Lets say the Waste Gate is set to open fully at 20 PSI.

Does it work like so: the waste gate flap stays fully shut from 0-19.9 PSI and as soon as it reaches 20 PSI, the flap opens, allowing the gasses to bypass the hot side of the turbo, thus controlling how much boost a car will run, OR

When the car reaches 5 PSI, the Waste Gate flap is 25% open, when it reaches 10 PSI, the waste gate flap is 50% open.. etc.

???

 

From what I understand, the wastegate opens when you're turbo is pushing more psi than you're target boost. If you're target boost is 20 psi, as soon as it goes over that, the wastegate opens to relieve the excess exhaust gas.

 

from my understanding, it slowly opens untill the desired boost.

 

The spring rate of the actuator, is typically the point at which the actuator will open bypassing exhaust gas away from the impeller preventing the turbo from generating more boost.

Most actuators will have a base spring pressure, from which you can adjust up with a secondary boost controllin device.

If, for example, you have a base spring pressure of 10 PSI, and you are NOT using any type of secondary boost control, once the turbo is making 10 PSI, the actuator should open and the turbo will stop increasing boost output beyond that point.

If you add a secondary boost controlling device, manual or electronic, the setting you adjust to will become the new target setting. Once that boost level is reached, the pressure signal will be sent to the actuator, subsequently opening it again preventing boost from increasing further.

There is not a sequential manner in which the actuator opens. The base actuator pressure is refered to as "cracking pressure", meaning it won't open AT ALL until that boost level is reached. Once cracked open, the increase in boost output will level off and exhaust gas pressure will, to a certain extent, hold the flapper open until throttle lift.