Boost is actually the pressure that is produced by the turbo.
Which means the pressure of the air that is going into the engine. Boost is measured by
the use of a boost meter.The boost meter measure the pressure of the air inside the intake manifold of the engine right after the throttle body.There is two measurement unit
commonly used which is the Bar/kPa and PSI.
*For conversion purposes,14.7PSI =1bar or 100kPa
Boostmeter is a gauge which measure the boost pressure.There are two types of boostmeter.First is the mechanical boostmeter where the vacuum is supplied directly to the gauge itself.So you will have one vacuum hose going into you car and to the gauge.And the second one is the electrical boost meter where the pressure is measured by a pressure sensor. The pressure sensor is located in the engine bay and the sensor will convert the pressure into an electrical signal and then the signal will be transmitted to the gauge through a wire and the electrical signal will move the pointer to indicate the readings. Often the electrical one will com with a preset boost warning and a
peak/maximum boost recall function.Some electrical boostmeter will require a separate control unit to make it function.
WHAT DETERMINE THE BOOST
If you remembered in the previous article on wastegate, the wastegate is used to regulate
the boost pressure.So to adjust the boost pressure, something should be done to the
wastegate.By right, we should change the spring rate of the wastegate to get a different
boost.But this is not practical, so instead of changing the spring, we can manipulate the
wastegate by altering the pressure that is going to the wastegate.This is where the handy
boost controller comes in.
The function of the boost controller is to reduce the pressure that is going to the
wastegate.This can be achieved by bleeding the air to the atmosphere,so the wastegate
will detect a lower boost from what the turbo is producing.For example, the wastegate spring is rated at 1 bar, by right the wastegate should open when the turbo is producing 1 bar of boost. But because of the air that is going to the wastegate had been bleed out a little, it will receive a slightly lower pressure(let say 0.8bar) and it will not open when the actual boost is at 1 bar.The wastegate will open whenever it detects 1 bar of boost so this means that the turbo have to produce 1.2 bar of boost in order for the wastegate to get the 1 bar signal.So, when more air is bleeded, wastegate will open much later,this will gives a higher boost.
So to increase the boost pressure, you will need to bleed the air by using a manual boost controller or an electronic boost controller which we will discuss later on.