EFI

[Alexander M. Lichstein]

Roughly speaking, you must know VE at all engine speeds.  Let's assume 100% VE
(that is, that every cylinder is able to completely fill itself with air
attaining equilibrium with the air outside the valve.)  This means that by
PV=nRT we can determine that  (for example),  with 150cc cylinders, and a wide
open throttle with perfect manifold design, we will have:

P = 1atm
V = 150cc
R = constant
T = measured (293K)

then we can get n (moles or # of molecules) which with a bit of plugging will
tell us mass.  In reality, the VE is not constant (f(RPM)) and is not 100%.  In
that case, we can adjust our calculation on the left hand side since the air
will not necessarily be at the same pressure as that outside the port. 

If you close the throttle, then the pressure in the manifold will drop
accordingly.  We plug this new value for pressure at the ports into the above
equation (adjusted for VE) and voila... air mass.

I haven't done the conversions above and I'm mixing my units, but you get the
idea.

- Z