The importance of the phasing of the combustion event for internal-combustion engines is well appreciated, but quantitative details are sparse. The objective of the current work was to examine the optimum combustion phasing (based on maximum bmep) as functions of engine design and operating variables. A thermodynamic, engine cycle simulation was used to complete this assessment. As metrics for the combustion phasing, both the crank angle for 50% fuel mass burned (CA(50)) and the crank angle for peak pressure (CA(pp)) are reported as functions of the engine variables. In contrast to common statements in the literature, the optimum CA(50) and CA(pp) vary depending on the design and operating variables. Optimum, as used in this paper, refers to the combustion timing that provides the maximum bmep and brake thermal efficiency (MBT timing). For this work, the variables with the greatest influence on the optimum CA(50) and CA(pp) were the heat transfer level, the burn duration and the compression ratio. Other variables such as equivalence ratio, EGR level, engine speed and engine load had a much smaller impact on the optimum CA(50) and CA(pp). For the conventional engine, for the conditions examined, the optimum CA(50) varied between about 5 and 11 degrees aTDC, and the optimum CA(pp) varied between about 9 and 16 degrees aTDC. For a high efficiency engine (high dilution, high compression ratio), the optimum CA(50) was 2.5 degrees aTDC, and the optimum CA(pp) was 7.8 degrees aTDC. These more advanced values for the optimum CA(50) and CA(pp) for the high efficiency engine were largely due to lower heat losses. (C) 2013 Elsevier Ltd. All rights reserved.