Combustion in modern spark-ignition (SI) engines is increasingly knock-limited with the wide adoption of downsizing and turbocharging technologies. Fuel autoignition conditions are different in these engines compared to the standard Research Octane Number (RON) and Motor Octane Numbers (MON) tests. The Octane Index, OI = RON - K(RON-MON), has been proposed as a means to characterize the actual fuel anti-knock performance in modern engines. The K-factor, by definition equal to 0 and 1 for the RON and MON tests respectively, is intended to characterize the deviation of modern engine operation from these standard octane tests. Accurate knowledge of K is of central importance to the OI model; however, a single method for determining K has not been well accepted in the literature.This paper first examines four different methods for determining K, using literature results from a modern SI engine operating with direct injection (DI), port fuel injection (PFI) and homogeneous, upstream fuel injection (UFI). The test fuels were ethanol-gasoline blends spanning a wide range of RON and MON, together with isooctane as a reference. The quality of the K results from some of these methods is particularly dependent on the design of the test fuel matrix, with unreliable K values resulting in some cases.One of the more reliable methods is then used to examine how K varies with the intake pressure, fueling strategy, engine speed and compression ratio, with throttled conditions considered in detail. Several of the observed trends are consistent with prior studies, including K being consistently negative at higher loads for DI. In contrast to other studies, however, K is also observed to approach 0.5 at part load, throttled conditions, irrespective of whether the engine is fuelled by DI, PFI or UFI. Preliminary analysis of the autoignition chemistry for different fuelling methods then suggests plausible reasons for these results.