If I measure the airflow into an engine using a volume flow meter and want to know what the mass airflow is I think I need to use the following equation:

M = mPV / nRT

where:

M = mass flow (g/m)
V = volumetric flow (cm-cubed/min)
P = pressure (atm)
n = number of molecules (mole)
m = mass of 1 mole of air (g)
R = Universal gas constant
T = Temperature (K)

Which looks fairly straighforward, however the thing I'm not sure about is what the pressure will be at the flow meter.
On a naturally aspirated car will this stay close to atmospheric, or will it vary wildly with RPM/throttle/load/etc?

Thanks!

Not entirely sure, but NA cars usually have a volumetric efficiency of a bit less than 100%. I would say the pressure to use would therefore also be the same percentage less than atmospheric pressure.

It is the pressure and temperature at meter inlet need to know, a lot depends on how much pipe work precedes the meter.
Also you should be working in S.I. units. You can convert the mass flow rate back to Kg/s. at end of the calculation.


The calculation for the density of dry air very simple

Density = P/rT

r the specific gas constant and T is in Kelvin P is in Pascals

The specific gas constant R. is different from the universal gas constant. Engineers generally use the specific gas constant not the universal gas constant.

The specific gas constant for dry air is 287
As a sanity check air at STP has a density of about 1.2kg per cubic metre.

this might help