The effects of combustion chamber shape and spark location on exhaust NO and HC emissions were investigated using a single-cylinder, 4-stroke, reciprocating, spark ignition engine. A total of 21 spark locations were evaluated in three combustion chamber configurations. The chamber shapes included a modified wedge, a disc, and a modified hemisphere. The engine was operated at constant speed and airflow, with optimum spark timing and peak NO air-fuel ratio.Both combustion chamber shape and spark location had substantial effects on NO and HC emissions. Deductive prediction of these effects was not possible due to incomplete knowledge of the patterns of pollutant distribution and mixture movements within the combustion chamber.It is recommended that chamber shape and spark location effects be evaluated experimentally in any new engine design.