Boost Pressure Measurements
The British measured boost pressure as lbs./sq.inch (or psi). The normal atmospheric pressure at sea level is 14.6959 psi, although this can vary from day to day: a reading of +6 meant that the air/fuel mix was being compressed by a supercharger blower to 20.7 (rounded figure) psi before entering the engine; +25 meant that the air/fuel mix was being compressed to 39.7 psi. However, there was a problem with the British system of measuring boost, in that in an aircraft the pressure gauges should measure absolute pressure within the engine's supercharger, rather than showing atmospheric pressure at sea level, plus the supercharger's pressure; at sea level this was a reasonable measure but, in engines that were used through different altitudes this method becomes completely arbitrary. Because the Americans measured their boost ratings using inches of Mercury (" Hg), their boost gauges more accurately recorded the absolute pressures being generated by the superchargers at all altitudes.
| Inches of Mercury (" Hg) | Pounds of Boost |
| 80.9" of mercury= | +25 lb boost |
| 66.6" of mercury= | +18 lb boost |
| 60.5" of mercury= | +15 lb boost |
| 48.3" of mercury= | +9 lb boost |
| 42.2" of mercury= | +6 lb boost |
Read more about this topic: Supermarine Spitfire Variants: Specifications, Performance And Armament, The Rolls-Royce Merlin and Griffon Engines
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