A comparative study of flame kernel development, flame propagation, and energy release rates for port fuel injected, lean engine combustion reveals effects of intake valve configuration. High speed schlieren motion pictures of combustion in a square piston engine simulator record the flame development histories for three different cylinder heads at two mixture ratios. The three cylinder heads are a four valve head with centralized spark plug, a two valve head with helical swirl intake port, and a two valve head with straight intake port. Observation at two pentane/air mixture ratios, stoichiometric or slightly rich and near the lean limit, compares intake flow and fuel/air mixture effects on combustion processes.Color schlieren motion pictures reveal qualitative features of both the intake flow and combustion event. Image processing of selected frames from black and white schlieren motion pictures produce quantitative burn rate data. Early flame development patterns are markedly different for the different intake valve configurations. Stoichiometric flame kernel development is more than 1.6 times faster than the lean burn case for the four valve head, 1.3 times for the swirl port, and 1.8 for the straight port. Volume burn data show cycle to cycle variations near the lean limit is due to poor flame kernel development rather than poor flame propagation.