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# Absolute vs Gauge Pressure

### Tech Articles

Discussion and questions related to the course EFI Tuning Fundamentals

Hi, so gauge pressure reads how higher or lower the measured pressure is compared to atmospheric pressure, but the absolute will be always the same number under standard conditions? 14,7 psi. I didn´t understand that well can someone explain me, please?

So to put it in simple term, gauge pressure is compensated for standard conditions. It's basically just an offset number. 0 PSIa (absolute) would be a perfect vacuum, while 0 PSIg (gauge) would be standard conditions. Boost gauges usually reads PSIg as the name implies.

The formula is PSIg = (PSIa - 14.7)

Example :

0 PSIg = 14.7 PSIa

10 PSIg = 24.7 PSIa

20 PSIg = 34.7 PSIa

And so on.

It is important to realise that "14.7" PSI correction ONLY applies under standard conditions, ie. ~sea level.

As one increases altitude* it will reduce, ie. at 10,000 feet, the atmospheric pressure is 10.2 PSI, absolute. This means the correction examples above would be 10.2, 20.2 and 30.2 PSIa, respectively, and are what the engine will actually be experiencing.

*You may find this correction chart of use, for your altitude(s).https://www.mide.com/air-pressure-at-altitude-calculator

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