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Discussion Starter · #1 ·
I am curious how a knock sensor works and where it is installed. More importantly is it a nomal open/closed switch, and can it be used to trigger a timing retard box?
 

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On my '68 EL Camino (396 engine) I have two Delco knock sensors hooked up to a "J&S Electronics" computer (which is connected to the HEI distributor). The computer can momentarily retard the timing in one or more cylinders individually if it hears them pinging. I have one sensor in the front end of the passenger's side head, and another in the back end of the driver's side head. The two are connected in series. I believe that they act as resistors and the amount of resistance varies with certain vibrations. the systems works fairly well, but it is not fool-proof. You still have to have a reasonable advance curve (timing) and appropriate air/fuel ratio.

I installed this setup nearly 10 years ago, so I don't know if the J&S unit is still available, or where the company is located now. They were in California.



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Dan Carr
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Not sure what is inside those things, whether it is resistive or piezo electric stuff. I had a unit similar to dcarr's but mine was a "Carter Knock Eliminator" and it worked well, once the sweet spot for the sensor was found. Turned out to be the rear end of the left head. The sensor is tuned to react to vibrations of 3000 to 5000 Hz and then do its thing. The brain box will then put in a time delay on the signal to the coil to retard the spark timing. This became important when the octane rating of gas was falling and we were still driving old cars that need "good" gas.
 

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Discussion Starter · #4 ·
I think I am going to have to get my hands on one of these and check it out. If it simply closes or opens an electrical circuit (like any switch) This could be as simple as a MSD starter retard and maybe a relay. The starter retard will drop 10 deg of timing when activated,(usually by the ignition switch) but rather a knock sensor. Then deactivated when the knock stops, instead of 800 engine RPM as advertised.
 
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