There is a rattle noise in engines, when we run it with wrong octane grade (commonly lower than specified), or the ignition timing is not correct, or a higher gear is selected but not corresponding with the vehicle load, speed and/or street angle. This also happens when the engine overheats, the oil is too thin or there is a failing spark plug. The sound is like a metallic and "mute" clank.

Is that called "piston slap"? And where it happens, piston hitting the cylinder walls; where...on top, at the bottom? Or is it the rod bearings; which ones?

  • Pinking is not piston slap...
    – Solar Mike
    Jan 20, 2018 at 16:11
  • So pinking...where it happens? Jan 20, 2018 at 17:49

3 Answers 3


The article I read from HotRod Magazine states pretty clearly what is causing the noise:

In an SI engine, auto-ignition of the end gas in the combustion chamber causes a knocking or pinging sound. When this occurs, engine damage is created by the sharp pressure rise and rough combustion taking place. Localized pressure peaks and the explosion, rather than the burn of the fuel, causes the piston, rings, and rod bearings to shake and make a sound similar to a ball-peen hammer hitting a metal surface.

Basically they are saying the noise from the air/fuel mixture exploding and hammering all these parts, reverberates through the engine to create the sound you hear.

  • Too add on I have a premium only car also, and when you put regular gas in the knock sensors pick it up so the ecu can retard or advance timing to compensate for the knock that lower octane fuel gives. So it could be just that your ecu cannot do something like that, and the knocking just occurs without any solution @Aram Alvarez Jan 21, 2018 at 13:28

You are talking about different noises

The rattle/knocking noise when using too low octane fuel, incorrect timing or bad tuning etc is called 'knock' also known as detonation and this is where the fuel/air mixture is ignited when it isn't supposed to / at a different time to when the spark plug is fired.

Piston slap is another noise usually caused by an excessive gap between the piston and cylinder and this is where the piston skirt (the bottom edge) hits the cylinder wall. You can also get a similar noise when the inside of the piston hits the con rod if the rod is bent after damage such as sucking in water.

Bottom end knocking is usually caused by excessively worn big end bearings and this can be made worse by using too thin oil as thicker oil will better fill the gap between the crank and conrod bearings.

You can also get tappet noise and this can also be make worse by using too thin oil as thin oil escapes too quickly from hydraulic tappets.

  • Understood..."knocking" is then. Where it happens? Jan 20, 2018 at 17:50

Mr Gould's answer is excellent. To elaborate, the detonation or knocking he mentions is an uncontrolled burning of the fuel often referred to as an explosion. Normal combustion is a more controlled burning of the fuel, albeit very rapidly. The knocking is directly inside the combustion chamber or cylinder. It is very destructive and should be avoided.

  • Yes, I know the "why". I'm just asking the "where" it happens. What parts are involved and in which point they hit each other, if that's the case, or is it just sound waves... Jan 20, 2018 at 17:58
  • It happens on the compression stroke I believe (someone can correct me here if I’m wrong) there is no collision of parts, just the untimely detonation of a certain cylinder without spark Jan 21, 2018 at 13:30

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