The second is that you lower the opening range of the wastegate door. One is that you increase the boost pressure needed to start opening the wastegate. When you increase the preload you do two things. Shortening the wastegate rod using the adjustment screws adds preload to the internal spring.
Therefore, the only aspect that we can change on the OEM actuator is preload. On aftermarket actuators and external wastegates we can change the internal spring. On OEM wastegate actuators we can’t change the spring inside the canister because they are sealed. Some of these are the turbo itself, the engine displacement, the flow parts installed, the ambient temperature and more importantly for this discussion the spring inside the wastegate actuator and the preload on this spring. In spite of the ECU controlling boost pressure going to the actuator, the amount of boost a turbo can hold at any engine speed (RPM) is determined by a large number of factors. The amount of boost pressure this canister sees is regulated using the boost control solenoid which is controlled by the ECU and the tune running on the motor. It starts to open it at a cracking pressure (also known as spring pressure) and then the more pressure you apply to the canister the more it opens the wastegate door. The actuator doesn’t open the wastegate fully at a given single pressure. Boost pressure is applied to the canister and once the boost is higher than the spring pressure inside the canister, the wastegate actuator rod starts to move and open the wastegate. The actuator is a boost actuated diaphragm inside a canister with a rod sticking out as shown below. This wastegate is pushed open or held closed by the wastegate actuator and a number of other variables such as exhaust pressure. The more exhaust you divert through the wastegate (the more you open the wasetgate door), the more you slow the turbo down and this is how you lower boost. The faster the turbine spins, the more boost the turbo generates. It does this by regulating a pathway/door which diverts exhaust gases AROUND the turbine.
The purpose of the wastegate is to maintain a particular boost pressure at the intake manifold by regulating the speed at which the turbo spins. Let’s first go over how the wastegate and actuator work.
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