Engine Knock Frequency Calculator

Example data table

Formula used

This calculator estimates the dominant knock resonance with: f = M · a / (2 · B)

• f = knock frequency (Hz)
• M = mode factor (dimensionless)
• a = speed of sound in end‑gas (m/s)
• B = cylinder bore (m)

If you choose the temperature method, then a = √(γ · R · T), where T is in Kelvin and R is in J/kg·K.

How to use this calculator

1. Enter your cylinder bore and select the correct unit.
2. Choose a speed of sound method: temperature-based is usually better.
3. Set end-gas temperature, γ, and R to match your assumptions.
4. Adjust the mode factor if your logged peak is consistently higher/lower.
5. Pick a band width (±%) to create a starting filter window.
6. Calculate, then export CSV or PDF for your tuning notes.

Article

1) What engine knock frequency represents

Knock is a fast pressure wave that rings inside the combustion chamber after auto‑ignition. A knock sensor “hears” this ringing as a narrow band tone, not a random noise, so estimating the dominant frequency helps you choose filters and sampling rates.

2) Why bore size is the main driver

The chamber’s characteristic length is closely related to cylinder bore. Larger bores create a longer acoustic path, lowering the resonant frequency. That is why many 70–75 mm bores land in higher kHz bands than 95–105 mm bores. On most modern inline engines, a rough rule is 1 mm more bore lowers the estimate by about 1–2%.

3) Formula used in this calculator

A common estimate is f = M · a / (2·B), where B is bore (meters), a is speed of sound in the end‑gas (m/s), and M is a mode factor that compensates for chamber geometry and the dominant acoustic mode.

4) Speed of sound from temperature

If you enable the temperature method, the calculator uses a = √(γ·R·T). With γ ≈ 1.30–1.40 and R ≈ 287 J/kg·K, end‑gas temperatures around 700–900 K often produce a between ~540 and ~600 m/s.

5) Typical frequency examples

With B = 86 mm, T = 800 K, γ = 1.35, and M = 1.2, the estimate is about 4.4 kHz. With B = 72 mm under the same conditions, it rises to ~5.3 kHz. Increasing M shifts results upward proportionally.

6) Harmonics and band selection

Real sensors can respond to the fundamental plus harmonics (2×, 3×). Many tuners place a band‑pass window around ±20% of the predicted fundamental, then check nearby harmonics if the signal is weak or noisy. Start light filtering, then narrow as needed.

7) Practical notes and limitations

This is a planning tool, not a guarantee. Chamber shape, piston bowl, deposits, mixture, and mounting location can move the peak. Use the estimate to start logging, then refine by finding the strongest peak during controlled knock events.

FAQs

1) What input affects the result the most?

Bore dominates because frequency scales inversely with chamber size. Temperature and γ change the speed of sound, shifting the estimate, but usually less than a major bore change.

2) Should I use constant speed of sound or temperature?

Use temperature when you can estimate end‑gas heat under load. Use constant a for quick comparisons, or when you only need a consistent reference across engines.

3) What is the mode factor (M) for?

M is a practical adjustment for chamber geometry and the dominant acoustic mode. Start around 1.0–1.3, then nudge it until the predicted peak matches your logged knock peak.

4) Why show harmonics?

Sometimes the fundamental is masked by mechanical noise. Monitoring 2× or 3× can give a cleaner indicator, especially on noisy valvetrain or gear-driven setups.

5) How wide should my band-pass filter be?

A common starting window is ±15–25% around the predicted fundamental. Narrow it after you log data and confirm where the strongest knock peak stays across RPM and load.

6) Does engine RPM change knock frequency?

RPM changes operating conditions and noise, but the resonance is mostly geometric. The peak frequency may drift with temperature and mixture, so logging at several loads is still important.