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A boost controller is a device to control the boost level produced in the intake manifold of a turbocharged or supercharged engine by affecting the air pressure delivered to the pneumatic and mechanical wastegate actuator.
A boost controller can be a simple manual control which can be easily fabricated, or it may be included as part of the engine management computer in a factory turbocharged car, or an aftermarket electronic boost controller.
Principles of operation
Without a boost controller, air pressure is fed from the charge air (compressed side) of the turbocharger directly to the wastegate actuator via a vacuum hose. This air pressure can come from anywhere on the intake after the turbo, including after the throttle body, though that is less common. This air pressure pushes against the force of a spring located in the wastegate actuator to allow the wastegate to open and re-direct exhaust gas so that it does not reach the turbine wheel. In this simple configuration, the spring's springrate and preload determine how much boost pressure the system will achieve. Springs are classified by the boost pressure they typically achieve, such as a "7 psi spring" that will allow the turbocharger to reach equilibrium at approximately 7 psi (0.48 bar).
One primary problem of this system is the wastegate will start to open well before the actual desired boost pressure is achieved. This negatively affects the threshold of boost onset and also increases turbocharger lag. For instance, a spring rated at 7 psi may allow the wastegate to begin to (but not fully) open at as little as 3.5 psi (0.24 bar).
Achieving moderate boost levels consistently is also troublesome with this configuration. At partial throttle, full boost may still be reached, making the vehicle difficult to control with precision. Electronic systems can allow the throttle to control the level of boost, so that only at full throttle will maximum boost levels be achieved and intermediate levels of boost can be held consistently at partial throttle levels.
Also to be noted is the way in which boost control is achieved, depending on the type of wastegate used. Typically manual "bleed type" boost controllers are only used on swing type (single port) wastegate actuators. To increase boost, pressure is taken away from the actuator control line, therefore increasing boost. Dual port swing type wastegate actuators and external wastegates generally require electronic boost control although adjustable boost control can also be achieved on both of these with an air pressure regulator, this is not the same as a bleed type boost controller. To increase boost with an external or dual port wastegate, pressure is added to the top control port to increase boost. When boost control is not fitted, this control port is open to the atmosphere.
Manual boost control
A bleed-type manual boost controller simple mechanical and pneumatic control to allow some pressure from the wastegate actuator to escape or bleed out to the atmosphere or back into the intake system. This can be as simple as a T-fitting on the boost control line near the actuator with a small bleeder screw. The screw can be turned out to varying degrees to allow air to bleed out of the system, relieving pressure on the wastegate actuator, thus increasing boost levels. These devices are popular due to their negligible cost compared to other devices that may offer the same power increase.
A ball & spring type boost controller uses the force of a spring acting against the boost pressure to control boost. This is installed with one boost signal line coming from the intake somewhere after the turbocharger, and one boost signal line going to the wastegate. A knob changes the force on the spring which in turn dictates how much pressure is on the ball. The tighter the spring, the more boost that is needed to unseat the ball, and allow the boost pressure to reach the wastegate actuator. There is a bleed hole on the boost controller after the ball, to allow the pressurized air that would be trapped between the wastegate actuator and the ball after it is seated again. These type of Manual boost controllers are very popular since they do not provide a boost leak, allowing faster spool times and better control than a "bleed type" boost controller. 
There are several different designs of ball-and-spring controllers on the market that range greatly in terms of cost and quality. Common body materials are brass and aluminum vary from inline to 90 degree designs. Another design aspect is the ball valve seat which is critical for performance stability.
Generally a manual boost controller will not be located within the cabin of the vehicle as the lengthy vacuum piping run between the turbo/wastegate & controller can introduce response issues into the system. It is possible to use two manual boost controllers at different settings with a solenoid to switch between them for two different boost pressure settings. Some factory turbocharged cars have a switch to regulate boost pressure, such as a setting designed for fuel economy and a setting for performance.
Manual boost controllers cannot be used to set a specific boost level at a given throttle position (& therefore be used to optimise driveability & control issues), although a ball-spring type boost controller does allow the boost threshold to be as low as is possible on a given engine configuration, and also keeps turbo spool as fast as is possible as the wastegate remains completely shut until the desired boost pressure is reached, ensuring 100% of the exhaust gases are diverted through the turbocharger exhaust turbine. They can be used in conjunction with some electronic systems.
