• De-compress the engine. This leads to side effects such as: Sluggish performance in low RPM before the turbo works. Extensive mechanical work on the engine.
• Control the turbo pressure through the RPM range. This requires that the “canister” be exchanged for a low pressure one. The theory is that you can raise the pressure slightly (0.2bar) at low RPM without knocking, but can raise the pressure to 0.5bar or higher at 6000 RPM, because the air does not flow properly. Most modern engine with a turbo use the “high compression” method. The XMS can perform this control function. Thus it is an ideal solution for engines with turbo conversion.

REQUIREMENTS

• A open function (wire)
• A turbo “canister” with relative low pressure. The pressure must be low enough to avoid knocking. A pressure of 0.2 bat (3lb) is normally sufficient.
• A “bleed” valve. This is a little solenoid with 3 air connections, which is operated from the XMS directly. The exact placement of the pipes depends on the type of bleed valve. It is placed in line with the pipe to the canister and is closed when the wires come off (failsafe!).
• A AMP(MAP) sensor which covers the range.

CONTROL PRINCIPLE

Without any control (XMS not connected!) the turbo pressure is controlled by the canister construction. The XMS can INCREASE the pressure, but can’t decrease it below the failsafe pressure. The controlled “target” pressure is settable for each RPM point. If the target pressure exceeds the AMP value, then the XMS outputs a PWM signal, which partially OPENS the bleed valve, reduces the EFFECTIVE canister pressure, and closes the waste gate, thus increasing the manifold pressure.

SETTINGS

• Enter boost TARGET pressures for each RPM point
• Choose a some settings in the PWM table in PERCENT. The top of the table should have zero or low values because the pressure is HIGHER than the target and the waste gate must be open.
• Assign a wire in the SETPOINT screen to the BOOST CONTROL function
• Set the turbo BOOST time. This is the loop control time constant. A small value gives a faster response.

PRACTICAL

Choosing a target value for each RPM is easy. You may choose the same values for each RPM point.
The BOOST TIME and PWM values are a little more difficult and require experimenting. Start of with a steep gradient at the target PWM scale and fast boost time (5).
This will lead to “oscillation”. Then reduce the gradient and check again.
Patience is the word here!
The control loop response depends on:

• Length of pipe from manifold to bleed valve.
• Inside diameter of pipe.
• Bleed valve construction
• Canister size (volume)
• Calculation speed (boost time)
• Waste gate friction
• Size of turbo, and how fast is spins up.
  

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