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# help understanding the density formula?

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Discussion and questions related to the course EFI Tuning Fundamentals

Why do we use the 2.7 constant in the air-density formula: D=2.7(P)/T?

This formula is derived from the ideal gas law: PV = nRT

Where:

P is the pressure of the gas,

V is the volume of the gas,

n is the amount of substance of gas (in moles),

R is the ideal, or universal, gas constant, equal to the product of the Boltzmann constant and the Avogadro constant,

T is the absolute temperature of the gas (Kelvin).

If we want the density we are looking for the mass (moles) per unit volume. In the case of the ideal gas law, grams per liter.

So the ideal gas law would look like this:

PV=nRT

becomes

P/RT = n/V

We now have pressure over temperature.

(1/R) * (P/T) = n/V

The 1/R term becomes 2.7 when the moles, grams and liters are solved for imperial units.

Please note that currently there is an error in the way the formula is represented in the module in the course. We will be fixing this very soon.

thank you guys.

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