I've been told that because diesels are detonation engines (more properly spontaneous ignition) the shape of the combustion chamber is irrelevant. The RJL 40 head contrapiston has a shallow bubble and was said to idle better. When we replaced the contrapiston after it broke, the piece of aluminum we used happened to have a shallow cone in the center. Both contrapistons ran fine for my purpose, which does not include idling. Most diesels have either a flat crown piston and a flat contra piston, or a slight cone on the piston and a matching cone in the contrapiston. We know that combustion chamber shape is important in a glowplug engine because combustion originates at the glowplug. Any thoughts on diesel?

Jim