Electronic boost control
Electronic boost control adds an air control solenoid and/or a stepper motor or servo motor controlled by an electronic control unit. The same general principle of a manual controller is present, which is to control the air pressure presented to the wastegate actuator. Further control and intelligent algorithms can be introduced, refining and increasing control over actual boost pressure delivered to the engine.
At the component level, boost pressure can either be bled out of the control lines or blocked outright. Either can achieve the goal of reducing pressure pushing against the wastegate. In a bleed-type system air is allowed to pass out of the control lines, reducing the load on the wastegate actuator. On a blocking configuration, air traveling from the charge air supply to the wastegate actuator is blocked while simultaneously bleeding any pressure that has previously built up at the wastegate actuator.
Control for the solenoids and stepper motors can be either closed loop or open loop. Closed loop systems rely on feedback from a manifold pressure sensor to meet a predetermined boost pressure. Open loop systems have a predetermined control output where control output is merely based on other inputs such as throttle angle and/or engine RPM. Open loop specifically leaves out a desired boost level, while closed loop attempts to target a specific level of boost pressure. Since open loop systems do not modify control levels based on MAP sensor, differing boost pressure levels may be reached based on outside variables such as weather conditions or engine coolant temperature. For this reason, systems that do not feature closed loop operation are not as widespread.
Boost controllers often use pulse width modulation (PWM) techniques to bleed off boost pressure on its way to the reference port on the wastegate actuator diaphragm in order to (on occasion ) under report boost pressure in such a way that the wastegate permits a turbocharger to build more boost pressure in the intake than it normally could. In effect, a boost-control solenoid valve lies to the wastegate under the engine control unit´s (ECU) control. The boost control solenoid contains a needle valve that can open and close very fast. By varying the pulse width to the solenoid, the solenoid valve can be commanded to be open a certain percentage of the time. This effectively alters the flow rate of air pressure through the valve, changing the rate at which air bleeds out of the T in the manifold pressure reference line to the wastegate. This effectively changes the air pressure as seen by the wastegate actuator diaphragm. Solenoids may require small diameter restrictors be installed in the air control lines to limit airflow and even out the on/off nature of their operation.
The wastegate control solenoid can be commanded to run in a variety of frequencies in various gears, engine speeds, or according to various other factors in a deterministic open-loop mode. Or by monitoring manifold pressure in a feedback loop- the engine management system can monitor the efficacy of PWM changes in the boost control solenoid bleed rate at altering boost pressure in the intake manifold, increasing or decreasing the bleed rate to target a particular maximum boost.
The basic algorithm sometimes involves the EMS (engine management system) "learning" how fast the turbocharger can spool and how fast the boost pressure increases. Armed with this knowledge, as long as boost pressure is below a predetermined allowable ceiling, the EMS will open the boost control solenoid to allow the turbocharger to create overboost beyond what the wastegate would normally allow. As overboost reaches the programmable maximum, the EMS begins to decrease the bleed rate through the control solenoid to raise boost pressure as seen at the wastegate actuator diaphragm so the wastegate opens enough to limit boost to the maximum configured level of over-boost.
Stepper motors allow fine control of airflow based on position and speed of the motor, but may have low total airflow capability. Some systems use a solenoid in conjunction with a stepper motor, with the stepper motor allowing fine control and the solenoid coarse control.
Many configurations are possible with 2-, 3-, and 4-port solenoids and stepper motors in series or parallel. Two port solenoid bleed systems with a PID controller tend to be common on factory turbocharged cars.
Since less positive pressure can be present at the wastegate actuator as desired boost is approached the wastegate remains closer to a completely closed state. This keeps exhaust gas routed through the turbine and increases energy transferred to the wheels of the turbocharger. Once desired boost is reached, closed loop based systems react by allowing more air pressure to reach the wastegate actuator to stop the further increase in air pressure so desired boost levels are maintained. This reduces turbocharger lag and lowers boost threshold. Boost pressure builds faster when the throttle is depressed quickly and allows boost pressure to build at lower engine RPM than without such a system.
