Abstract Using the geometric compression ratio, calculated without considering crevices, usually ensures good accuracy when performing heat release analysis and other evaluations based on recorded in-cylinder pressure traces. This is due to the fact that top-land, head gasket, spark plug thread and other crevices amount to a small percentage of the clearance volume in production engines. Particular cases such as engines with optical access can however feature special geometry with relatively high crevices volume. Increased blow-by rates also represent an issue that can have a significant impact on first-law analysis performed on such engines. Special requirements for the optical access sometimes results in the separation of the lower crankcase and the cylinder, thus making measurements of gas leakage extremely difficult. Therefore, an existing methodology for estimating compression ratio based on the motored in-cylinder pressure trace was improved in order to ensure good accuracy with reduced computational effort, and a new procedure was developed for modeling blow-by rates.