In-Cylinder Pressure Analysis

What the trace actually measures

A pressure transducer in the spark plug hole records cylinder pressure through all four strokes. The plug is out, so the test cylinder doesn't fire: you're looking at a motored cylinder, a sealed volume compressed and expanded by the crank, with the valves opening and closing on top of it.

That makes the trace a direct readout of four physical things:

• Sealing — peak pressure, tower symmetry, and expansion pocket depth.
• Valve timing — where the exhaust and intake events land in crank degrees.
• Exhaust flow — the pressure the piston pushes against on the exhaust stroke.
• Intake flow — how closely cylinder pressure follows manifold pressure on the intake stroke.

A compression gauge gives one number. This trace gives all four, without removing the head.

What you'll need

• Lab scope, at least 2 channels. A 3rd channel for the crank sensor is ideal.
• In-cylinder pressure transducer with a DC (static) output.
  • Gasoline engines: a range of about 16 bar (232 psi) or more. Open-throttle and snap-throttle peaks on high-compression engines can approach that ceiling; a flat-topped peak is clipping, not a plateau.
  • Never use a gasoline-range transducer on a diesel. Diesel compression exceeds its overload rating.
• Spark plug adapter hose with the Schrader (check) valve removed. A check valve blocks everything below atmosphere and destroys the pocket, the exhaust ramp, and the intake readings.
• An angle reference: the crank sensor signal (best), or an ignition signal from another cylinder.
• The engine's bore, stroke, rod length, and compression ratio, plus its cam timing if you have it.

Sample rate: degrees per second = RPM × 6, so 0.1° resolution needs 60 × RPM samples per second.

Sensor response time. A transducer with a 1 ms (10–90%) rise time lags the true pressure by about 0.45 ms. That's about 1.9° of crank at 700 RPM and about 5.5° at 2,000 RPM, the same size as the timing faults you're hunting. Compare valve-event angles only between captures taken at the same RPM with the same sensor.

Step-by-step capture

1. Warm the engine to operating temperature.
2. Disable the test cylinder.
   • Unplug the injector so no fuel enters the cylinder or the catalyst.
   • Coil-on-plug: unplug the coil's electrical connector.
   • Plug-wire systems: connect the lead to a spark tester so the coil still has a path to ground.
3. Install the adapter and transducer. Make sure the adapter's O-ring or washer seals: an adapter leak looks exactly like a valve or ring leak.
4. Zero the transducer at atmosphere with the engine off. It should read 0 psig; vacuum reads negative.
5. Connect the angle reference, and record the operating state: RPM, manifold pressure, coolant temperature, and the commanded cam position on variable-timing engines.
6. Capture 10 seconds per state:
   • Idle
   • Steady 2,000–2,500 RPM
   • Three quick throttle snaps (not held)
   • Optional: cranking with all cylinders disabled and the throttle open, for sealing only
7. Keep each run short — a few minutes at most — so the transducer doesn't heat-soak.

The 720° map

Angles are measured from compression TDC (0°/720°).

The physics baseline

WaveAssist Pro calculates what this cylinder should do from its geometry instead of comparing it against a stored waveform.

Cylinder volume vs crank angle (a = stroke / 2, L = rod length, B = bore, CR = compression ratio):

Vd   = (π·B²/4) · stroke
Vc   = Vd / (CR − 1)
V(θ) = Vc + (π·B²/4) · [ L + a − ( a·cosθ + √(L² − a²·sin²θ) ) ]

Compression after the intake valve closes (polytropic):

P(θ) = P_IVC · ( V_IVC / V(θ) )ⁿ

• P_IVC is about manifold absolute pressure at the moment the intake valve closes.
• n is the polytropic exponent. With no heat loss, a compressed mixture at a 10:1 compression ratio reaches about 20–21 times its starting pressure, an equivalent exponent of about 1.30–1.32. Air alone runs a little higher. Wall heat loss lowers the measured n, and leakage lowers it further.

Measure n from your trace with two points on the compression side after the intake valve closes:

n = ln(P₂ / P₁) / ln(V₁ / V₂)

Plotting ln P against ln V turns the compression stroke into a straight line of slope −n. The point where it starts being straight is where the intake valve has effectively closed.

The adapter adds clearance volume

The adapter hose and transducer add dead volume to the combustion chamber, which lowers the measured peak. On a 6.2 L V8 (103.25 mm bore, 92.0 mm stroke, 10.41:1), an 8 cc hose drops the effective compression ratio from 10.41:1 to about 9.57:1 and costs roughly 10–13% of peak pressure. Enter the hose volume with the upload, and never compare captures taken with different hoses without correcting for it.

Worked example: what the physics predicts for a 6.2 L V8 at idle

These are calculated values, not measurements. They show the range a healthy cylinder should land in, and how much each input matters.

