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Turbo Boost Control

> - 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.

Boost Controls.
 - 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.

The other 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 consumption.
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.

Wastegates.
 - Most popular form of boost control.
 - 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.

Exhaust 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.
Both 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 overcome.
The 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.
To 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.
Therefore, 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.
A 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 boost control.
 - Allows an overboost facility.

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.
An 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.

External Wastegates.
 - Best option for performance.
 - Heavy, bulky, takes up a lot of engine bay space.
 - Must be kept cool.
 - Plumbing can be complicated.

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.

Boost Guages.
 - Large and easy to read is best.
 - HKS generally make best guages.

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.