I used knock detectors on my SVOs and Supercoupe. They are simply tuned crystal microphones that listen in for certain frequency vibrations in the block. I know you know that James but I am sharing with others. On my Mustang SVOs they are located on the intake manifold and are too sensitive. I ran it on the street but disconnect it for 1/4 mile runs. The knock detector on the Thunderbird Supercoupe screws into the block above the starter and did not pick up on knock soon enough on many occasions.
I find the fins on most air cooled engines amplify the noise of detonation and preignition enough to hear it well IF you have a quiet enough muffler on.
I read a paper (SAE?) on quench distance vs power production effects many years ago. This was on a 4 stroke engine. It mentioned there is a power loss (to heat) and deceleration of the piston as the squish area increases and the gap decreases. I remember it had graphs of power loss (pumping losses?) on a driven (not running) engine that increased exponentially as the quench gap decreased. As I remember quench area was more a linear relationship with pumping losses.
The noise you are hearing may be the deceleration of the piston against the air/fuel trapped by the squish area. The piston decelerates, and you hear the gear backlash taken up. Possible?
I find the fins on most air cooled engines amplify the noise of detonation and preignition enough to hear it well IF you have a quiet enough muffler on.
I read a paper (SAE?) on quench distance vs power production effects many years ago. This was on a 4 stroke engine. It mentioned there is a power loss (to heat) and deceleration of the piston as the squish area increases and the gap decreases. I remember it had graphs of power loss (pumping losses?) on a driven (not running) engine that increased exponentially as the quench gap decreased. As I remember quench area was more a linear relationship with pumping losses.
The noise you are hearing may be the deceleration of the piston against the air/fuel trapped by the squish area. The piston decelerates, and you hear the gear backlash taken up. Possible?