Inputs:

• Bore 103.25 mm, stroke 92.0 mm, compression ratio 10.41:1 (published specification)
• Rod length 154.89 mm (family value; not on the published specification sheet)
• Idle manifold pressure 28–35 kPa absolute
• Effective intake closing assumed between 40° and 60° after BDC (this engine's valve timing is not published)
• n = 1.30, and an 8 cc adapter

What this tells you:

• The peak range is wide because idle manifold pressure and intake closing both vary. Measure MAP at capture time and the uncertainty shrinks.
• The "half pressure at about 30° after TDC" checkpoint used in the trade holds up: the model lands at about 28°.
• Pressure at 90° before TDC is still well below atmosphere at idle. A rule that says the cylinder should be near atmospheric there only holds at higher manifold pressure, so don't use it at idle.
• Snap-throttle peak runs roughly three times the idle peak, close to the ratio of manifold pressures. The trade's "about three times idle" rule agrees.

Reading the pattern

Healthy pattern

• Peak within the physics range for the measured manifold pressure, and repeating within a few percent cycle to cycle.
• Tower close to symmetric about TDC, with the expansion side slightly below the compression side (heat loss).
• Expansion pocket at idle at least as deep as the intake-stroke pressure. With no leakage, expanding back to a similar volume returns to roughly the starting pressure, and heat loss makes the pocket slightly deeper. In the model, the pocket at 50° before BDC is −10.0 psig against −9.6 psig on the intake stroke at 35 kPa.
• Exhaust ramp centered near BDC (180°).
• Intake ramp centered roughly 20° after exhaust TDC (about 380°); later on engines with variable intake timing, depending on the commanded position.
• Intake closing complete somewhere around 560–600°.
• Exhaust plateau close to atmosphere at idle.

The ramp and closing windows above are field screening windows used in the trade. When the engine's own effective valve timing is known, measure against that instead.

Low peak, shallow pocket

Pressure leaks out on compression and gas leaks back in during expansion: rings, a valve, a head gasket, or the adapter.

• Re-seat the adapter and capture again before condemning anything.
• Then run a leakdown test to find the path.
• Leakage has more time to act at low piston speed, so a marginal leak shows more clearly on a cranking capture than on a running one.

Low peak, but normal pocket and normal n

The cylinder seals, but traps less air. Look at the intake closing point. At low speed, cylinder pressure stays near manifold pressure until the valve closes, so a late intake closing lets the rising piston push charge back into the intake. Causes: retarded cam timing, or a worn intake lobe. Check the intake ramp too.

Exhaust and intake ramps shifted

Shifts are in crank degrees; one cam degree equals two crank degrees.

Engine computers only flag cam/crank correlation errors above a set limit (on some engines about 11 crank degrees). A smaller phase error can sit undetected by the module and still be clearly visible on this trace.

No exhaust ramp; a second tower at 360°

The exhaust valve never opened. The cylinder compresses and expands its gas like a spring, so a tower appears every revolution instead of every other one. Causes: a collapsed lifter, a broken rocker or pushrod, or a flat lobe. On engines with cylinder deactivation, confirm deactivation wasn't commanded during the capture first.

Exhaust plateau high

The piston is pushing against a restriction: a plugged catalyst, a crushed pipe, or a failed muffler.

• At idle: under about 2 psig is a common field limit; a plateau of 5 psig or more usually comes with a noticeable loss of power.
• At higher speed and load, exhaust-stroke pressure rises normally with flow. Several psig at high speed and load can be normal, so an idle limit doesn't apply there. Judge restriction by how steeply the plateau climbs from idle to 2,500 RPM, and compare it with a known exhaust backpressure test if in doubt.
• A motored cylinder reads slightly high. Its exhaust gas is cooler and denser than a firing cylinder's, so the pressure difference across the exhaust valve is larger late in the stroke.

Intake plateau well below manifold pressure

On the intake stroke, cylinder pressure normally runs a little below manifold pressure because of the pressure drop across the valve. A cylinder that sits well below the others is starving: deposits on the intake valve, low valve lift, or a restricted runner.

Cycle-to-cycle variation

Peaks or ramps that move from cycle to cycle at a steady idle point to a sticking valve, a lifter pumping up and bleeding down, or a timing drive that is moving. Compare consecutive cycles in the overlap region around 360°.

Why the reference channel matters

Without a crank signal, angle has to be estimated from time:

θ   = 720 · (t − t_peak) / T_cycle
RPM = 120 / T_cycle        (T_cycle in seconds)

This assumes constant crank speed. The test cylinder isn't firing, so the crank slows and speeds up within every cycle, and time-based angles can drift by a few degrees. A crank sensor channel removes that error, because every tooth edge is a true angle mark. Use it whenever valve timing is the question.

Common mistakes

• Leaving the Schrader valve in the adapter.
• Comparing an idle running peak against a cranking compression spec: the idle peak is low by design.
• Ignoring adapter volume when comparing captures taken with different hoses.
• Leaving the injector connected.
• Judging valve timing to a degree or two without a crank reference, or across different RPMs.
• Condemning a valve before re-seating the adapter.
• Calling a deactivated cylinder a failed lifter without knowing the commanded state.
• Applying an idle exhaust-backpressure limit at high RPM.

Frequently asked questions

Related guides

• Relative compression test →
• ROTKEE sensors on PicoScope →
• Known good waveforms →
• Triggers & timebase →
• Belt & timing chain noise →