How to use THE KNOCK BOX

How does THE KNOCK BOX help?

As I illustrated before, the ringing of the knock event is a resonance of the hot gas in the confined clearance volume. It has a resonant frequency that depends on the density of the gas and size of the enclosed volume and when this volume is disturbed in a knocking engine cycle, it will ring like a bell for a short instant.

For most practical engines this ringing frequency is broadly between 5 and 12 Kilohertz (about 5,000 to 12,000 cycles per second). Older people cannot hear much usefully above 12 KHz, however almost everyone with normal hearing can discriminate up to 10 KHz.

THE KNOCK BOX has sensors that fit to the engine block, usually in two widely spaced places to get better coverage. The engine tuner will advance the timing until best torque output is found, or he can hear a faint irregular spattering against the background noise of the regular sound of reciprocating engine components in the headphones. This widely spaced irregular spattering sound is the onset of knock and most un-tuned road engines will sustain light knock for extended periods without damage.

It is the irregularity of light knock that we are listening for, rather like the sound of the first drops of rain on a tin roof. Advancing the ignition to light knock has the same characteristic irregularity as sparse raindrops, then passing into more knock events until there is a storm of spatters as ignition is advanced. When there are so many knocking events that it sounds like a hailstorm, back off immediately! The idea is to listen for the onset of knock, (this first few spits per second) and then back off a degree of timing or so and run again to check the setting; but more of that later.