Not sure if it applies to the mechanical injection systems but on the EFI systems if you know your fuel pressure at the injector, and jet area you can work out changes with simple math.
(actually you need to work with the pressure drop across the injector nozzles so it is fuel rail pressure gauge - manifold pressure gauge to find the effective pressure across the nozzle.)
If you increase boost and the fuel supply pressure stays the same, it will lean you out.
Flow changes directly proportional to the jet area just like a carburetor and as the square root of the change in fuel pressure across the orifice.
If you go from 8 psig manifold pressure to 12 psig manifold pressure your fuel flow with identical nozzles and fuel supply pressure should change by the square root of (fuel line pressure - 8/ fuel line pressure -12). This assumes there is no boost referenced fuel pressure regulator. If your line pressure goes up in lock step with your boost then you can ignore the manifold pressure for this calc.
A typical fuel line pressure in EFI is 43 psi, so the effective fuel pressure across the nozzle at 8 psi boost would be 35 psi , and at 12 psi 31 psi. your flow should vary as the square root of 31/35 = 0.94 or it would lean you out by 6%. (common EFI systems on production cars use boost referenced fuel pressure regulators so at 20 psi boost the actual line pressure would be 63 psi and the pressure drop across the injector would in theory stay at 43 psi.)
How stable is the fuel pressure on those systems?
Does it change throughout the engines rev range or do you have a regulator to hold a fixed base supply pressure.
(never really had any exposure to them so the EFI method may not be at all appropriate)
From looking at schematics of the mechanical injector systems it looks like it is setup as a positive displacement pump feeding in effect 2 orfices, the nozzle to the intake manifold which will be influenced by the manifold pressure, and the bypass pill that will bypass all the additional fuel. As I see it, fuel supply pressure should rise to what ever pressure is necessary to move the fuel displaced by the pump through both nozzles (injector and bypass).
Increased boost should cut flow on the manifold injector and that should cause a matching increase in line pressure until the bypass pill can flow the equivalent amount of fuel as the drop in flow through the injector.
It looks like you could look at it as a parallel resistor problem in electronics. (Current in must equal current out --- pump flow must equal injector flow plus bypass flow). Any change in one orifice (injector nozzle or bypass pill) would create an equal and opposite change in flow at the other.
Over very small changes in mixture, increased flow in the bypass would create an equal and opposite change in flow at the injector (over small size changes the system pressure should stay very nearly the same.)
So following that logic, if you wanted to go from a 9.45:1 AFR to a 12:1 AFR you would want to flow 78% of your original flow through the injector, that would need an increased flow on the bypass pill to 128% of its previous flow,(assuming fuel pressure changes were very small). Then you would have to compensate for changes in boost pressure. That would imply an increase in bypass pill orifice area of 28% would be about right if you stayed at 8 psi boost.
Being Conservative, take only about 1/2 or 1/3 the expected change, and see what your mixture does. --- rinse and repeat as necessary.
(this is just a mental learning exercise for me, so looking for the guys with hands on experience to tell me what I forgot or got stupid wrong
)
Is this a reasonable approach?
Larry
