Line flux ratio between HCO⁺ and HCN as a function of the infrared luminosity. The black squares are the individual values obtained for our sources (only sources with two > 3σ lines are shown) and the red star is the average value of our five objects. The gray diamond is the mean ratio found by Spilker et al. (2014) using a stacking of the cycle-0 SPT SMG spectra. The green triangles are the ratios of the 3-2 transitions in the local sample of Graciá-Carpio et al. (2008) and blue filled circles are the 4-3 ratios in the low-z sample of Zhang et al. (2014). The cyan pentagons represent the 3-2 ratios measured in local active nuclei by Imanishi et al. (2016). The purple crosses shows the 3-2 ratio measured in z ∼ 1.5 lensed star-forming galaxies by Oteo et al. (2016). The orange downwards-facing triangles are two H-ATLAS sources provided by Yang et al. (priv. comm.). The yellow plus is the 5-4 flux ratio found by García-Burillo et al. (2006) in the APM08279+5255 quasar (see also Riechers et al. 2010 for the 6-5 transition).