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air density

EFI Tuning Fundamentals

Relevant Module: Fundamental Engine Principles > Engine Principles > Air Density

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just clarifying I understand the calculation fully. D=2.7 x 14.7/519 = 0.076

so if I were to calculate the density for where I am currently with the temperature being 105F it would be D=2.7 x 14.7/565=0.070 with the constant of the equation being 2.7 and 14.7 correct?


The module mentions standard temperature and pressure, but when trying to figure out your current local air density, you'll want to use your local temperature in pressure.

The 2.7 value can be considered constant.

564.67 rounding to 565 is correct for your local temperature you stated, converted to Rankine.

14.7 psia may or may not represent your local pressure at the moment.

When I'm thinking about air density, it's usually in relation to the vehicle I'm working on that day, so I take a look at the barometric pressure reading in the engine computer to see what the approximate ambient pressure is, and then you can use that in your calculation in place of the 14.7.

If you don't have access to a baro pressure sensor reading from a car at the moment, it's good to be aware that in the USA and perhaps other areas, local air pressure on the news, weather reports, etc. is altitude compensated.

For example, I just checked the current air pressure in Denver, Colorado and the weather report states 30.16 in HG which converts to 14.8 psia, but Denver is at about 5280 feet altitude so I know they added about 5.2 in HG to that value, and the actual air pressure is about 12.3 psia.

To get accurate and up to date meteorological data, look up the METAR for your local airport, this information is updated hourly for most airports.

If you're serious about it, and especially if you're travelling to diffent locations for racing, you can also buy good quality "weather stations", normally used for personal/home use, that work well and you can check their calibration at an airport, or similar installation.

gentleman, thank you for your time. Your answers and combined knowledge opened my eyes to what I was overlooking.

You're welcome! Thanks gents.

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