I will present results from a comprehensive and systematic study of the fraction of dense gas and its star formation efficiency as traced by several high critical density molecular lines. The results are based on the IRAM-30m large program EMPIRE, a ~500h survey that provides full maps of high critical density tracers like HCN or HCO+ across the entire star- forming disks of 9 nearby disk galaxies. My thesis work addresses one of the key goals of the survey: understanding how dense gas fractions and star formation efficiencies vary across and among galaxies. In particular, we find the star formation efficiency in the dense gas to vary systematically, at odds with a whole class of models based on Milky Way work. Another particular focus is to relate the fraction of star-forming gas and its ability to form stars to local ISM and dynamical conditions by studying their dependence with local conditions such as stellar surface density or ISM pressure. I furthermore present results of analyzing optically thin isotopologues (e.g. H13CN) and high signal-to-noise 13CO and C18O in EMPIRE and ancillary ALMA data, to constrain optical depths of high critical density tracers and abundance variations in our disk galaxies. Both quantities are crucial to understand the ISM density distribution.