VB here's something of interest!
I've ditched the bar graph volatage display in favour of the >$8 multimeter Jaycar have. When you wire this up you now an output in Volts so can use that as your degree of knock; anything under 1.00V is safe, anything over needs attention
You basically free-rev the car, set the pot so free revving = 0.00v and it eliminates background noise, even a slight knock will spike well over 1.00v instantly as it's actively listening in that frequency range.
It works great and can all fit in a jiffy box with 3 inputs (knock sensor, 12V + & -) just have a rotary dial to set sensitivity and you've got an LCD display from the cheapo voltmeter, an accurate and sensitive det-box for $30!!!