This also allows the use of a much softer spring in the actuator. For instance, a 7 psi (0.48 bar) spring together with a boost controller may still be able to achieve a maximum boost level of well over 15 psi (1.0 bar). The electronic control unit can be programmed to control 7 psi (0.48 bar) psi at half throttle, 12 psi (0.83 bar) at 3/4 throttle, and 15 psi (1.0 bar) at full throttle, or whatever levels the programmer or designer of the control unit intends. This partial throttle control greatly increases driver control over the engine and vehicle.
Limitations and disadvantages
Even with an electronic controller, actuator springs that are too soft can cause the wastegate to open before desired. Exhaust gas backpressure is still pushing against the wastegate valve itself. This backpressure can overcome the spring pressure without the aid of the actuator at all. Electronic control may still enable control of boost to over double gauge pressure of the spring's rated pressure.
The solenoid and stepper motors also need to be installed in such a way to maximize the advantages of failure modes. For instance, if a solenoid is installed to control boost electronically, it should be installed such that if the solenoid fails in the most common failure mode (probably non-energized position) the boost control falls back to simple wastegate actuator boost levels. It is possible a solenoid or stepper motor could get stuck in a position that lets no boost pressure reach the wastegate, causing boost to quickly rise out of control.
The electronic systems, extra hoses, solenoids and soforth add complexity to the turbocharger system. This runs counter to the "keep it simple" principle as there are more things that can go wrong. It is worth noting that virtually all modern factory turbocharged cars, the same cars with long warranty periods, implement electronic boost control. Manufacturers such as Subaru, Mitsubishi and Saab integrate electronic boost control in all turbo model cars.
Availability and applications
Electronic boost control systems are available as aftermarket stand-alone systems such as the HKS EVC and VBC, Apex-i AVC-R, or Gizzmo IBC / MS-IBC as a built-in feature of modern factory turbocharged vehicles such as the Subaru Impreza WRX STi and often as built-in features in full aftermarket stand-alone engine management systems such as the Hydra Nemesis, AEM EMS and MegaSquirt.
Dangers in use
Installing a boost controller in a vehicle that is already well tuned, such as a factory turbocharged car, may allow higher boost pressure than tolerable by the engine or turbocharger, reducing life and reliability. Care should be taken to avoid exceeding the limits of any engine system components such as the engine block, fuel injectors, or engine management system. This is as true with boost control as it is with fuel and timing controls, or any number of other engine system modifications.
In particular, users may find the extremely low cost and ease of adding a manual boost controller a particular draw for extra power at low cost compared to more comprehensive modifications. Users should carefully consider how installing any boost controller may affect and interact with existing complex engine management systems. Additional boost levels may not be tolerated by the existing turbocharger, causing faster wear. Fuel injectors or the fuel pump may not be able to deliver additional fuel needed for higher air flow and power of higher boost pressure. Or the engine management system may not be able to properly compensate for fuel or ignition timing, causing knock and/or engine failure.
Past and future
There are other outdated methods of boost control, such as intake restriction or bleed off. For instance, it is possible to install a large butterfly valve in the intake to restrict airflow as desired boost is approached. It is also possible to actually release large amounts of already compressed air similar to a blowoff valve but on a constant basis to maintain desired boost at the intake manifold. The currently popular exhaust gas bypass via wastegate is quite superior if compared to creating intake restriction or wasting energy by releasing air that has already been compressed. These methods are rarely used in modern system due to the large sacrifices in efficiency, heat, and reliability.
Other methods may come into widespread use in the future, such as variable geometry turbochargers. With a sufficiently large turbine, no wastegate is necessary. Low speed response and faster spool up are then obtained using variable turbine technologies rather than a smaller turbine. These systems may replace or supplement typical wastegates as they develop. Control methods for the variable mechanical controls, such as the principles of closed loop will still apply even if they no longer involve pneumatics.
- MBC Concept Overview, Installation, and Setup
- How To Guide - Manual Boost Controller Installation at addictiveperformance.com.