Turbocharger operation (petrol)

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

Petrol intake manifold turbocharger evaluation without variable valve lift, including WOT snap test.

The purpose of this test is to rapidly confirm the operation of the turbocharger; from idle speed through to a wide open throttle (WOT) snap test, and returning to idle speed.

This procedure is not a definitive test for your turbocharger. It does, however, confirm the presence of a positive pressure (above atmospheric) during the WOT snap test, confirming the rotation of the turbine and compressor shaft.

The turbocharger boost pressure is directly related to the:

  • intake condition and flow
  • throttle position
  • valve timing and lift
  • engine condition
  • exhaust flow
  • boost pressure control or override circuits

All numerical readings quoted in this help topic are typical and are not applicable to all engine styles.

All the values below are obtained with the WPS500X and referenced to gauge pressure.

The intake pressure before the throttle (air inlet side) is described as atmospheric pressure = 0 mbar.

The intake pressure after the throttle (engine side, negative pressure) is described as vacuum = below 0 mbar.

The intake pressure after the throttle during the WOT snap test is described as a positive pressure = above 0 mbar, Boost pressure

Make sure that the WPS500X is fully charged before you start this test.

How to perform the test

Accessories

 

PicoScope settings

  • WPS500X: Range 2
  • Channel A: ± 1 bar
  • Timebase: 5 s/div

 

  1. Connect the BNC to BNC test lead to Channel A on your scope and to the BNC socket on the WPS500X pressure transducer.
  2. Connect the vacuum hose to the inlet port on the pressure transducer.
  3. Securely attach the selected compression hose adaptor to the standard compression hose.
  4. Switch on the pressure transducer and wait for the self-test to complete (LEDs will scroll from range 1 to 3 and revert to 1).
  5. Fit the pressure transducer to any convenient inlet manifold vacuum source (use the vacuum adaptor if required).
  6. Press the range button and select Range 2 (–15 to 50 psi) (do not select zoom functions).
  7. Run the PicoScope software by pressing either the space bar on your keyboard or the Go button.
  8. Crank and start the engine, and let the idle speed stabilize.
  9. When the engine is at correct operating temperature, you can carry out a momentary WOT snap test and let the engine’s idle speed stabilize. (The manifold pressure waveform will now be present on the screen.)
  10. Let the engine idle for another 20 seconds to reduce the turbocharger speed.
  11. Repeat steps 7 to 9 if required.

We advise you to recharge your WPS500X after use to ensure it is ready for future measurements.

Example waveform

Figure 2

Channel A shows the intake manifold vacuum/pressure during the various stages of the test (steps 8 and 9).

The green line (at marker 3) denotes atmospheric pressure 0 mbar.
The black ruler (at marker 1) denotes idle intake vacuum, approximately -560 mbar

The orange arrow (at marker 4) denotes the initial delay from the start point of WOT to the commencement of positive pressure (at marker 5) building inside the intake manifold, referred to as Turbo lag. The example waveform shows that the test vehicle measures to approximately 600 ms.

The orange arrow (at marker 6) denotes the time taken for the intake manifold pressure to build to its peak of +560 mbar (at marker 7), from the initial increase at marker 5.

By adding the time indicated by the orange arrows at markers 4 and 6, you can calculate the total time taken to achieve maximum manifold pressure (boost pressure) from the initial WOT at marker 2. (Approximately 1.1 seconds on our test vehicle.)

The purple arrow (at marker 7) denotes the maximum positive manifold pressure (above atmospheric) produced by the turbocharger. The boost pressure.

The blue line (at marker 9) denotes the maximum manifold vacuum, approximately -820 mbar.

Waveform notes

Typical values (when the engine is at correct operating temperature).

 

  1. Stable idle speed – note the stable manifold vacuum.
  2. WOT applied – rapid decrease in manifold vacuum to atmospheric pressure (0 mbar).
  3. WOT held – Atmospheric pressure
  4. Turbo Lag
  5. Commencement of positive intake manifold pressure – Boost pressure
  6. Elapsed time from commencement of boost to maximum boost pressure
  7. Maximum boost pressure
  8. Rapid decrease in manifold boost pressure
  9. Increased intake pocket during over-run.

