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The reduction of emissions of future SI engines is of prime importance for their development. Hence, investigations of the formation of unburned hydrocarbons in SI engines have been the subject of intensive research for many years. The scope of this work was to investigate several pistons with different top land geometry with respect to the potential of HC reduction. The observation of flame intrusion into the top land crevice was enabled by an optical fiber measurement technique. For this, six optical probes were inserted into the cylinder liner of a production four-cylinder SI engine. The emissions were detected with a conventional exhaust gas measuring system. The results of the investigation show reductions of about 30% in the HC emission. The flame intrusion depth and the frequency of intrusion are clearly dependent on the top land geometry.