Knock sensors

[Robert W. Hughes]

The only knock sensor I have personally tested was for a 1986 Buick GN.
I recorded several taps and digitized them with a sound board. The
response was essentially white to above 10kc. Apparently the response
tuning is done with filters in the electronics. I have also seen
information that the knock frequencies peak in the 6-9kc band for most
engines.

The information on the Harris chip supports this also (broadband sensor
and signal in the 6-9kc band). Things that hit the engine or rattle can
cause false responses which is the reason for the elaborate filtering
and two channel design of the Harris chip.

Incidentally this GN sensor is convenient for testing various engines.
It mounts with a 1/4 MPT stud so it can be used to replace a block drain
plug. A decent cassette recorder can be used (with an attenuator -
output is up to 5-6 volts PP for a tap and .1-.5 volt for normal running
with the possibility of much more for things like rods hitting the
block) to record the output, the sound board on your PC can digitize the
recordings, and there are many FFT type programs available to see the
results.
-- 
Robert W. Hughes (Bob)
BackYard Engineering
Houston, Texas