We discontinued knock sensor use in the 1st generation EFI due to false knocks caused by engine noise. The old GM knock sensor was a resonate based sensor when basically generated voltage when it vibrated at a certain frequency. The ECM would see the indication from the sensor controller and reduce timing until the knock controller stopped indicating a knock. This frequency covered a wide frequency area and allowed other items to be included (noise) that the GM sensor was not intended for (the Corvair air-cooled engine).
We are now using a new type of (flat) sensor which narrows the frequency that the sensor can hear and is adjustable for sensitivity. The controller and sensor has been tuned to a frequency that matches the Corvair cylinder bore size and eliminates the detection of things like timing gear noise.
This knock sensor will only work on the 2nd generation EFI that uses the MS type of ECM. It also has an LED that can be wired in and will flash when a knock is detected by the controller. Cost is $150 with a Bosch sensor.
21 Sep 2018: We tested this knock sensor by using 87 octane gas which produces slight ping/knock. During a 1 hour drive, only 2 times did I hear a ping before they knock sensor fired and reduced timing. We currently have 6 degrees with a .5 degree per second recovery. According to many experts, you can’t hear the knock without amplification and isolation, so by the time we hear it over engine and road noise, I guess it must be pretty bad. Will keep up updated and informed here.