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
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
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.
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
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 boot 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.
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
Robert Lucky Arnold