boost pressure under varying circumstances. Advantages By keeping the wastegate in a closed position more often, a boost controller causes more of the exhaust gas to be routed through the turbocharger, thus reducing turbo lag and lowering the boost threshold. Disadvantages Regardless of the effectiveness of the boost controller, wastegate actuator springs that are too soft can cause the wastegate to open before desired. This due to the exhaust gas backpressure pushing against the wastegate valve itself, causing the valve to open of the actuator at all. Therefore, there is an upper limit to the effectiveness of a boost controller for a given spring stiffness in the wastegate actuator. To prevent excessive boost pressures in the event of a failure, the boost controller needs to be designed such that failure mode do not result in any pressure being bled off. For instance, a solenoid-type boost controller should direct all air to the wastegate when it is in the non-energized position (the common failure mode for a solenoid). Otherwise, the boost controller could get stuck in a position that lets no boost pressure reach the wastegate, causing boost to quickly rise out of control. Also, the electronic systems, extra hoses, solenoids and control systems add cost and complexity. Nonetheless, in recent times most automobile manufacturers use boost controllers on their turbocharged engines. Alternatives In the past, boost pressures have been controlled by restricting or bleeding off some intake air before it reaches the intake manifold. Designs which restrict the intake air can use a butterfly valve in the intake to restrict airflow as desired boost is approached. Designs which bleed off the intake air functioned similar to a blowoff valve, but on a constant basis to maintain the desired boost pressure. These methods are rarely used in modern system due to the large sacrifices in efficiency, heat, and reliability. Variable geometry turbocharger can be used to manage boost levels, negating the need for an external boost controller. Also, the wastegate itself has a similar function to a boost controller, in that it is used to manage the turbocharger's boost pressure. References Automotive engine technologies Turbochargers