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Wastegate Adjustment, A Closer Look
Wastegate adjustment may not be the most exciting topic when it comes to performance and power but it’s one of the most important base settings when it comes to boost development in a turbocharged engine. Improperly set, you’ll find even a tuned ECU brings in lethargic and tepid power. So let’s take a closer look at the unsung hero of boost management, the wastegate actuator!
The Wastegate Actuator
First let’s fully understand the wastegate actuator, how it works, and what it ultimately does. The wastegate actuator is metal can with a diaphragm in the middle of it that is crimped between the can halves. The diaphragm is also connected to a rod that exists one side of the wastegate actuator. On one side of the diaphragm is a spring and on the other a vacuum port where pressurized air is fed. As boost pressure rises and pressurized air is fed into the wastegate actuator, the pressure on the diaphragm increases and at some point will overcome the spring tension on the other side of the diaphragm and cause the rod to start moving. This rod is in turn connected to the wastegate arm on the turbo, as it moves the wastegate inside the turbo begins to open and exhaust gasses start to bypass the turbine. When this occurs, less exhaust gas is acting on the turbine and so consequently it slows down in its rotational speed. This reduced rotational speed is directly felt by the compressor wheel as well since the compressor wheel and turbine wheel are directly and mechanically connected to one another. The slow-down of the compressor wheel results in reduced boost pressure fed to the engine. This is the fundamentals of wastegate operation and boost control.
Every wastegate actuator has a spring in it and that spring has a set amount of tension. If more boost is required then more tension is necessary to hold the wastegate closed until that boost pressure is reached. Without changing the spring, how can this be done? The answer is by adjusting the amount of pretension the spring has against the diaphragm. That is accomplished by the threaded adjustment on the end of the wastegate actuator rod. This adjustment allows for considerable changes in wastegate spring tension that is required for higher levels of boost as well as better early boost onset.
The Simple Early Years
Wastegate control in early turbocharged cars was as simple as you might imagine, a vacuum port on the intake manifold was plumbed to the wastegate actuator and when the boost pressure in the intake manifold was higher than the total spring tension in the actuator then the wastegate would start to open and hold boost at that preset level. It’s as simple as that.
A Job Done Two Different Ways
In later cars with more advanced electronic fuel injection, the ECU (engine computer) was put in charge of wastegate operation by virtue of a solenoid (an electrically controlled valve). In the 850 models with a BCS (boost control solenoid) the pressure normally fed to the wastegate actuator is instead bled off when the BCS is activated by the ECU. In later models (1998 and up) the TCV (turbo control valve) blocked boost pressure to the actuator when it was activated by the ECU. The have the same job but do it in different ways.
In both instances the BCS and TCV, are designed so that if the solenoid fails the system will by default feed pressure to the wastegate in all conditions, resulting in a car that will only create boost to what the base setting (adjustment) of the wastegate actuator is. The system is designed to “fail safe” and prevent potential over boosting that could damage the engine.
Boost for the General Public
With that said, the reason the base setting is so important is due to the nature of the ECU programming. The ECU will not start activating the BCS/TCV until after the first 1psi of boost is generated, in some models it doesn’t start boost control until after 2psi. Which means if the solenoid will not start blocking pressure to the wastegate actuator until over 2 psi, it will start to open and reduce boost pressure far before it is necessary. With factory wastegate actuator pretension commonly under or around 2 psi a condition exists where the boost is bled off inside the turbine housing before it is desirable to do so and early boost development is greatly diminished. This results in a very flat, slow responsive power production.
The short answer is to increase the base setting above the minimum TCV/BCS operation range. In most cases 3.2 psi is a good starting point but you can refer to our recommendations at ardtuning.com for your specific vehicle and setup. For many though you can go higher even still for added midrange performance and efficiency. My basic rule of thumb is as follows; increase wastegate actuator spring tension until you start to see slight overboost, then back off the adjustment until the overboost is gone. So if you’re ECU is set to 10psi, when you start seeing consistent 11-12psi overshoot, go back a touch on the adjustment until the overshoot is gone. This is the most precise method since each actuator will vary some with regard to the spring tension internally since they age differently and the heat cycles from the turbo can harden the spring in some cases.
An Added Thought
With regard to overshoot I’m referring to on throttle, sustained boost levels. It is not to be confused with the normal and acceptable boost pressure surge between shifts, specifically with automatic transmissions. So don’t let that confuse you!
Robert Lucky Arnold
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