The semantics of knock, pre-ignition, or detonation can be argued among automotive engineers, however it is basically an uncontrolled ignition of the air fuel mixture. The uncontrolled part means that it doesn't occur at the desired time in the combustion cycle. Normally the timing of the ignition is designed to maximize pressure in the cylinder so that it will push the piston down with the most force. Knock can cause high cylinder pressures at times when it actually counteracts the force and also cause shockwaves within the cylinder. It is the shockwave that emits the characteristic "tinkling" sound you often hear when the engine is knocking. In reality by the time you hear knock the chances are that the engine has been knocking for a while. Car manufacturers are continuously researching better ways of detecting knock and in the future we may see more direct methods such as pressure sensors and ionized current measuring however the majority of cars still use an indirect method of detecting knock by listening to the ringing sound of the cylinder using a piezoelectric knock sensor.
Turbo or supercharger? Well, they are two different beasts. Turbochargers generally allow for much higher bhp numbers as boost can be cranked up depending on your engine setup. Chargers are more reliable but generally have lower hp numbers. What are you looking for?
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