Knock sensor

[Dave Williams]

-> I was gonna notch-filter the thing for 5-15 khz.

 SAAB's Trionic engine management system drops voltage after the initial
spark, then looks at the resistance of the plug gap.  SAAB claims they
can tell if the cylinder is running into detonation by looking at this
value.

 If there's an SAE or IMechE paper on this anywhere it'd be interesting.
It would save trying to find a place to mount and adjust a sensor.
Dunno if the electrical/software part would be harder or not.