This research aims to experimentally analyze and evaluate the influence of varying the spark timing on the performance and emission characteristics of a single cylinder, four-stroke spark ignition (SI) engine fueled by two grades of gasoline used in Saudi Arabia, RON91 and RON95. The experiments were conducted at stoichiometric conditions for five different spark timings (18 degrees CA BTDC, 20 degrees CA BTDC, 22 degrees CA BTDC, 24 degrees CA BTDC, 26 degrees CA BTDC and 28 degrees CA BTDC) at fixed compression ratio 9.5: 1, wide throttle open (WOT) and engine speed 2500 RPM. The spark timing is controlled electronically using an engine control unit (ECU). The experimental results showed that as the spark timing increases, the brake power (bp) and the brake thermal efficiency (BTE) increase for both fuels. It is observed that bp and BTE of RON95 is higher than that of RON91. The specific fuel consumption (BSFC) decreases with increasing in spark timing for both fuels. However, RON95 shows lower BSFC than RON91. Combustion analysis showed a mixed response to different research octane number (RON). Generally, combustion of RON95 displayed higher peak rate of heat release (ROHR) at earlier occurrence compared to RON91. It is also found that the coefficient of variation of the indicated mean effective pressure (COVIMEP) decreased with increases the spark timing for both fuels. However, RON95 displayed lower COVIMEP than RON91. It is observed that NOx and CO2 emissions increased for both fuels as the spark timings increases. It is also detected that THC and CO emissions from both fuels decreases as spark timing increases. (C) 2016 Elsevier Ltd. All rights reserved.