WPS500X - Petrol intake manifold pressure (snap WOT)

The purpose of this test is to evaluate the intake manifold pressure under varying engine running conditions from cranking, idling, wide open throttle (WOT) "snap" test, idling, to engine off.

Connection guidance

Manifold pressure is directly related to: intake condition and flow, throttle position, valve timing and lift, engine condition, exhaust flow and any boost pressure applied by forced induction.

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

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

Intake pressure before the throttle (air inlet side, positive pressure) is described here as atmospheric pressure = 0 mbar.

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

Ensure that the WPS500X is fully charged before starting this test.

How to perform the test


  1. BNC to BNC test lead
  2. WPS500X Pressure Transducer
  3. TA085 Vacuum Hose
  4. TA129 Vacuum Adapter

PicoScope settings

  1. Channel A Option WPS500X Range 2
  2. Channel A ± 1 bar
  3. Timebase 5 s/div
  1. Connect BNC to BNC lead to Channel A of your scope and the BNC socket of the WPS500X transducer.
  2. Connect the vacuum hose to the inlet port of the WPS500X.
  3. Switch on the WPS500X and wait for the self-test to complete (LED will scroll from range 1 to 3 and revert to 1).
  4. Fit to any convenient inlet manifold vacuum source, using the vacuum adapter if required.
  5. Press the range button and select Range 2 (–15 to 50 psi) (do not select zoom functions).
  6. Run your scope software by pressing either the space bar on your keyboard or the "start" button in PicoScope.
  7. Crank and start the engine, and allow idle speed to stabilise.
  8. With the engine at the correct operating temperature carry out a momentary WOT snap test and allow engine idle speed to stabilise.
  9. Switch off engine and monitor intake pressure (vacuum) decay.
  10. Repeat steps 8 to 10 if required.

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

Waveform notes

Channel A shows the intake manifold vacuum during the various stages of the test (Steps 8, 9 and 10).

Green line (at markers) denotes atmospheric pressure 0 mbar
Black ruler (at markers) denotes idle intake vacuum, approximately -710 mbar
Blue line (at marker) denotes maximum manifold vacuum, approximately -970 mbar

Key to Example waveform

  1. ENGINE OFF - Atmospheric pressure
  2. ENGINE CRANK AND START - Note rapid increase in manifold vacuum
  3. IDLE SPEED STABILISING - Varies between idle control systems
  4. STABLE IDLE SPEED - Note the stable manifold vacuum
  5. WOT SNAP TEST - Rapid decrease in manifold vacuum
  6. INCREASED INTAKE "POCKET" DURING OVERRUN - Note the increase in manifold vacuum below the stable and even idle at 4
  7. ENGINE SWITCHED OFF - Monitor the rate of decay of manifold vacuum

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

Typical values (when engine is at correct operating temperature)

1. Engine off

Before engine start the intake manifold vacuum should be identical to atmospheric pressure (zero mbar on our scope scale).

2. Engine crank and start - Approximately 1.5 seconds from minimum to maximum manifold vacuum

During engine crank and start the manifold vacuum will rapidly increase, indicating sufficient cranking speed with reduced pumping loss. Refer to "Intake pressure cranking" preset under the "Automotive" menu for closer analysis during cranking.

3. Idle speed stabilising

Idle stabilisation should settle reasonably quickly depending on engine idle control systems. "Fly-by-Wire" systems will have a larger effect on idle stabilisation vacuum than "Idle air bypass control" systems. Any loads applied here will also have an effect on idle stabilisation vacuum such as air conditioning, cooling fan operation, and alternator output/load after cranking.

4. Stable idle speed

Once idle speed stabilisation has been achieved by the engine management and all loads applied to the engine have settled, the idle speed manifold vacuum should remain reasonably stable on this timebase. For more accurate analysis at idle speed refer to "Pressure Sensors > WPS500X Pressure Transducer > Intake Manifold Pressure-Idle Speed" preset under the Automotive menu.

5. WOT snap test

With the engine at the correct operating temperature, a WOT snap test should result in an instant fall in manifold vacuum, reverting to the atmospheric pressure recorded at step 1 (engine off, 0 mbar). On release of the throttle, the manifold vacuum should rise rapidly at the same rate as the fall. For more accurate analysis of the WOT snap test refer to "Pressure Sensors > WPS500X Pressure Transducer > Intake Manifold Pressure-WOT Snap Test" preset under the Automotive menu.

6. Increased intake "pocket"

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 to form the "intake pocket". Here we are further able to 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" preset under the Automotive menu.

7. Engine switched off - Approximately 1.8 second decay time maximum to minimum manifold vacuum

During the engine shutdown period, the rate of decay is all–important and should be progressive as opposed to a rapid fall in vacuum to atmospheric pressure measured at "Engine off" (0 mbar). Once again a rapid fall here would indicate a potential engine efficiency issue or intake leakage. Refer to "Pressure Sensors > WPS500X > Pressure Transducer > Intake pressure additional tests", cylinder pressure and crankcase test to identify the area of decay.
Note: Leaks in vacuum auxiliaries such as the brake servo and vacuum switching valves will contribute to a rapid decay in vacuum.

Further guidance

The internal combustion engine can be likened to a mechanical air pump, where air is drawn in through the intake and forced out through the exhaust. Engine efficiency relies heavily on this process, which is often referred to as "Engine breathing". During the intake stroke on our petrol engine below, air is drawn into the relevant cylinder, but the flow of air is met with a restriction in the form of our throttle butterfly valve. The butterfly valve will be held near to the closed position leaving a very small area for air to be drawn in and reach the cylinder on the intake stroke. A comparison can be made here with a bicycle pump, where placing your finger over the inlet to the pump while drawing back on the grip will restrict the air flow into the pump and generate a vacuum under your finger.

This test will provide you with an overview only of the sequence of events and vacuum values present within the intake manifold during the conditions 8, 9, and 10 mentioned in "How to perform the test". Should you discover an area of concern, the zoom function of your scope will enable you to analyze the waveform further. You will then need to select the relevant preset test under "Pressure Sensors > WPS500X Pressure Transducer > Intake Manifold Pressure" within the Automotive menu.


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


  • PicoScope WPS500X Vacuum tap


  • Vacuum hose


  • Universal vacuum adaptor


  • WPS500X Pressure Transducer Kit (with carry case)


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Guided test: Intake Manifold - Intake Pressure (Snap Wide-Open Throttle)