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Discussion Starter #1
Just wondering what engine knock is I kinda have an idea what it is...but my friend who sold me the car said the knock sensor has to be changed out..I was just wondering what actually causes Knock :confused: and would I really need to change out the sensor. thXns

274 Posts
I was just reading the EFI Basics manual from the AEM EMS and here's what they have to say about knock. I just copy and pasted the section from the PDF file.

Engine Knock (Detonation) and Preignition
It is important to understand the mechanisms that cause knocking and preignition to set up an ignition map that is suitable for the engine. Auto ignition, also known as knocking, pinging, or detonation, is generally caused by improper combustion in an engine. An internal combustion engine runs properly when the spark-initiated combustion wave expands rapidly but smoothly throughout the combustion chamber. Combustion knock is caused by spontaneous ignition in the hot unburned portion of the fuel mixture (typically referred to as end gas) in the combustion chamber. The remaining charge portion is compressed first by the upward piston movement and then by the moving flame front. Knocking is the almost instantaneous ignition of part of the remaining mixture. This mixture auto ignites because the rapidly rising pressure and temperature caused by the piston movement and the expanding gas from the flame front are sufficient to ignite the remaining gasses. To illustrate the loads imposed on the engine components by knocking, note that normal combustion speeds are about 12-25 m.s-1 while knocking combustion speeds may be as high as 250-300 m.s-1. The next illustration depicts the difference between a normal and abnormal combustion process.

If the gasoline-air mixture auto-ignites somewhere in the cylinder (other than at the spark plug) just after spark ignition, the auto-ignition combustion wave can collide with the spark-initiated combustion wave, causing the vibration we hear as a knock or ping. Depending on its intensity, knocking combustion may range from barley audible “pinging” to a rather violent thumping. The point at which the knocking becomes damaging to the engine is dependent on the components used in the engine. If sustained knocking occurs, then the pistons may be damaged. When knocking reaches a violent thump, engine operation should be ceased or at minimum the load and temperature reduced to prevent engine damage. Light knocking that happens during acceleration is less harmful and may not damage the engine. Knocking tendency is increased by the following design or operational characteristics:
• High Engine loads encountered while towing a vehicle.
• Using low octane gasoline in a high-compression engine.
• Too much timing advance for the type of fuel being used.
• Higher air density, (this can be caused by starting a calibration at high altitude and then traveling to a lower one, or the addition of forced induction).
• Increased temperatures and pressure in the combustion chamber due to inadequate engine cooling.
• Excessive inlet air temperature.
• Spark plugs with an improperly high heat range.
• A non-central spark plug location in the combustion chamber.
• An elongated combustion chamber design.
• Too lean of an air/fuel mixture.

The following tuning adjustments can be performed on an engine to reduce or eliminate knocking:
• Reduce ignition timing.
• Verify that the air/fuel mixture is adequate for your engine set up.
• Verify that the spark plugs are of proper heat range.

Preignition is the ignition of the charge in the combustion chamber before the spark occurs. This type of ignition is caused by a very hot, or even incandescent surface in the combustion chamber. These “hot spots” can be an overheated spark plug, a glowing remnant of carbon in the chamber or even a hot exhaust valve edge. The preignition condition flame front rapidly expands while the piston is still on its way up the bore. Due to the very high pressure generated by the expanding flame front and the piston approaching TDC, the combustion chamber pressure rises rapidly causing audible knocking. Detonation and preignition typically have a cause and effect relationship; when detonation is prolonged and overheats the spark plug to the point where the tip glows, preignition occurs. Preventative measures can be taken to avoid preignition by using spark plugs with the correct heat range, avoiding detonation by using fuel with the correct octane rating for your application, and when building an engine, ensuring that there are no machined components with sharp corners in the combustion chamber. Also, the cooling system must be in good working condition to effectively cool the combustion chamber. Sustained operation of an engine in either of these conditions can result in severe engine damage.
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