The purpose of the Knock detector is to detect the amount of knock (aka Detonation or Pinking) within the car's cylinders. With air being pumped into the cylinders under pressure by the turbocharger, and then being further compressed by the piston, there is more danger of knock. Knocking happens because as you compress air, the temperature of the air increases. The temperature may increase enough to ignite the fuel before the spark plug fires.
Knock is a form of abnormal combustion (called pre-detonation) that can cause engine damage. The fuel ignites spontaneously and explosively, causing a sudden rise in cylinder pressure that produces a metallic pinging or knocking sound (like a marble in a tin can). Detonation can be caused by excess spark advance, low-octane fuel, lean air/fuel mixtures, and/or overheating. Carbon buildup inside the combustion chambers and on the piston face can also increase compression sufficiently to cause detonation.
Mild detonation is not harmful, but heavy detonation can damage valves, pistons, rings and rod bearings. It can also cause elevated HC emissions. Detonation can be caused by over advanced timing, a lean fuel mixture, overheated engine or faulty knock sensor.
The Impreza has a factory-fitted knock sensor installed and the ECU does perform knock correction to prevent engine damage. Unfortunately, the OEM sensor isn't the best and can miss some detonation in some circumstances and above 5,500rpm doesn't appear to listen at all.
You can install the Knock detector monitor using the OEM sensor, but it is advisable to use the optional sensor (shown right). This is manufactured by Bosch (part# 0-261-231-006) and is a much more reliable and sensitive. I sourced the sensor separately from another group buy on Scoobynet and the Knocklink monitor was supplied by Jap Innovations.
The monitor comes with a wiring loom for connection to a constant +12v supply, -12v ground supply and to the sensor itself (two wires). The sensor part of the loom does not have a connector and you need to source this separately, although I'm told that if you buy the Knocklink display and sensor as a kit, you should get the connector as part of the kit.
The connector is a seperate Bosch kit (part# 1-287-013-003, Jetronic Connector Kit) and is available from any Auto Electrical outlet. I sourced mine from Teeside AutoTech for the princely sum of £10 inc. VAT and delivery.
The purpose of the Knock detector is to detect the amount of knock (aka Detonation or Pinking) within the car's cylinders. With air being pumped into the cylinders under pressure by the turbocharger, and then being further compressed by the piston, there is more danger of knock. Knocking happens because as you compress air, the temperature of the air increases. The temperature may increase enough to ignite the fuel before the spark plug fires. 
You can install the Knock detector monitor using the OEM sensor, but it is advisable to use the optional sensor (shown right). This is manufactured by Bosch (part# 0-261-231-006) and is a much more reliable and sensitive. I sourced the sensor separately from another group buy on 
