- 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
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
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
- 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
- Not so easy on the
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
- 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
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.
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
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
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.