers) and in the use of ball bearings to reduce frictional drag. The most exciting technology to emerge however was the use of variable nozzle technology.
The Garrett VNT turbo uses multiple vanes. These close when exhaust gas flow is low to provide a small nozzle area which maximizes turbine wheel acceleration and speed. The vanes then gradually open as exhaust flow increases to minimize exhaust back pressure, control boost pressure, improve fuel economy and increase horsepower. It acts as a small turbo as low rpm and a large turbo at high rpm.

Therefore, turbo lag is quite markedly reduced. When response times of multi vane VNT turbos are compared to conventional turbos of similar size, it is found that the standard turbo requires 70-90% more time to come up to full boost. Mitsubishi and IHI also manufacture multi vane turbos as do most big turbo manufacturers. Aerodyne aerocharger is probable the best known of these (apart from Garrett of course). In some installations a wastegate inst even required. Obviously these turbos are more complex and therefore more expensive and potentially less reliable.

The most widely adapted multi vane turbo to be adopted is the twin scroll turbo. Two different sized scrolls are used for the primary and secondary. A small primary scroll is open for low speed operation and the secondary scroll is opened for high speed, high gas flow operations.

In 1992 Garrett introduced another take on the twin scroll concept. It was called the VAT25 (Variable Area Turbine). It was initially used on the Peugeot T16. A single movable vane controls the exhaust gas speed as it approaches the turbine wheel. In the closed position gas velocity increases which accelerates the turbine wheel faster. As exhaust gas increases, the flap opens and allows all the gas in which lowers exhaust gas pressure and increases flow through the turbine.

Mitsubishi came up with a double vane design that is nearly as effective as the multi vane design but is simpler and more reliable. The two vanes operate together to provide continually variable nozzle area.