RE: Diesel combustion chamber shape.
ReCycled flyer Some good points, in some full size diesels they do have a dished piston to take advantage of this , wants to expand in this area first, even in munitions the charge is shaped by its container to direct it straight forward to penetrate the thick armor of say a tank,
to punch thru it rather than just a random blast
any disturbance to this area poor design or geometry can be an issue this may explain the issue with the pings in 4 stroke conversions
I do remember a film clip of a full size engine with that had a quartz window in the head or cylinder and the combustion pattern filmed with a high speed camera martin