They are a closed chamber design, that is, the chamber in the head is not as big as the cylinder bore so a portion of the deck surface of the head protrudes over the bore. When combined with a flat top piston that comes up flush to the deck surface of the block it is the thickness of the head gasket that keeps the piston from contacting the cylinder head. If this gap is maintained in the .035-.045" range the mixture in the cylinder will be very violently squeezed over to the other side of the cylinder creating a very uniform mix of fuel and air. The uniform mixture eliminates hot spots that cause detonation. Virtually every current engine design uses this principle and it allows for higher compression without detonation.
