The fiber optic spark plug was used in conjunction with a piezoelectric pressure transducer to collect combustion diagnostic data on four production engines designed to generate quiescent, swirl, and tumble charge motions. Spark advance was varied under low speed, low load conditions to investigate changes in flame kernel behavior and in-cylinder charge motion as functions of crank angle and spark advance. Two flame kernel models were filled to the data and a critical comparison of the models was conducted. Flame kernel behavior was represented by three values: convection velocity, growth rate, and convection direction. Convection velocity was highest in the swirl chambers. It also varied considerably among cylinders in the same engine. Growth rate correlated well with 0-2% burn but showed negligible correlations with later burn or IMEP. Convection direction proved useful in determining flow direction near the plug. Furthermore, changing flow characteristics, such as tumble breakup and turbulence generation, were evident from the fiber optic data as functions of crank angle degree, implying that the fiber optic spark plug may have increased diversity as a diagnostic tool.