Boost gauges read PSI in the US, bar everywhere else, kPa on factory dashboards, inHg for vacuum, atm in some engineering specs. They're all the same physics — just different units.
Conversions to remember
- 1 atm = 14.696 PSI = 1.013 bar = 101.3 kPa = 29.92 inHg (standard atmosphere at sea level)
- 1 bar = 14.504 PSI (close enough to 14.5 for shop math)
- 1 PSI = 6.895 kPa
- 1 inHg ≈ 0.491 PSI (vacuum gauges)
Engine pressure references
- Vacuum (idle, healthy engine): 18–22 inHg · ~9 PSI vacuum
- Oil pressure (warm idle): 20–40 PSI · 1.4–2.8 bar
- Oil pressure (under load): 50–80 PSI · 3.4–5.5 bar
- Fuel rail (port injection): 43.5 PSI · 3 bar
- Fuel rail (direct injection): 2,000–3,000 PSI · 138–207 bar
- Boost (mild street): 8–15 PSI · 0.55–1.0 bar
- Diesel rail (modern common rail): 28,000+ PSI · 1900+ bar