Boost Control

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With turbo charged engines a pulse width modulated (PWM) output from the ECU can be used to drive a solenoid to control boost via the wastegate. The advantages of using a boost controller over a fixed spring or bleed system are a more consistent boost pressure, and the ability to run different boost pressures under different conditions (by gear for example, or with different octane fuel).

 

How A Boost Controller Works

 

There are multiple configurations of how a boost controller can act on a wastegate to control boost, but basically they reduce the pressure differential on the wastegate diaphragm to hold the wastegate closed. The wastegate diaphragm normally has boost pressure acting to open the wastegate valve against a spring, and most external wastegates also have a chamber on the other side of the diaphragm which acts to hold the wastegate valve shut.

 

Factors acting to open the wastegate (decrease boost):

Air pressure on the valve side ('bottom') of the wastegate diaphragm.
Exhaust back pressure on the wastegate valve.

 

Factors acting to close the wastegate (increase boost):

The wastegate spring.
Air pressure on the non valve side ('top') of the wastegate diaphragm.

 

Notes:

The minimum boost is determined by the wastegate spring. The boost controller will not be able to reduce boost, but can only raise boost above the minimum level.
The maximum boost is limited by the exhaust back pressure opening the valve against the wastegate spring, even with no pressure differential on the wastegate diaphragm. The exhaust back pressure increases with engine speed, so it is common to see a drop in boost at high rpm.
Boost creep is when the boost rises at high rpm despite the wastegate being fully open. Boost creep is caused by using a wastegate valve which is too small, or (more commonly) placing the wastegate off center in the exhaust manifold so that it does not effectively reduce exhaust back pressure from all cylinders.
A drop in boost at high rpm occurs without a boost controller because the progressively increasing exhaust back pressure open the wastegate earlier as the rpm rises. Changing the position of the wastegate pressure source often eliminates this problem.

 

Using the ECU to control boost

 

1.Install the boost control solenoid. Boost Control Installation shows some information about installing and wiring the boost control solenoid.
2.In KManager set the Boost Control Parameters for boost control.
3.Dyno or otherwise tune the vehicle to set the desired level of boost. Boost Control Example Graphs contain dyno graphs showing the effect on boost and engine output from varying solenoid duty cycles.

 

Important notes:

Use a boost limiter to prevent engine damage if there is a wiring or component failure.
Do not exceed the stock MAP sensor's pressure range (approx 10.5 lbs).
With the boost control disabled, the solenoid will not be energized. With most solenoid configurations this will result in maximum boost. For this reason do not disable the boost control output nor disconnect the solenoid without changing the wastegate so that pressurized air is not connector is not supplied to the top of the diaphragm.