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EFI Tuning Fundamentals

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Hi everyone, so how can I know at what air pressure my car will be operating at 12 meters above sea level for example? Is there any formula to calculate?

Some ECUs have a barometric sensor, so you can read the ambient air pressure directly. Others determine the air pressure from the Manifold Absolute Pressure (MAP) sensor, since it will read the ambient air pressure when the engine isn't running (so they remember this).

As for your specific question, yes there is a formula to calculate air pressure from altitude (and vis-versa). See this link for the details:


As the partial pressure of oxygen - basically how much oxygen is in each unit of air volume - is the main point of interest, temperature and humidity will also be factors.

You can buy 'home weather stations' which will give you all that information, plus wind speed - https://www.weatherstationadvisor.com/. The quality may vary a bit with price, but if you're near an airport, you should be able to calibrate the 'station against their rather more precise equipment for a small fee.

If you're just interested in the pressure, you can simply purchase a barometer and have it calibrated, or use a meter long, closed end tube and some mercury to make your own mercury barometer. I did one at school using this same basic method - https://www.youtube.com/watch?v=GgBE8_SyQCU&t=9s

You could do it with water, but it will be susceptible to the ambient temperature affecting the readings so need a correction factor, be susceptible to freezing in cold weather, and need a vertical tube ~10m long.

That said, if you're only looking at using the vehicle in the same area, and not significantly changing elevation, it isn't really that important as it is what it is. With forced induction, it's the manifold pressure the engine sees that's really the important bit for tuning.

thank u