Boost behavior of VAG TDI engines, engine code AHF (Articles)

Translated on 04-07-2026, 13:49.

Here are some screenshots showing the turbocharger pressure behavior on the VAG 1.9l TDI engine AHF.
However, the engine was not original, but...
It was upgraded to 101 kW (137 hp) using an auxiliary control unit. Otherwise, only minor flow optimizations were made to the connections between the turbocharger and the intercooler. The charging curve looks similar, but at approximately 2000 minutes^-1, the pressure is only about 0.85 bar.
At approximately 3200 minutes^-1, the pressure is approximately 0.9 bar, which gradually decreases to 0.7 bar starting at approximately 4100 minutes^-1.
Looking back, it can be said that the values may be considered too low, as the regulated boost pressure...
It should be at approximately 0.95 bar.
If someone measures their car and doesn't achieve the values shown in the diagrams, that's not necessarily a problem. The important thing is that the actual boost pressure roughly follows the target value. If the actual boost pressure is more than 15% (~a limit determined by experience) below or above the specified target value, even after several seconds of full load, a defect should be suspected.
In the meantime, ADAC Fürth has performed another power measurement. However, the results are only comparable to a limited extent because the first measurement was performed on a different test bench.
The measured value is now 103 kW (140 hp). This is an increase, but only within the range of measurement accuracy. However, it must be said that during the first measurement, a fan was blowing directly onto the intercooler, while in the new measurement, the fan was positioned relatively far in front of the car and was blowing very weakly; it was not producing more wind than a speed of 40 km/h. Well...

To explain the operation of the boost pressure control system

Maximum boost pressure is approximately 2000min^{-1}

The color of the measured values corresponds to the color of the curves, the purple line.
For example, the engine speed is indicated, and at the vertical red line, it's approximately 4100.
min^-1 reached.

However, the program has a few bugs regarding the units and labels (it recommends KDataScope, which did not exist at that time).

Mbar are correctly mbar, 'Engine Speed' (revolutions per minute) is correct, but the others are not.
The values represent the target boost pressure, the actual boost pressure, and the duty cycle of the magnetic valve used for boost pressure control.

'Left Cursor' is the green line on the far left, and 'Right Cursor' is the...
at the peak of the yellow curve (actual boost pressure).

This information can be read in the control unit under measurement block 11.

Slow decrease at 2700min^-1

Further decrease at 3800min^-1

From 3800min^-1 it remains constant until 4116min^-1, further measurements were not taken.

How the turbocharger pressure regulation works.
during the TDI with variable turbine geometry

. This description was provided by the forum participant.
I would like to express my sincere thanks to Ulf for his assistance.

Before I give you an important note about measuring boost pressure: Only values obtained after at least 3 seconds of full throttle (held down!) and an engine speed above 2000min^-1 are meaningful.
Under idling conditions or at partial load, the boost pressure drops to values slightly above atmospheric pressure.
Measured values for the regulated boost pressure are approximately 0.9-0.95 bar for the 90hp TDIs, and approximately 1 bar for the 110hp TDIs. The higher-performance pump-injector versions of the 1.9-liter engines are significantly more powerful.

The engine + turbocharger system has a tendency towards self-destruction:

Since the exhaust flow is, in a way, the product of the air-fuel mixture and the engine speed,
represents, this cycle occurs below a certain minimum speed.
Despite being at full throttle, it doesn't engage, but on the other hand, it's driven by high RPMs.
especially when operating at partial load.

In order to prevent this cycle from causing overload of the charger and / or
To prevent damage to the engine, the turbocharger's boost pressure must be limited. At the VTG loading station.
(Variable turbine geometry) is achieved through...
Adjusting the guide vanes around the gas turbine. This will result in...
Altering the airflow to the turbine changes the impeller speed, and consequently...
The charging pressure is affected.

The force used to adjust the guide vanes (via...
A ring mechanism in the charger is provided by a membrane dose.
This is powered by a vacuum, which in turn is generated by a pump located on the engine.
(which also supplies the power brake booster).

increases with sufficient exhaust gas flow

To limit or The boost pressure is continuously regulated by the engine control unit.
measured. If the pressure exceeds the target value specified by the characteristic curves,
The engine control unit then electrically actuates a control valve (pressure converter valve).
which reduces the vacuum in the VTG membrane cell, thereby decreasing the charging pressure.
sinks.

The electric pressure-reducing valve has inlets for the vacuum.
the motor pump and the (atmospheric) external pressure, as well as a...
Starting point. It can not only be opened and closed, but also...
depending on the control signal provided by.
the engine computer (duty cycle between voltage on and
Voltage drops can also cause intermediate states to occur in a very rapid sequence.
so that its output is any pressure value between atmospheric pressure and vacuum pressure.
mixed and thus the VTG mechanism can be continuously adjusted.

Without tax pressure, the outside pressure is present at the outlet of the valve.
In the event of a power failure affecting the pressure-regulating valve, the VTG (Variable Turbine Geometry) mechanism remains operational.
in its resting position, and no boost pressure can be built up.
Since the loader requires quick response (small turbines with low
The turbine speed would decrease significantly in every case.
The VTG mechanism adjusts almost instantaneously and usually travels too far upwards.
or, conversely, decrease, which would then require further adjustments.
etc.

fluctuating

more careful

Below the target boost pressure, the pressure control valve operates depending on...
opened only as far as necessary so that, in the event of a sudden...
Accelerating the boost pressure until the control system intervenes should not cause it to shoot up excessively.
However, under sudden full acceleration, the turbocharger pressure can increase from approximately 1,000...
min^-1 in 1st gear due to the rapidly increasing engine speed for fractions of a second.
increase to approximately 1.3 bar (target: about 1 bar) before it rises within approximately...
The system is adjusted to reach the target value in 2 seconds.

^-1

reaches

Shortening the VTG linkage only increases the aforementioned.
Pressure peaks until the control system intervenes, against which the level of the regulated pressure is set.
The boost pressure will not be changed (as long as a regulation is still possible).
is), because the engine computer simply controls the pressure relief valve with a different,
appropriate duty cycle to achieve.