Bendy,
never is all fuel burnt any place near TDC or much later at BDC
My post wasn't about rich or lean mixtures. It was about incorrect timing for a particular fuel and how the head could get hot enough to cause valve recession. Without good data relating to where peak pressure occurred at various revs, what that pressure was and the actual CHTs at different revs it's drawing a long bow to blame lack of lead in LNG or propane for internal damage to heads. Taxi cab owners aren't noted for being silly with their money yet they continued to do gas conversions until gas became too expensive and hybrids became available.