The details of a model which predicts friction mean effective pressure (fmep) for spark-ignition engines are described. The model, which was based on a combination of fundamental scaling laws and empirical results, includes predictions of rubbing losses from the crankshaft, reciprocating, and valvetrain components, auxiliary losses from engine accessories, and pumping losses from the intake and exhaust systems. For some predictions, it was possible to derive terms which were proportional to fmep based on lubrication theory. For other predictions, phenomenological terms which described the results of the processes rather than the processes themselves were used. Each of the predictions was “calibrated” using fmep data from published sources. The sum of these predictions gave reliable estimates of spark-ignition engine fmep and serves as a useful tool for understanding how the major engine design and operating variables affect individual component friction.