"Now for a couple of SAE papers to get you guys started.

SAE 871911 "Individual Cylinder Knock Control by Detecting Cylinder Pressure" by Kunifumi Sawamoto, Yoshiaki Kawamura, Toru Kita and Kenjiro Matsush*ta of Nissan Motor Co. Ltd. This paper will show you the relationship of knock frequency with acoustic velocity in the combustion gasses, bore diameter and resonance mode. It will also characterize the frequency multiples of the modes, and it 'taint x2, x4 ,x8 etc. Paper will also show where in crank angle knock typically occurs.

SAE 890156 "Characterization of Knock in a Spark Ignited Engine" by Kwang Min Chun and John B. Heywood of MIT Fame. This paper has a couple of nice graphs and drawings that will show you why knock collapses the ring lands.

SAE 982477 "Knock in a S.I. Engines: A Comparison between Different Techniques for Detection and Control" by F. Millio and C. V Ferraro of the Politecnico di Torino, Departmento di Energetica... like in Italy. This work will show you that the resonances are Bessel wave functions ... also that predominant resonances are are engine speed dependent. Also has some really nice graphs that show you how valve chatter and piston slap overlay the knock signatures."

As some of you may be aware, I'm in the process of developing a box of tricks to use the standard knock sensor and provide a bargraph of knock level, a 0-5V analogue signal for ecu's which support knock control and a digital 'knock present' output to switch maps on ecu's which dont support knock control.

The system I have so far is just a crude filter & level detector, using the inbuilt resonance of the knock sensor to provide the main filtering.

It has been suggested that it would be better to use a 'sample & hold' system which measures the level only during the time knock is likely in order to reduce the effect of engine noise. I was concerned that in order to receive fast enough response to make this worthwhile, would require reducing the 'Q' of the filtering and could therefore mean any advantages of sampling could be lost by allowing other frequencies through.

I decided to run some tests on my old knackered engine (No. 4 ringlands) before replacing it rather than abuse the new one.

Firstly, checking whether the spark is on rising or falling edge of IGT signal: (answer appears to be falling edge)
Trace 1: G1 from dizzy. Trace 2: IGT. Trace 3: Probe next to king lead.


Secondly tests at 4K RPM with no load, with load, with load and 27 degrees extra ignition
Trace 1: G1 from dizzy. Trace 2: IGT. Trace 3: knock sensor (10M load, disconnected from ecu)
No load:


With load:


With load & extra advance:


Thirdly tests at 7K RPM (rev limiter) with no load, with load, with load & 17 degrees extra advance (trace 3 scale changed)

No load:


With load


With load & advance



The traces show that using the standard knock sensor, the base noise level is reasonably flat (apart from knocking ringland), and det. is by far the loudest thing heard making time gating somewhat un-necesary.