In this work, an experimental study on the performance and exhaust emissions of a commercial hydrogen fueled spark ignition engine (HFSIE) was performed at partially and full wide open throttle (50% and 100% WOT) positions. The engine is a four-stroke cycle six-cylinder, engine volume of 4.9 L, port fuel injection, hydrogen fueled SI engine with a bore of 102.1 mm, a stroke of 101.1 mm and a compression ratio of 13.5:1. The experiments were performed using 3 different spark plug gaps (SPG) (0.4, 0.6 and 0.8 mm), varied engine speeds of 1000-3000 rpm and two ignition timing values (10 and 15 degrees CA BTDC) at 50% and 100% wide open throttle (WOT). SPG is a factor affecting the performance of the engine depending on the engine structure. Maximum power values were obtained at 0.6 mm SPG for both 50% and 100% WOT at ignition timing values of 10 and 15 degrees CA BTDC. The maximum efficiency values were obtained with a 0.8 mm SPG at 50% WOT. At 100% WOT position, the maximum efficiency values were obtained with a 0.6 mm spark plug gap (SPG) at ignition timing values of 10 and 15 degrees CA BTDC. A significant decrease in NO emission was observed using hydrogen for all WOT and SPGs. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.