HIS Variable Boost Control is a simple circuit to modify the factory boost levels. It was designed in the first instance to suit a Subaru WRX but it could be adapted to many cars and FWDs fitted with turbochargers. It employs a small PC board and requires only four wire connections to the car’s electrical system.

Before we go any further we should warn that if you want to fit this project, you must have a turbo boost gauge permanently fitted. Operating variable boost above factory set levels can blow your engine if you don’t know what the boost level is.

How it works

Fig.1: the circuit is based on just two transistors and a couple of diodes. It intercepts the standard boost signal from the car's engine management computer (ECU) and stretches it so that the signal to the boost control solenoid has a longer duty cycle

The Variable Boost Control is a simple interceptor – it "intercepts" the standard boost signal to the Boost Control Solenoid from the car’s engine management computer (ECU) and stretches it so that the solenoid signal has a longer duty cycle.

Most factory boost control systems use a variable width pulse signal to control the boost solenoid. This operates at about 14Hz to suit a Subaru WRX (1998 model).

While the circuit of Fig.1 looks simple, it works quite well and has been fitted to several cars.

The ECU boost pulse waveform is fed to the input which has a 180W 1W pull-up resistor. This is sensed by the ECU as the boost solenoid’s coil. From there, the signal is fed via a 4.7kW resistor to the base of transistor Q1 which inverts the signal at its collector.