A number of proposals for fuel rating in spark ignition (SI) engine have been suggested till date and still no consensus on this has been reached. The automobile industry is continuing to use RON and MON tests for rating fuels, and still there exists a need to come up with new fuel rating systems. The fuel?s knocking tendency in SI engines is mainly governed by the end-gas autoignition. Another combustion mode, homogeneous charge compression ignition (HCCI), is also driven by autoignition of the complete charge inside the cylinder. Fundamentally, the combustion process in both combustion modes is driven by autoignition, and HCCI combustion mode can be used to understand the knocking behavior in SI engines. The lean combustion environment in HCCI mode provides a good platform to replicate the operating conditions of modern SI engines, which are operating at boosted pressures and low intake temperatures, and understanding of the fuel knocking behavior under such conditions is vital for achieving high-efficient engines. Therefore in this study, HCCI combustion will be used in the standard CFR engine to understand the autoignition behavior of the fuels for SI engines. For this purpose, three fuel blends were selected, which had the research octane number equal to 90. The standard CFR engine was operated with varying intake pressures and temperatures under HCCI combustion mode. The Lund-Chevron HCCI fuel number was used to rate the fuels and this was compared with RON and MON of the blends. It was found that HCCI combustion could be used to rate the fuels with the standard CFR engine with minor modifications to accommodate the boosted conditions without affecting the geometry and the flow inside the CFR engine. Low temperature reactions were observed and were correlated with the Lund-Chevron HCCI fuel numbers.