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In this paper the results and main conclusions of a study concerning BGF at low loads, its relationship with valve timing and load, and its influence on engine instability and emissions, are presented. The study covered a wide range of load and valve timings for both intake and exhaust valves. Measured values of engine performance, emissions and instability were used. BGF was estimated by a global engine code based on a full wave action model. The good correlation found between calculated BGF with measured parameters proves the capability of wave action models to predict the influence of design modifications on engine performance at low loads. A very interesting relationship, found among BGF, volumetric efficiency, IVO and permeability is presented. As well, the relationship between BGF and manifold pressure, with engine instability and NOx emissions are discussed and a correction of the manifold pressure which describes very well both aspects of engine performance is presented.