> - All installations should
have careful boost control.
- The standard practice is to
use a wastegate.
- A precautionary fuel-cut is
sometimes used as is an inlet blow off.
- Excessive boost will
quickly destroy an engine.
- Operating at the maximum
boost that the engine can accept will not
give maximum power if operated well beyond the turbos efficiency band.
Turbo boost must be limited to keep detonation
at bay, to preserve ceramic turbo internals and to prevent engine blow-up! In fast road setups with high levels of max boost, care must be taken to
prevent cylinder head bolts from stretching, pistons from collapsing, and
conrods and crankshafts from breaking. Another reason to limit boost is to limit traction problems, particularly in
lower gears on wet and greasy roads. Excessive boost can also cause the turbo to operate beyond its efficiency
causing the inlet air to be excessively heated. Boost control is necessary to
keep the turbo at max efficiency. Finally boost must be limited to comply with competition rules which limit
boost to set levels.
- Exhaust and inlet
restrictors were used as the first boost restrictors.
- They are not
suitable for performance engines unless regulations demand it.
The first boost controls were a form of exhaust
restriction. Small bore mufflers or washers were used. High exhaust
back-pressure after the turbo was the result. This caused the usual problems
associated with excessive back pressure – cylinders retained a lot of exhaust
gas, limiting the amount of fresh air/fuel entering the cylinders, leading to
lower hp and excessive heat.
method used was on the inlet side. Inlet restrictors were used (similar to
grpA restrictors) to limit boost – grpA teams have overcome this by using
massive charge cooling. Inlet manifold blow-off valves and exhaust wastegates were soon used in place
of these crude early methods.
Inlet Vent Valves.
- Inlet side valves were
used before wastegates and are still used sometimes.
- Cheap and accurate boost
control is possible with careful design and application.
- Only suitable for
competition cars not running CATs.
Most tuners these days
use wastegates to control boost, but some tuners still swear by inlet vent
valves. Some of the most powerful competition engines use inlet boost dump control
arrangements. This system is very accurate and responsive, and a lot cheaper
than exhaust wastegates.
Often two blow off valves (dump valves) are used to provide the necessary
venting characteristics. Valve control is operated by solenoid valve control
via the EMU. Vent valve location is an important consideration. If it is placed after the
intercooler, then efficiency drops because charge cooled air is being wasted.
It is better to place the valves before the intercooler but after the
compressor outlet, that way the air is vented before entering the cooler.
Affect on Air/Fuel Ratio. If a MAP sensor is used to meter the air/fuel ratio then the use of a vent
valve won’t have any affect on the AFR. However, if a throttle position sensor
is used to meter the AFR or an air-flow meter is being used, then the mixture
will go rich whenever the valve opens. In competition engines this may not be
a major problem because fuel efficiency is not a major consideration and
because the excess fuel is providing valuable cooling which might have to be
otherwise mapped in anyway. This excess fuel will also pass into the exhaust
and burn, helping to maintain turbine speed and thus reducing turbo lag. It
also makes for spectacular flames out the exhaust. However, in competition
cars that employ a cat, the ceramic structure will melt and restrict gas flow.
The same applies to road cars with the added concern of excess fuel
So for road cars air flow metering and blow off valves don’t do hand in hand.
Of course you could modify the engine map to lean the mixture when the valve
opens but any slight change is opening times or if the valve malfunctions in
any way then a lean condition will exist in the engine and detonation and
pre-ignition won’t stay away for long.
- Most popular form of
- Highly accurate,
responsive and reliable due to years of development.
- Internal is light, cheap,
used by car most manufacturers.
- External is free flowing
and is suitable for maximum performance engines.
bypass is by far the most popular way to limit boost. Originally, the only
wastegates available were the external type. Later manufacturers developed the
cheaper and more compact internal wastegate – which can be found on all new
stock turbo cars today.
internal and external wastegates operate in the same way. Boost is controlled
by diverting a portion of the exhaust flow away from the turbo turbine wheel
at any time the max desired boost pressure is reached. By regulating the
volume of exhaust flow through the turbine, rpm is restrained and the
compressor speed is in turn kept at bay. This sounds simple in theory but in practice several problems have to be
traditional wastegate is held closed by using a spring which is tensioned to
suit the required max boost level. However, the spring doesn’t snap or
collapse instantly. Instead it begins breaking open at 65-70% boost. Long
before max manifold pressure is reached; the wastegate is leaking exhaust
energy which should be used to keep driving the turbine to max boost. In lower gears this isn’t a major problem because the car is struggling for
traction on full boost anyway. However, in higher gears this is an important
factor to consider and we don’t want to be wasting exhaust gas because of this
premature opening of the wastegate.
combat this, especially on 4wd rally cars, hydraulic wastegates are used
whereby oil pressure is used to open the wastegate, instead of using air
pressure. With an EMU directly controlling the hydraulic valve which directs
hydraulic pressure to the wastegate, the operation is more in sync with what
is required. This permits more direct and precise control.
Boost Signal to the Wastegate.
- Signal from the
compressor outlet is the safest but least responsive.
- Boost can be tweaked
upwards by relocating the pickup point.
The source of the actuator pressure signal is
an important consideration when dealing with wastegate installation. Pressure
is not uniform between the compressor outlet and the inlet valve. Any
restriction in the air flow will show an increased pressures just before the
restriction. Factory installations usually have the pressure signal line
located at the compressor outlet. This is definitely the safest place for the
signal line but it is also the least accurate place. The advantage of having
the signal line here is that it is very close to the wastegate actuator.
there is minimal delay between the max boost pressure being reached and the
actuator opening. Air in the signal line is less likely to compress and allow
the turbo to overboost etc. the downside is that the wastegate begins to open
at 50% boost.
compromise is to locate the pickup point somewhere after the intercooler, but
before the throttle plates. The pressure here is lower than at the turbo, so
moving the line to this point will immediately result in higher boost without
having to fiddle with the wastegate. Also the wastegate will not crack open as
early. Another location for the line is in the inlet manifold after the throttle
plates. This sometimes exhibits improved response, but is to be generally
avoided and doesn’t work at all when using multiple throttles.
