With the publication of NFPA 68 (2013); a major change is in progress in venting area calculation methods for gas explosions. Old methods referring to the Kg parameter proved to be inappropriate for real applications. The present work provides new data to correlate real gas concentration and initial turbulence conditions to flame propagation and explosion overpressure during a vented gas explosion. Explosion tests were performed in a 4 m3 rectangular chamber equipped with transparent walls and vented (0.49 m2 square vent) on one side. The chamber is filled with a turbulent or quiescent hydrogen-air mixture with a purposely built injection system that allows to vary the turbulence intensity and the length scale. Gas concentration and turbulence parameters are measured with concentration gauges and Pitot probes distributed in the chamber (Duclos, 2017). Then the flame propagation is fully characterized with high speed video and explosion overpressure is measured inside and outside the chamber. The paper presents the parametric study performed by varying the initial turbulence and focuses on its influence on the inside explosion overpressure. Then physics of vented gas explosion is discussed, results are compared to developing phenomenological model.