Sylvie Willis
A boost compensator is a device that increases the maximum fuel injection volume in accordance with the increased intake air volume, in order to maintain optimal combustion conditions and increase the engine output at all times. It is installed above the governor of the injection pump and has a diaphragm and push rod that move up and down by the boost pressure. The boost compensator fitted to the IP used on some engines has a diaphragm in the canister with a spring on the front side, so that boost pressure opposes the spring. The mechanism actuated by the boost pressure, or the spring acting on either side of the diaphragm, allows the stop cam to travel all the way to the full load stop screw when boost pressure is high.
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What You'll Learn
Boost compensator function
Boost compensators are fitted to the back of the injection pump and have a diaphragm inside connected to the intake piping. They are designed to modify the pump in a 3D fashion, depending on the extra flow or pressure provided by the turbocharger.
When the boost pressure is increased by the turbocharger, the boost compensator increases the maximum fuel injection volume to maintain optimal combustion conditions and increase engine output. The diaphragm and push rod move up and down by the boost pressure, and the push rod's tapered construction rotates the control arm via the connecting pin. This movement becomes tension lever movement distance, and the spill ring movement distance (injection volume).
The boost compensator can be adjusted to suit the extra flow, which can enhance throttle response by allowing more off-boost fuel to be injected. This is generally not adjusted unless significant airflow improvements are made to the engine, requiring more fuel off boost. Spring tension on the pin adjusts how far a specific amount of boost pressure pushes the compensator pin down. Lower tension means more fuel for the same boost pressure, and more tension means less fuel.
The boost compensator also reduces the fuel rate when the boost pressure is low, preventing the engine from smoking under load at low RPM.
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Boost compensator construction
The boost compensator is a can fitted to the back of the injection pump. Inside is a diaphragm connected to the intake piping. The diaphragm is attached to a rod that pushes the rack to increase fuel injection quantities and hence torque and power.
The boost compensator can be seen at the very rear of the injector pump. There are three short bolts on the back of the compensator (front of the vehicle) and one long bolt on the front, hidden down close to the engine, which has a safety wire tied through it. It is not necessary to remove the compensator to adjust it.
The diaphragm can wear out and will need to be replaced if it has holes in it. The first check is to blow down the tube and if you feel it leaking, then a new diaphragm is needed.
The boost compensator is used to fine-tune fuel delivery, allowing you to adjust the amount and timing of fuel delivery as the turbo goes from idle to being on boost. The amount of fuel being used will have an effect on the amount of boost produced.
When the boost pressure is high, the diaphragm is pushed by the boost pressure to move the push rod downward. The tension lever is pulled by the control spring, therefore the connecting pin moves to the right toward the tapered portion of the push rod, and the control arm also rotates clockwise. The tension lever and control lever rotate counter-clockwise around fulcrum A and move the spill ring to increase the maximum fuel injection volume.
When the boost pressure is too high, the connecting pin is pushed back to the left by the tapered portion of the push rod, reducing the maximum fuel injection volume. This prevents the boost pressure from rising too high in the case of a failure.
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Boost compensator adjustments
Boost compensators are fitted to the IP used on some engines and have a diaphragm in the canister with a spring on the front side. The diaphragm is connected to the intake piping and has a rocker with an adjustment screw on the end that pushes a plunger into the injection pump body. This plunger limits rack travel, which limits fuel injection quantities and, therefore, torque and power.
The boost compensator can be adjusted to tune out the smoke when boost comes on. There are three adjustment points:
- Fuelling with no boost (tuning of the spacer length)
- Fuelling during the transition when boost arrives
- Fuelling maximum (the fuel screw on the pump)
The boost compensator can be adjusted to get more boost at lower RPMs. Adjusting the screw in or out changes the RPM at which boost is achieved. Winding the screw out reduces the effect of the boost compensator.
Fueling adjustments are best made in coordination with boost adjustments. The overall fuel pressure can be increased by turning the screw on the side of the pump. Spring tension on the pin adjusts how much boost pushes the compensator pin down. Lower tension means more fuel for the same boost pressure, and more tension means less fuel.
It is important to note that adjustments to the boost compensator will not directly change the amount of boost. Before adjusting fuel settings, it is important to understand why there is a smokey exhaust.
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Boost compensator tuning
Boost compensators are used to fine-tune fuel delivery, allowing adjustment of the amount and timing of fuel delivery as the turbo goes from idle to being on boost. The amount of fuel used will impact the boost produced, but adjusting the boost compensator will not directly change the amount of boost.
The boost compensator can be used to reduce the fuel rate when the boost pressure is low, preventing smoking under load at low RPM. It can also be used to prevent over speeding by reducing fuel delivery at around 3000 rpm.
To tune the boost compensator, adjustments can be made to the following:
- Fuel Screw/Main Metering Screw: This is the overall adjustment for the pump.
- Compensator No Boost Screw: This adjusts the off-boost fueling.
- Compensator Pin Spring Tension: Adjusting the spring tension controls the turbo boost. This can be done by tuning the spring on either side of the diaphragm.
- Compensator Pin Shims: The number of shims underneath the pin will impact how far down into the compensator the pin will travel under full boost conditions. More shims result in less full-boost fueling, while fewer shims mean more full-boost fueling.
- Compensator Pin Rotation: This adjusts the overall on-boost fueling.
It is important to note that adjustments to the boost compensator should be made in coordination with boost adjustments. Increasing overall fuel pressure can be achieved by turning the screw on the side of the pump. It is recommended to have a boost and pyro gauge installed before making any adjustments to prevent engine damage.
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Boost compensator and fuel pressure
A boost compensator is a can fitted to the back of the injection pump with a diaphragm inside connected to the intake piping. Inside they have a rocker with an adjustment screw on the end that pushes a plunger into the injection pump body. This plunger limits rack travel, which limits fuel injection quantities and, therefore, torque and power.
The boost compensator increases the maximum fuel injection volume in accordance with the increased intake air volume when the intake air volume (boost pressure) is increased by the turbocharger. This maintains optimal combustion conditions and increases engine output.
The diaphragm and push rod move up and down due to the boost pressure. The push rod has a tapered construction, which rotates the control arm via the connecting pin. The amount of movement becomes tension lever movement distance, and the spill ring movement distance (injection volume).
When the boost pressure is low, the spring pushes the diaphragm upward. The injection volume does not increase because the connecting pin is in contact with the tapered bottom portion of the push rod.
The boost compensator can be adjusted to suit the extra flow. This is used to adjust the amount of fuel added off-boost on a compensated pump. It can enhance throttle response by allowing more off-boost fuel to be injected. This is generally not adjusted unless significant airflow improvements are made to the engine, which will require more fueling requirements off-boost.
Spring tension on the pin adjusts how far a certain amount of boost pushes the compensator pin down. Lower tension means more fuel for the same boost pressure, and more tension means less fuel for the same boost pressure.
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Frequently asked questions
A diesel fuel boost compensator is a device that modifies the pump in a 3D fashion, depending on the extra flow or pressure provided by the turbocharger.
A diesel fuel boost compensator works by increasing the maximum fuel injection volume when the intake air volume (boost pressure) is increased by the turbocharger. This is done to maintain optimal combustion conditions and increase the engine output at all times.
One of the main benefits of using a diesel fuel boost compensator is that it allows for more precise adjustments to the fuel injection volume, resulting in improved engine performance and efficiency. Additionally, it helps to prevent smoking under load at low RPM.
