Left: Ratio of HCN(1–0) and FIR luminosities (a proxy for star-forming efficiency) as a function of star-formation rate. The dotted lines indicate the mean and 1σ scatter for local galaxies from the EMPIRE sample (Jiménez-Donaire et al. 2019). We infer HCN(1–0) luminosities for our DSFGs using the following excitation coefficients: r₃₁ = 0.41 and r₄₁ = 0.59. Right: Ratio of HCN(1–0) and CO(1–0) luminosities (a proxy for the dense-gas fraction). The inferred HCN/CO ratios span ≈1 dex, suggesting that dense-gas fraction in high-z DSFGs vary significantly source-to-source.