Knocking is caused by the burning of the gas inside the cylinder BEFORE the piston goes down. Some people call it detonation. In any case, the piston gets a big knock, which may eventually destroy it. This is bad news ! Bad fuel, or advanced timing, or high temperatures cause knocking.

Some cars employ knock sensors to retard the timing if knocking is detected. It then advances the timing again until it detects knocking, thus adjusting the timing to the optimum point all the time. Fantastic! I like it!

If you want to retrofit an engine with a knock sensor, you are faced with extreme difficulties. Normal engine noise is un-distinguishable from knocking above 4000 RPM. Even by ear.

Where do you place the knock sensor? This is a major problem, because the sound of one cylinder knocking travels a different path as other cylinders and may eventually cancel itself and you detect nothing. Some manufacturer has 5 different locations for knock sensing.

The electronic side of knock sensing is relatively simple. The placement of the knock sensor should be left to the original engine manufacturers, who have the means to verify its location and operation. Perfect Power fuel injection systems don't use knock sensors due to the problem involved in fitting one. If the system relies on a wrongly placed sensor, it would not detect any detonation, advance the timing, and blow up the engine.

To counteract knocking if low octane fuel is used, the second MAP can be programmed for low octane fuel and retard timing.

Premature ignition can be avoided by "rounding" all corners of the combustion chamber.