Autoignition in modern spark ignition (SI) engines occurs at different conditions to those in the standardized Research (RON) and Motor (MON) Octane Number tests. The Octane Index (OI) has been proposed to account for these differences, with OI=RON–K(RON-MON) related to the occurrence of knock in the modern engine by several methods. The so-called K value then quantifies the deviation of the modern engine operation from the standard RON and MON tests. This paper presents a comparison of different methods for calculating the K value for the same modern engine operating with direct injection (DI), port fuel injection (PFI) and homogeneous, upstream fuel injection (UFI). The test fuels used span a wide range of RONs and fuel sensitivities (S=RON-MON). The quality of the results obtained using some of these methods were 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 fuelling approach, the intake pressure, the engine speed and the compression ratio. Several of the observed trends are consistent with prior studies, including K being consistently negative at higher loads for DI. Importantly, however, K is also observed to be roughly 0.5 at low load, throttled conditions for all three types of engine fuelling. Given the importance of operating modern engines efficiently at all conditions, the Motor Octane Number (MON) therefore still appears to play an important role in fuel quality standards.