Pressure Ratio

The ratio of absolute outlet pressure to absolute inlet pressure across a compressor, used to place an operating point on the compressor map.

Pressure ratio (PR) is compressor outlet absolute pressure divided by inlet absolute pressure. At sea level, 14.7 psi boost is roughly a 2.0 PR (29.4 psia / 14.7 psia). It matters because turbo compressor maps are plotted as pressure ratio versus mass airflow, and your operating point determines compressor efficiency, surge margin, and outlet air temperature. A turbo pushed past its efficient island heats the charge dramatically and makes less reliable power, even if the boost gauge looks good. PR also explains why the same boost is harder to make at altitude: lower inlet pressure raises the required PR. HP Tuners does not show a PR channel directly, but you can compute it from logged MAP and barometric pressure in VCM Scanner using a math channel. Use it to confirm your turbo is sized correctly before chasing more boost, and to interpret rising intake air temperatures.