Knock Detection 101

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What is Engine Knock?

Normal combustion occurs when a mixture of air and fuel is ignited by the spark plug and the flame front progresses smoothly

from the point of ignition to the cylinder walls.

Engine knock, or detonation, occurs during combustion when the temperature and pressure in parts of the unburned air/fuel

mixture (end gases) exceed a critical level, causing auto ignition (detonation). This causes a rapid increase in cylinder pressure

that produces a shock wave across the combustion chamber which excites a resonance in the cylinder at a characteristic frequency.

This frequency is dependent primarily on cylinder bore diameter,combustion pressure and temperature.

Damage to pistons, rings, bearings and exhaust valves can result if sustained heavy knock occurs.

What Causes Engine Knock?

Automotive manufacturers invest in hundreds and even thousands of hours to fine tune knock detection and management

on their engines, as engine knock is a complex process that is governed by many engine specific and external variables:

Knock is also heavily dependent on the tuning parameters ignition timing, air/ fuel ratio and boost for forced induction engines.

How to Effectively Detect Knock Frequency?

The characteristic knocking sound is created by the rapid rise in pressure due to uncontrolled combustion of one or more areas

of the combustion mixture, which create higher than average pressure waves just like shock waves of an explosion. These waves run

across and bounce-off the combustion chamber walls, creating a vibration and noise that we can also hear and monitor through the

engine block.

The combustion chamber geometry and especially its diameter play a major role in the specific frequency of these vibrations.

The larger the chamber the lower the frequency. The PLEX KNOCK MONITOR offers the user the ability to adjust the window of the audio

spectrum that is used to listen to and detect knock.

This window has a center frequency that is directly related to cylinder bore size and its primary vibration mode or mode 0. As this frequency

can vary +- 1KHz due to combustion pressure, temperature, rpm, engine structural configuration and other parameters, a sufficient width of

this window is set for detection whose properties are programmed by the factory proprietary algorithm. It may be the case that some engines

may have other vibrating parts that are overlapping the mode 0 frequency. In these cases using the mode 2 vibration frequency will yield a better

signal to noise ratio.

The comprehensive features of the PLEX KNOCK MONITOR offers its operators the ability to accurately detect knock and fine tune engine

management in a fraction of the time regularly required

Setting the Knock Frequency:

- Use the recommended settings for your bore size as found in Menu 2.2, or the first step in the BASIC MODE wizard

- Advanced users may consult with Plex Tuning regarding methods of determining the exact knock frequency.


- Verifying the knock frequency requires inducing knock to the engine for a period of several seconds. This can only be achieved in a controlled

environment like a load type dynamometer. Use steady state control with low rpm and as light a load as possible to induce light detonation.

It will help to use a low octane fuel for this test ONLY!  It might not be possible to induce detonation at a light load on some engines.

The engine can be damaged by detonation!

- Engine knock or detonation can very quickly destroy an engine. Determining if the engine is detonating is a specialized job requiring

experience and correct evaluation of the feedback from the device. In certain cases it will not be possible to detect detonation accurately with

the PLEX KNOCK MONITOR. PLEX TUNING will take no responsibility for any damage occurring when using this device.