Boost Control Basics

Every modern turbocharger uses a wastegate to regulate boost. The wastegate is simply a valve that allows exhaust to bypass the turbine, limiting the energy available to drive it . When designed into the turbine housing, they're called internal wastegates, the kind found on nearly all production cars. External wastegates are located separately from the turbocharger, somewhere between the exhaust ports and turbine housing. The remote location allows the valve to be bigger and flow path smoother, resulting in better boost control and the ability to safely manage a more powerful engine.

Photo of "internal" wastegate. The gold-colored can is the wastegate actuator. Boost pressure is this can pushes on a diaphragm to open the wastegate.
The wastegate is held shut by a spring, and as boost builds, the wastegate actuator diaphragm pushes the wastegate open against this spring. The size of the diaphragm and strength of the spring determine how much boost it takes to open the wastegate.

Boost controllers act on the pressure signal that opens the wastegate. Reducing this pressure will keep the wastegate closed, allowing more boost to build. When the desired boost setting is reached, the controller lets pressure into the diaphragm, opening the wastegate.

Electronic controllers use either stepper motors or solenoids to do this. Stepper motor assemblies vary the opening of a precision valve, similar to a water faucet. This action is accurate, though somewhat slow. Solenoids, however, operate in either a fully open or closed position, but can change this state very quickly, feeding and releasing full pressure so quickly that the pressure in the diaphragm can be accurately controlled. Dual solenoids will give an even greater degree of precision.

If a wastegate is set to 10 psi, it may start to open around 6 psi, slowing the turbo's acceleration and finally reaching equilibrium at 10 psi. Boost controllers can improve turbo response by keeping pressure off the wastegate as long as possible, making more energy available to drive the turbine. Just as boost reaches the set point, pressure is dumped on the diaphragm to rapidly open the wastegate, and hopefully keep from spiking the boost.