High frequency variations in crankcase pressure have been observed in Inline-four cylinder (I4) engines and an understanding of the causes, frequency and magnitude of these variations is helpful in the design and effective operation of various engine systems. This paper shows through a review and explanation of the physics related to engine operation followed by comparison to measured vehicle data, the relationship between crankcase volume throughout the engine cycle and the observed pressure fluctuations. It is demonstrated that for a known or proposed engine design, through knowledge of the key engine design parameters, the frequency and amplitude of the cyclic variation in crankcase pressure can be predicted and thus utilized in the design of other engine systems. In addition to the prediction of pressure fluctuations for I4 engines this paper discusses the variation of crankcase pressure of various other engine architectures and the observations made from I4, V6 and V8 engine crankcase pressure measurements at various engine speeds. The sources of discrepancies between predicted variation and measurements are discussed and additional noise factors in these observations that should be explored with further modelling are identified.