Diesel Tuning Box – How It Works – Fuel Pressure Signal Manipulation

Tuning boxes for diesel engines work primarily by manipulating the fuel pressure signal that goes to the ECU from the fuel rail pressure sensor.

The graph below shows fuel pressure against expected engine torque and engine rpm for an example engine.

The numbers in the table show the fuel pressure requirements to achieve this torque figure at this RPM.


A common rail diesel engine will be constantly changing the fuel pressure at the fuel injector to get the best possible power with the minimum amount fuel.

Because common rail diesel engines have the ability to adjust fuel rail pressure (the fuel pressure at the injector, the more pressure equals more fuel into the engine) it gives a tuning box an easy method of increasing engine power.

Trigger Threshold

There is no need for a tuning box to manipulate the fuel pressure sensor at all times. The fuel pressure only needs to be manipulated when the driver wants more power.

And this is how tuning boxes will differ between manufacturers.

The threshold at which the tuning box starts to manipulate the fuel pressure signal will normally be different, and the amount of manipulation can vary between manufacturers.

Going back to the table above. Some cars may trigger a check engine light at high RPM because they see too much air going into the engine. In this case the tuning box manufacturer may manipulate the fuel pressure sensor less, when actual fuel pressure gets to 1500 BAR for example. ie they will tail off the amount of signal manipulation as maximum fuel rail pressure is reached.

But they may also be able to manipulate the fuel pressure more between 1000 BAR and 1500 BAR fuel pressure to give more power in the middle of the power curve.

A tuning box manufacturer may have various stages built into their software. For example fuel pressure between

600 BAR and 1000 BAR

1000 BAR and 1500 BAR and

1500 BAR and 1600 BAR

could get their own amount of signal manipulation.

The number of stages and how much the pressure signal is altered will be down to the specific tuning box manufacturer.

This information will be kept confidential as it is proprietary to each individual tuning box manufacturer.

Diesel Tuning Boxes – Summary

  1. Tuning boxes for diesel engines work by manipulating the fuel pressure signal entering the ECU

2.  There will be a fuel pressure threshold where the tuning box starts working as it is unnecessary for the tuning box to be working all the time.

3.  A tuning box manufacturer may break down fuel pressure into zones in order for each zone to have its own program/degree of manipulation.

4.  The number of zones and the degree by which the fuel pressure signal is manipulated in these zones is confidential information, it is what gives a tuning box manufacturer an advantage over another

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