In light- and medium-duty diesel engines, piston bowl shape influences thermal efficiency, either due to changes in wall heat loss or to changes in the heat release rate. The relative contributions of these two factors are not clearly described in the literature. In this work, two production piston bowls are adapted for use in a single cylinder research engine: a conventional, re-entrant piston, and a stepped-lip piston. An injection timing sweep is performed at constant load with each piston, and heat release analyses provide information about thermal efficiency, wall heat loss, and the degree of constant volume combustion. Zero-dimensional thermodynamic simulations provide further insight and support for the experimental results. The effect of bowl geometry on wall heat loss depends on injection timing, but changes in wall heat loss cannot explain changes in efficiency. Late cycle heat release is faster with the stepped-lip bowl than with the conventional re-entrant bowl, which leads to a higher degree of constant volume combustion and therefore higher thermal efficiency. This effect also depends on injection timing. In general, increasing the degree of constant volume combustion is significantly more effective at improving thermal efficiency than decreasing wall heat loss. Maximizing thermal efficiency will require a deeper understanding of how bowl geometry impacts late-cycle flow structure, turbulent mixing, and combustion.