Boost pressure is the amount of air pressure above atmospheric pressure that a turbocharger or supercharger forces into your engine. Atmospheric pressure at sea level is approximately 14.7 PSI (1.013 BAR). Absolute pressure is what the turbo or blower is actually pushing — boost is the difference between absolute pressure and atmospheric pressure. More boost means more air, which allows more fuel to be burned and more power made.
Enter the atmospheric pressure (14.7 PSI at sea level — adjust for altitude if needed) and the absolute manifold pressure your setup is running. The calculator returns boost in both PSI and BAR. If you are reading a boost gauge, note that most boost gauges already read in gauge pressure (above atmospheric), so you can use that number directly without this calculator.
More boost increases cylinder pressure, which also affects your effective compression ratio.
Most stock small engines with cast internals can handle only 3-5 PSI safely. Beyond that, the risk of detonation, ring failure, and bearing damage increases rapidly without forged pistons, lower compression, and proper fuel tuning.
Gauge pressure is measured relative to atmosphere (what a boost gauge reads). Absolute pressure includes atmospheric pressure on top. At 7 PSI boost, absolute pressure is 7 + 14.7 = 21.7 PSI absolute.
Yes. At higher altitudes, atmospheric pressure is lower, so the same absolute pressure produces less boost over atmosphere. Adjust the atmospheric pressure field for your elevation.
As a rough estimate, each additional PSI of boost adds about 3-4% more power on a naturally aspirated engine. A 5 HP engine at 7 PSI boost could theoretically make around 7-8 HP, depending on fueling and tuning.
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