The influence of the shape of a combustion chamber in S.I. engines on the knock phenomenon has been investigated. The possibility of application of the theory of combustion in a tube for the tubular combustion chamber has been considered. An experimental head with a tubular combustion chamber has been designed and tested on a two-stroke loop scavenged S.I. engine. A comparative investigation of the tubular combustion chamber in comparison with the conventional pentroof combustion chamber on the same production two stroke engine has been done. The performances and tendency to knock have been studied on the experimental engine with each combustion chamber. The variation of the compression ratio of the tubular combustion chamber has also been investigated. The superiority in resistance to knock and therefore in fuel tolerance of the tubular combustion chamber in comparison with the pentroof combustion chamber has been noted.