Port fuel injection - PFI - engines are widely available and the calibrations of fuel models are well defined. Modern flex-fuel engines are designed to work with different fuels and a mix of them. However, Brazil’s ethanol is composed of anhydrous Ethanol mixed with 8% distilled water, making it distinct in relation of composition. In a PFI, the fuel mass is mixed to the air mass before entering the combustion chamber in order to keep the mixture most uniform as possible. In this process, heat exchange and partial pressure variation are present and this may lead to volumetric efficiency modification. However, measurement errors may be introduced from the indirect measurement method if it does not considers water content and combustion efficiency to determine volumetric efficiency. This paper has the objective to investigate the effects of hydrous and anhydrous ethanol on volumetric efficiency in a PFI spark ignition engine on low/medium loads where maximum break torque - MBT - spark advance is achievable. Indicated measurement and exhaust emission equipment were used to monitor and record combustion data in order to verify combustion efficiency, and an uncertainty analysis performed on the fuel flow meter to observe the water influence.