Once you find the most advantageous point for the signal line, connect it here
and then adjust the wastegate actuator for the desired boost.
Adjusting Boost Pressure.
- Never adjust the boost
outside of a tuners workshop.
- Moving the signal line
ups the boost.
- Adjusting the actuator
spring will affect the boost.
- Adjusting the actuator
rod or using a helper spring will adjust the boost.
- Intercepting or modifying
the signal line will adjust the boost.
There is any number of ways to up the boost on
a stock turbo car. As mentioned, moving the signal line can up boost slightly.
External wastegates often have an adjuster screw for varying the boost levels
4-5 psi. for further changes the wastegate must be dismantled and a stiffer
spring used (like changing the spring in a Forge dump valve). Internal wastegates do not provide easy ways to adjust the boost. Altering the
length of the actuator rod is one way to up the boost. Adding a helper spring
to the outside of the actuator is another way to up the boost.
Another approach to upping boost is by adjusting the signal line in some way,
so that less pressure is returned to the actuator and therefore it stays
closed longer. A crude but sometimes effective method is to cut the signal
line, place a hard plastic pipe either side of the cut in the line and place a
series of small perforations on the plastic pipe – the more perforations, the
higher the boost. Just remember that the bleed holes must be upstream of any
fuel input (i.e. air only). These methods of boost control must never be done outside of the workshop –
they require careful boost and speed monitoring on the dyno.
Electronic Boost Control
- Easily the best method of
- Allows an overboost
Electronic control is by far the best method of
boost control and provides many advantages over all other methods. With no
disadvantages. The most common method is by electronically controlling the signal line boost
return. A solenoid valve is installed in the signal line and controlled by the
EMU. The pulsing of this valve determines how much signal line boost is bled
off. If the knock sensor doesn’t detect detonation then the pulses are spread far
apart and vice versa.
overboost facility is made possible because of this valve. A 10% boost
increase is permitted of a few seconds to improve acceleration. Highly
accurate actuator control is also made possible because of the electronic
control valve. The valve can be kept constant until and instant before max
boost and then it’s kept fully open to force the actuator instantly open.
There is a danger here
because the temptation is there to delay opening the actuator until the last
second. This causes the
same problem as having a near square cam lobe acting
on an inlet or exhaust valve. Rather than opening
progressively, the actuator is slammed open and starts bouncing and
reverberating in an uncontrollable manner. A progressive response is a better
way to go. The amount of signal bleed off trails actual boost pressure by 30%
as soon as 70% of max boost is attained. By the time 90-95% of max boost is
reached the actuator should be fully opened.
Overboost Protection. To protect from electronic valve failure, actuator seizure or user misuse,
a backup safety valve is usually used.
The first valves to be used were simple pop-up valves that were similar to a
radiator cap. Later, electronic devices were used to limit ignition or fuel
when an overboosted situation occurred. There are overboost defencers on the
market to overcome these restrictors (e.g. HKS overboost defencer). If a fuel cut does occur, take your foot of the gas immediately otherwise you
will destroy the engine due to an over lean situation occurring.
Internal Wastegate Flow Problems.
- Internal wastegates will
never be superior to external units.
- Cheap, compact and light.
- Sometimes the only
solution because of space problems in the bonnet.
A major factor in turbo engine performance is
maximising exhaust flow energy to drive the turbine wheel up to speed as fast
as possible. A wastegate will always disrupt exhaust flow to some extent. But
internal wastegates are much worse in this respect than are external
wastegates. With internal wastegates, there isn’t a lot that can be done to maximise
exhaust flow. The web dividing flow between the turbine and the wastegate bypass passage can
be streamlined. The wastegate passage entry can be bell mouthed. This reduces
flow turbulence and increases the flow potential of the wastegate. If the flow
volume is insufficient then the engine can have several psi more than is
required. On the exit side of the turbo, more free flowing manifolds should be used when
there are excessive bends or too small bores to allow easy exit of the gas.
- Best option for
- Heavy, bulky, takes up a
lot of engine bay space.
- Must be kept cool.
- Plumbing can be
An external wastegate is a better option when
upgrading. However, it is expensive and consumes a lot of space in the engine
bay. Several areas need to be considered when choosing an external wastegate.
It should have an adjustable actuator screw to allow boost adjustment without
having to change the spring. The body material should be considered as well as
the cooling fins and the distance from the exhaust flow to the diaphragm
chamber. All these factors will influence the heat entering the spring and
diaphragm and therefore how long they will last. Stainless steel is superior
to cast steel. The more cooling fins the better. More space between the
diaphragm and the bypass passage is better (within reason). Also a wastegate
with a larger ratio of valve to diaphragm chamber diameter will be more
consistent and stable. How you orient and cool the wastegate will also affect its ability to give
good boost levels. Exhaust pressure should always push evenly against the flat
face of the valve. A cooling duct directed at the wastegate cooling fins is
also helpful in extending the service life of the wastegate.
- Large and easy to read is
- HKS generally make best
Any modified car should always have a large,
easy to read and accurate boost guage. Always check it for accuracy and
linearity. The needle should move smoothly and never get stuck or jagged when
returning to zero.