Injector flow rate table - clarification sought

[jfpilla]

XLR8NSS, you know Flashscan allows for selection of units by table?
Joe

[XLR8NSS]

XLR8NSS, you know Flashscan allows for selection of units by table?
Joe

Are you referring to the conversion page I posted?

I just put that up there for John.

[joecar]

The IFR table tells the PCM what flow capability the injectors have;

So, a lower injector flow value in the table means this:
to get a given amount of fuel to spray, the PCM has to keep the injector open for a longer time (flow rate (g/s) x time (s) = mass (g)).

And, a higher injector flow rate value in the table means this:
the PCM keeps the injector open for a shorter time.

So, in other words we are "tricking" the PCM into thinking that it needs to open injectors for longer or shorter times.

Actual injector flow rate depends on fuel rail pressure minus manifold absolute pressure.

Edited...

He he he, clear as mud, eh....
:lol:

[joecar]

GMPX,

EDIT: SEE MY LATER POST FOR A CORRECTIONS TO THIS POST
(fuel pressure is referenced to atmospheric, and I did not account for this here),
(idle vacuum is approx 10"Hg vacuum which corresponds to approx 10 PSI MAP, not 5 PSI).

It seems that the MAP pressures should be subtracted from the fuel rail pressures;

e.g. (edited a little since I posted this)

(assuming idle vacuum is 10"Hg, and 1 atm is 30" Hg)


In Imperial (using approximation 1 atmosphere = 15 PSI):
60 PSI minus 15 PSI at WOT, and
60 PSI minus 5 PSI at idle.

At WOT, the injector has 45 PSI difference,
At idle the injector has 55 PSI difference.


In Metric (using approximation 1 atmosphere = 100 kPa):
400 kPa minus 100kPa at WOT, and
400 kPa minus 33 kPa at idle.

At WOT, the injector has 300 kPa difference,
At idle the injector has 367 kPa difference.


The flow rate of the injector is proportional to the pressure difference.

i.e.
Low vacuum (high MAP (WOT)) impedes injector flow rate.
High vacuum (low MAP (idle)) assists injector flow rate.


[Actually, 1 atmosphere at sea level = 14.5 PSI = 101.4 kPa = 29"Hg, or something, but the approximations I used make for easier mental maths].

[2000ssnb]

joecar,

that's what I thought - there is more flow rate at idle than there is at
WOT -> of course this is compensated for by the pulse width actually
applied to deliver the right fuel mass into the air mass entering the cyl.

Gert

[joecar]

That's right, and the IFR table tells the PCM the actual injector flow rate capability over the MAP range (so the PCM can meter out the correct mass of fuel).

[joecar]

I made a slight mistake....

In Imperial (using approximation 1 atmosphere = 15 PSI):
60 PSI minus 15 PSI at WOT, and
60 PSI minus 5 PSI at idle.
At WOT the injector has 45 PSI difference,
At idle the injector has 55 PSI difference.

In Metric (using approximation 1 atmosphere = 100 kPa):
400 kPa minus 100kPa at WOT, and
400 kPa minus 33 kPa at idle.
At WOT the injector has 300 kPa difference,
At idle the injector has 367 kPa difference.

I just realized that fuel rail pressure is referenced to atmospheric and is not absolute;
to get absolute pressure, you must add atmospheric;
so to the above examples would be...

(and, correcting my other mistake, an idle vacuum of 10"Hg is equal to a MAP of 10 PSI or 67 kPa).

In Imperial (using approximation 1 atmosphere = 15 PSI):
60 PSI + 15 PSI - 15 PSI at WOT equals 60 PSI injector difference,
60 PSI + 15 PSI - 10 PSI at idle equals 65 PSI injector difference.

In Metric (using approximation 1 atmosphere = 100 kPa):
400 kPa + 100kPa - 100 kPa at WOT equals 400 kPa injector difference,
400 kPa + 100kPa - 67 kPa at idle equals 433 kPa injector difference.

Note: atmospheric minus MAP is the vacuum (as read by a vac guage).

[joecar]

Wait, let's tidy this junk up a bit...

Let:
    FAP   = fuel rail absolute pressure
    FP    = fuel rail pressure
    ATM   = atmospheric [absolute] pressure
    MAP   = manifold absolute pressure
    delta = pressure difference across injector

Where:
    FP  = 60 psi or 400 kPa (constant without MAP feedback)
    ATM = 15 psi or 100 kPa (approximation for easy numbers)

Now:
     FAP = FP + ATM

   delta = FAP - MAP
         = FP + ATM - MAP

Note: ATM - MAP equals vacuum (which is referenced from ATM pressure)

In psi:
   delta = 60 + 15 - MAP
         = 75 - MAP

In kPa:
   delta = 400 + 100 - MAP
         = 500 - MAP

Tabulated in psi:
   MAP   delta
   psi    psi
     0     75         
     5     70     
    10     65  
    15     60  (WOT)

Tabulated in kPa:
   MAP   delta
   kPa    kPa
     0    500         
    33    467     
    67    433
   100    400  (WOT)

[joecar]

Now if the fuel pressure regulator has MAP feedback...

Let:
   FAP   = Fuel rail absolute pressure
   SP    = Regulator spring pressure (constant over a small distance)
   MAP   = Manifold absoulte pressure
   delta = Pressure difference across injector

The pressure regulator has FAP on one side, and MAP and SP on the other, 
so we can write (keeping all pressures absolute):
   FAP = MAP + SP

Then:
   delta = FAP - MAP 
         = MAP + SP - MAP 
         = SP
         = constant

This means a fuel pressure regulator using MAP feedback will keep the
pressure difference across the injectors constant for all MAP values.

[LS1 Cobra]

Here's my best attempt to explain it from what I understand of it.

Because the pressue reg is referenced to manifold vac this is what happens.
Lets say you have a fuel rail pressure of 350kPa.
Manifold pressure is 70kPa, the injector pressure will be 420kPa.
OR
Lets say you have a fuel rail pressure of 350kPa.
Manifold pressure is 50kPa, the injector pressure will be 400kPa.

So therefore the table compensates for less pressure at the injector due to manifold vac.

Note: Return systems do not need this table.

Cheers,
Ross
P.S - I am pretty sure it is a 0-80kPa table.

Sorry if I'm asking stupid questions, I'm new to this and still learning.

Why do return systems not need this table? The return system will likely provide a more consisten source of fuel pressure but manifold vacume is still going to have the same effect on the pressure diferential as a non return system.

I've machined my own rails and plumbed in an externally adjustable regulator with a full return system. The main line is 1/2" and the return is a 3/8" line running via a SARD regulartor. The regulator has provision for connecting to manifold vacume but I haven't connected it under advice from a couple of tuners.

after reading this thread and thinking about it I would rather trust the ECU monitoring manifold vacume via the MAP than the vagaries of a mechanical spring and diaphram in the regulator. It's relying on the regulator being linear and able to respond quick enough to a rapid change in pressure as you get on and off the throttle. The ECU should be able to modulate the injector pulse widths almost instantly shouldn't it?

I thought the main reason for the manifold port on the reg was to give the injectors a bit more pressure to overcome boost pressure in forced induction applications.