Diagnosis

Refer to vehicle technical data for specific test conditions and results.

When idle speed stabilization has been executed by the engine management and all loads applied to the engine have settled, the idle speed manifold vacuum should remain reasonably stable on this time base.

For more accurate analysis at idle speed refer to the Pressure Sensors > WPS500X Pressure Transducer > Intake Manifold Pressure-Idle Speed pre-set on the Automotive menu.

Applying WOT, with the engine at the correct operating temperature, should result in an instant decrease of the manifold vacuum, reverting to the atmospheric pressure (0 mbar) indicated by the green line at marker 3.

There will be no restriction inside the intake manifold while the throttle is held open. As a result, the intake manifold will remain equal to atmospheric pressure until the turbocharger speed increases sufficiently to generate a positive pressure (boost pressure at marker 5).

At approximately 360 degrees of crankshaft rotation, you can see the commencement of the intake pocket dropping below 0 bar indicating the cylinder pressure to now fall into a vacuum. This indicates adequate sealing of the exhaust valve that will now close as the piston descends the cylinder during the intake stroke. Valve timing, the integrity of the air intake, piston compression rings and cylinder face, can also be confirmed by looking at intake pocket. The depth of the intake pocket (the vacuum level) can be measured with the signal ruler (marker 5) and the value displayed in the ruler legend (marker 4) at -698.6 mbar.

From the instant the throttle is held wide open, an inevitable delay will be experienced before the commencement of positive pressure building inside the intake manifold. This is referred to as Turbo lag. The example waveform shows that the test vehicle measures to approximately 600 ms. (indicated by arrow with marker 4).

As the engine speed continues to increase, the speed of the turbocharger’s turbine and compressor shaft will increase to a point where a positive pressure is generated inside the intake manifold. This initial increase in manifold pressure (above atmospheric) is indicated by marker 5.

The time for the intake manifold pressure to increase from the commencement of boost pressure at marker 5 to the maximum boost pressure at marker 7, is indicated by the arrow by marker 6. In the example waveform, the time was approximately 470 ms

The continued increase in the turbocharger’s turbine and compressor shaft speed during the WOT stage results in a rapid increase in manifold pressure to the maximum boost pressure recorded at marker 7 (highlighted with the purple arrow). Please be aware that the value recorded here must only be used as an indication of turbocharger activity, (confirming rotation of the turbine and compressor shaft) and not a definitive guide to the efficiency of the turbocharger. Various manufacturers will limit the turbocharger maximum boost values when the vehicle is stationary. This highlights the need to carry out a similar test under road test conditions. Make sure that the maximum boost pressure does not exceed the manufacturer’s stated value as this could indicate a sticking turbocharger waste gate/vane control mechanism or failure within the maximum boost control circuit.

When you release the throttle pedal, both the engine speed and the turbocharger’s turbine and compressor shaft speed will decrease, resulting in a fall in boost pressure to below atmospheric pressure, and returning to a vacuum. Be aware that the turbocharger speed and engine speed will fall at differing rates resulting in a progressive return into a vacuum as opposed to a rapid return with non-turbocharged engines.

Given the engine speed is high but falling after the WOT test (throttle now closed) the intake manifold vacuum continues to rise above the stable idle speed level (at marker 1) to form the intake pocket. Here you can further confirm the mechanical efficiency of the engine and the integrity of the intake system. A small intake pocket could indicate a mechanical defect or intake leak. For more accurate analysis of the intake pocket, refer to Pressure Sensors > WPS500X Pressure Transducer > Intake Manifold Pressure-WOT Snap Test pre-set on the Automotive menu.

Additional information

You can read more about Turbochargers in our online training section.

AT787-2

Disclaimer
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

Suitable accessories

  • WPS500X Pressure Transducer

    £749.00

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Guided test: Turbocharger operation (petrol)