The performance and emissions of gasoline engine using different research octane number (RON) gasolines (91, 93, 95 97, and 98 RON) at varying spark timing (ST) has been presented in this paper. For this work, a single cylinder, four stroke, naturally aspirated spark ignition engine requiring gasoline fuel with 95 RON was used. The original (ORG) ST of the engine is 23 degrees CA BTDC. The tests were conducted for three different STs (20 degrees CA BTDC, 23 degrees CA BTDC, and 26 degrees CA BTDC) by varying cam positions mechanically. Results showed that using RONs higher than the requirement of an engine not only decreased brake thermal efficiency (BTE) but also increased brake specific fuel consumption (BSFC), the emissions of carbon monoxide (CO) and hydrocarbon (HC) at ORG ST. On the other hand, with the increased ST (26 degrees CA BTDC); BSFC, the emissions of HC and CO decreased, and BTE boosted for higher RON. (C) 2012 Elsevier Ltd. All rights reserved.