A single cylinder research engine has been used to study a range of gasoline combustion chamber designs which have been divided into three categories open, compact and four valve. Measurements of fuel consumption and exhaust emissions are presented at full load and part load conditions, and a comparison is made of the best chamber from each category. At full load the performances of the three chambers were very similar at low speeds but at high speeds the superiority of the four-valve was clearly shown. At part load there were significant differences in the burn rate when operating at lean mixtures with the compact design showing the highest level and hence the most extended lean limit. The relationship between part load fuel consumption and full load octane requirement is presented as a method of comparing the relative performance of a range of combustion chamber designs.