The math behind this is as follows (for those who want to know)...
Definitions
R0 = injector's rated flowrate measured at pressure P0
P0 = pressure at which the injector's rated flowrate R0 was measured (see
Note below)
FP = regulator's
gauge pressure, also known as regulator's base pressure.
MAP = manifold
absolute pressure
BARO = barometric
absolute pressure = 101.3 kPa at sea level = 100 kPa approx. a little above sea level
MANVAC = manifold vacuum = BARO - MAP
gauge pressure =
absolute pressure - BARO = value shown by pressure gauge
absolute pressure =
gauge pressure + BARO
Pressure
difference across an injector: this is the difference in the
absolute pressures above and below the injector.
An unreferenced regulator has its pressure set constant; this pressure is FP (
gauge) or FP + BARO (
absolute).
A MAP-referenced regulator has a reference hose connected to the intake manifold,
so its pressure has MAP added to it; this pressure is FP + MAP (
gauge) or FP + BARO + MAP (
absolute);
with reference hose removed, this pressure becomes FP + BARO (
guage) or FP + BARO + BARO (
absolute).
The flowrate of an injector is directly proportional to the squareroot of the pressure difference across the injector (this is the Bernoulli pressure relationship).
Note on Injector Flowrate Measurement:
when
R0 is measured on an injector flow bench,
P0 is actually the pressure
difference across the injector:
absolute pressure above injector = P0 + BARO; absolute pressure below injector = BARO; so then difference =
P0.
Note regarding 1997-1998 Y-car:
The FPR is not MAP-referenced (follow the reference hose to verify it sees atmosphere/BARO).
Unreferenced Regulator (e.g. 1998-2002 F-car, 1997+ Y-car)
Pressure difference (of
absolute pressures) across injector = (FP + BARO) - (MAP) = FP + BARO - MAP =
FP + MANVAC
IFR = R0 * sqrt((
FP + MANVAC) / P0
) = varies/slopes with MANVAC
At the rail,
absolute pressure is FP + BARO, so
gauge pressure is
FP.
--> Directly measure
FP at rail with engine running (
FP should be constant).
The spreadsheet calculates this equation (Example 1, post #3).
MAP-Referenced Regulator (e.g. trucks)
Pressure difference (of
absolute pressures) across injector = (FP + BARO + MAP) - (MAP) =
FP + BARO
IFR = R0 *
sqrt((
FP + BARO) / P0
) = constant/flat regardless of MANVAC
At the rail,
absolute pressure is FP + BARO + MAP, so
gauge pressure is FP + MAP.
Remove reference hose, regulator is now exposed to barometric pressure (so MAP = BARO),
and rail
absolute pressure is FP + BARO + BARO, so
gauge pressure is
FP + BARO.
--> With reference hose removed, directly measure
FP + BARO at rail with engine running.
The spreadsheet calculates this equation at MANVAC=0 if measured FP + BARO is entered (Example 2, post #3).
Units
Rail pressure is measured in psi, and MAP or MANVAC is measured in kPa.
You need to convert one to the units of the other before calculating IFR.
Convert psi to kPa by multiplying by
(100/14.5).
14.5 psi = 100 kPa
If injectors are rated in lb/hr:
convert lb/hr to g/s by multiplying by 0.125998
(can round to 0.1260).
1 lb/hr = 0.1260 g/s
If injectors are rated in cc/min:
convert cc/min to g/min by first multiplying by density of gasoline
(typically 0.73 g/cc), and then dividing by 60.
1 cc/min = 0.0122 g/s (depending on density of gasoline)