A new method for instantaneous friction estimation in firing internal combustion engines has been developed in the Powertrain Control Research Laboratory (PCRL) at the University of Wisconsin - Madison. This Synthetic Variable approach, which has previously been used for combustion quality diagnostics, focuses on carefully measuring instantaneous engine speed and other easily measurable engine variables and combining them with dynamic models of other engine processes. This approach numerically strips away the dynamic effects that mask friction effects on engine speed and reveals friction estimates with clarity. This information could be useful for engine designers and developers to assist in accurately understanding the sources of instantaneous friction within the running engine.The friction results from these studies have been very encouraging. Accuracy was found to be within 0.5% on average in the data sets where friction was based on measurable engine variables, with this decreasing to within about 5% when the friction applied was more arbitrary. The exception to these numbers occurs in the immediate vicinity of piston TDC, where errors in crankshaft acceleration estimation (due to the steep transients at combustion) carry through the model and degrade the friction estimation. A statistical analysis method that was used to determine the relative contribution to overall friction from individual components or locations within the engine is also presented.