Do I need to buy extra parts to configure my WRX Hybrid as a plumb back or vent-to-atmopshere only BOV?

The blanking plate, plug and plumb back block off plug come standard with the WRX Hybrid, so that it can be configured in three ways; either as a hybrid, a plumb-back or full vent-to-atmosphere, for maximum noise.

Which blow off valve kit do you recommend for an MY98 WRX with a cat back exhaust and pod filter? Everything else is standard.

It all depends on how loud you want it! Our range includes a full plumb-back valve for silent operation, the twin trumpet Bovus Maximus for maximum noise, or the Hybrid, which offers the best of both worlds. For the best versatility, we recommend the Hybrid. Due to the modular design of GFB blow-off valves, it can easily be configured (using a supplied plug) for vent-to-atmosphere operation, or as a plumb back to solve any airflow meter related problems or excessive BOV noise issues. All of these valves will fit directly onto the factory blow off valve hose on the MY98.

How does a boost controller work?

There are 2 main components of a boost control system on a turbocharged engine: the wastegate with actuator and the boost controller. Note that some turbo cars do not use a boost controller as such, instead they rely on the spring rate in the wastegate actuator to achieve the desired boost level.

There are two types of wastegates; internal and external. Internal wastegates are found on the vast majority of turbo cars, where a flap-type valve is built into the turbocharger exhaust housing, and is opened by a remote diaphragm actuator. The external type usually use a poppet-style valve attached directly to the diaphragm actuator as one unit, which is plumbed into the exhaust header before the turbo, and dumps back into the exhaust after the turbo.

Both types of wastegate open a path for exhaust gas to bypass the turbo, thereby limiting the amount of gas available for making boost. The diaphragm actuators in both cases are controlled by the boost pressure itself, which creates a feedback loop boost control system; as the boost pressure increases, it pushes the diaphragm actuator (and therefore the wastegate valve) further open, which results in the boost stabilizing at a set level determined by the spring rate in the actuator.

A boost control device is commonly used in modern turbo engines which alters the boost signal that reaches the wastegate actuator. If the actuator sees a lower boost signal than is actually present, the wastegate will open less. This results in greater exhaust gas flow through the turbo, which increases boost. For example, if the actuator spring is set to 7 psi (i.e. when the valve is fully open, boost level is 7 psi) but the boost control device only allows 5 psi to reach the actuator, it will open the wastegate valve less, so more exhaust flows through the turbo, spinning it more quickly, so that the actual boost produced will be greater than 7 psi; say, 10 psi.

The boost control device is designed to reduce the pressure reaching the actuator, which it does by ‘bleeding off' a specific amount of air from the supply hose. This can be done electronically through the ECU by pulsing a solenoid-type valve in the supply hose, or mechanically by placing a ‘controlled leak' in the supply hose.

GFB boost controllers are the latter type, which may not sound incredibly scientific, but there are a few design criteria that are critical to performance. Essentially, a GFB boost controller receives boost pressure from the turbo compressor, passes it through a restrictor (which is necessary, or the turbo would simply fill the supply hose as fast as the controller could bleed it off), where a measured amount of air is bled out of the hose that leads to the wastegate actuator. Note that it is not possible to reduce the boost level below the setting of the actuator spring, only increase it. It is however possible to reduce boost below standard on cars that originally had ECU/solenoid controlled boost.

I'm having new intercooler piping made and I want to know where the best place to install the blow-off valve is?

I've not seen any conclusive results that prove that one position is better than another, but many people have different ideas about this. Some say that it's best to have it close to the throttle, since that's where the back-pressure builds from when the butterfly is closed and it will respond quicker. Others say that it's better to put it as close to turbo as possible so that the valve is venting hot air rather than post-intercooler cold air, so that the inlet pipes after the intercooler are still filled with cool air.

Both positions have their merits, but are really of minute benefit. There is a trap to watch out for when mounting close to the turbo however. There is often a measurable pressure drop between the turbo and throttle body, especially as the RPM and hence the airflow increases. A BOV uses two pressure signals to stay shut, one from the manifold and one from the inlet, which act on opposite sides of the piston and since they oppose each other they should balance out, and the spring then holds it shut.

The problem is when the revs rise and there becomes a pressure difference between the two signals. The pressure leaving the turbo may be say 16psi, while only 13 or 14 psi makes it to the manifold. So this means that there is 2 or 3 psi of pressure acting against the spring, which is enough to move the piston a fraction. In the Hybrid and Bovus Maximus valves, since the outlets are staged and one is placed very close to the seat for rapid response this slight spring compression can be enough to open the valve a small amount. It is not a problem with the other valves in the range, and it depends on the flow efficiency of your intercooler and pipes, but for the Hybrid and Bovus Maximus valves it is best to locate them after the intercooler.

If this problem does show up then it may also be a good idea to test the efficiency of your piping by measuring boost at redline just after the turbo, and after the throttle body. If there's any more than about 3 psi difference then your turbo is working harder than it needs to, and reducing this drop would help performance a lot.

Do GFB boost controllers use a ball and spring?

No. Some manufacturers use a ‘ball and spring’ arrangement in their manual boost controllers, with the claim that it brings the boost on faster by keeping the wastegate closed until the boost has nearly reached it’s peak level. This is a sound idea in theory, however it is worth noting that the "gated" ball-and-spring system offered by other manufacturers is incapable of holding back more than a paltry 2psi - certainly not enough to live up to the advertising claims.

Another nail in the coffin for the ball-and-srping design is poor boost stability. Since it is a dynamic system, and the ball can move slightly differently each time your engine comes on boost, the size and flow properties of the flow path through the restrictor and around the ball can change each time. This can lead to poor boost stability and random variations in peak boost level. The ball can even flap around like a pea in a referee's whistle whilst air is flowing through, resulting in a fluctuating output signal from the controller. Grab a boost gauge, a compressed air supply and a pressure regulator and see for yourself.

GFB boost controllers use a more consistent needle-valve bleed system, which brings brings boost on just as fast as any ball-and-spring system, and generally more quickly than a factory boost control system.