A transient 2-dimensional moving mesh CFD Computer model was created, validated against experimental data, and used to investigate the flow and resulting temperature fields in a rapid compression machine. The sensitivity of the horizontally opposed twin-piston RCM to nonsynchronized and non-uniform piston strokes was determined and the effect of non-uniform heating on resulting pressure profiles was investigated. Predictions of the ignition temperature in a rapid compression machine are made very difficult due to the existence of a highly non-uniform temperature field at the end of the compression stroke. An optimally designed piston head crevice, determined by a number of criteria, can largely overcome this problem by eliminating the mixing of the cool boundary layer gas with the hot compressed core gas. We used the CFD model to optimize the piston head crevices for our RCM and determined some new factors that are important when optimizing the piston head crevice design. Our best crevice design was then applied to a range of test gases and recommendations regarding the use of these as bath gases were made. © 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.