What do we want BOOST and when do we want to buy NOW!
The turbo is driven by the exhaust gases and the faster the exhaust flows the quicker the turbo spins and more air gets forced in to the engine.
If an excessive amount of air needs into the engine then it will run lean. This causes an intermittent loss of power and you also risk putting an excessive amount of pressure on the engine with a surge of power.
Additionally you risk detonation or knock and the car would be undrivable and unreliable. TorqueCars recommend getting a boost controller setup with a custom remap for reliable power especially on twin turbo engines.
An effective boost controller adds an electronic brain & allows far better control of the boost
To get around this the makers have fitted a spring valve (the wastegate actuator) to shut down the flow of exhaust on the the turbo which regulates the pressure. By way of example to get 1 bar of pressure the valve might fully open when 1.3 is achieved.
Then it starts to close again, and when we are back down to .7 bar of boost it closes completely. The wastegate actuator is additionally very gradual in its operation being partially open or closed more often than not effectively restricting the power on offer.
This closing and opening wastegate actuator helps keep a standard pressure at 1 bar. But it implies that power can be purchased in unsteady peaks, especially on highly tuned engines.
We need to look at the fact that if the foot is lifted off the accelerator the environment that the turbo sucks in cannot be burned and would likely fill the engine until something bursts so this pressure is released – see dump valve or blow off valve for a description of this in greater detail.
This means that the turbo effectively spins down because the 1 bar pressure will increase instantly mainly because that there is nowhere for that air to look, while the engine is at closed throttle (when you lift away from the accelerator) and so the spring valve driving it is closed.
If you re-apply the throttle and the engine starts burning air sucked in again the turbo spins up again when the exhaust gases are moving fast enough typically at 2500 rpm or greater – this delay is called turbo lag and can be quite annoying.
A good boost controller adds an electronic brain to the valve that controls the flow through the turbo and allows much better power over the boost. Instead of peaking at 1.3 and dipping to .7 and merely allowing full turbo flow at its its peak you can have a full boost of 1.3 at wide open throttle.
This means you have a much quicker response when the throttle is applied and the wastegate actuator will match the throttle position with the amount of boost given as opposed to just averaging around a safe setting.
Some boost controllers can effectively double the amount of boost at full throttle and give more power throughout the rev range.
Mechanical boost controllers are clunky things that do little more than allow a bit of tweaking to the threshold at which the wastegate is fully open and a switch can be added that allows you to affect the setting from inside the car but these will still be subject to fluctuation and are quite different from a fully mapped electronic boost controller.
Boost controllers will increase the power gains in the turbo however it is upto the engine management to set the fueling. It would use readings from the AFM, MAP or MAF and Lambda sensor to determine this. If these sensors are faulty the car will come across limp home mode at relatively low boost pressures.