Boost & Pressure Ratio

A boost gauge shows pressure above atmospheric; a compressor map is drawn in pressure ratio, the absolute outlet pressure divided by the absolute inlet pressure: PR = (boost + p_atm) ÷ p_atm. The same 100 kPa of boost is PR 1.99 at sea level and PR 2.25 at 80 kPa in the mountains — more work for the compressor and less air for the engine. p_atm is what you enter; the page has no altitude table.

Compressing air heats it. The ideal (isentropic) outlet temperature is T₂s = T₁ × PR^((γ−1)/γ) with γ = 1.4 for air and temperatures in kelvin; a real compressor at efficiency η reaches T₂ = T₁ + (T₂s − T₁) ÷ η, the efficiency read off the map island at your operating point. An intercooler with effectiveness ε cools the charge towards the ambient it uses: T₃ = T₂ − ε (T₂ − T_ambient). The density ratio PR × T₁ ÷ T₃ is the airflow gain relative to naturally aspirated at the same inlet — the number that actually feeds the engine.

Pressure drops through the intercooler and piping, heat soak, humidity, the compressor’s surge and choke lines, and engine-side effects (fuelling, knock, cam timing). The efficiency and effectiveness are yours from the map and the cooler’s data; nothing is assumed. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.