The drop in wall heat losses with increasing engine size is shown not to be due to the decreasing surface-volume ratio, because this influence is compensated by the engine speed, which also drops. Instead, the heat-transfer coefficient decreases as the cylinder bore becomes larger. The dependence of the heat-transfer coefficient on engine size is explained using Reynolds analogy. This dependence can be used to determine approximately changes in wall heat losses resulting from differences in engine size and the subsequent effects on indicated efficiency. It is found that only a minor part of the differences in efficiency between small and large engines is due to size-dependent changes in wall heat losses. © 1991, Institution of Mechanical Engineers. All rights reserved.