A transient, one-dimensional, two-phase (crankcase gases and liquids) flow network model was developed (and coded in FORTRAN) to calculate the crankcase pressures versus crank angle during engine operation and the consequent crankcase pumping mean effective pressure (CPMEP). The two-phase flow was represented by an empirical expression. Note, CPMEP is one of the components of engine FMEP (friction mean effective pressure) and is being introduced here as a new term. The model was calibrated with engine crankcase pressure measurements. The motivation for the present work was the fact that no commercial (or public domain) software is available to adequately address this subject in sufficient detail.The model also predicts that closing (i.e. sealing) the individual bays of an engine can result in (nearly) zero CPMEP. This was confirmed by motored single cylinder engine measurements. In practical terms however, the bays cannot be sealed completely since a minimum flow area is required to facilitate drain-back of the oil and removal of the blowby gases from each bay. Therefore, some CPMEP expenditure cannot be avoided. However, an engine with nearly closed bays could spawn other advantages such as improved cylinder block structure, reduced engine volume, lower hood line etc. Meanwhile, at present, the (nearly) closed bay concept may only have minimal value until methods to deal with blowby and oil drain back from closed bays are developed.