Injector flow rate table - clarification sought

[2000ssnb]

Just wanted to be sure about the injector flow rate table.

It shows injector flow (lbs/h) as a function of manifold vacuum (inHg)
As the vacuum increases the flow inceases because the total pressure
differential across the injector increases (58psi constant from the pump).
It seems that the total injector flow rate is the highest when at idle (23.5inHg) and the lowest at WOT (0 inHg), exactly the reverse of what I would like to have. Also, is 23.5 inHg the max vacuum pressure at closed throttle - that's what it looks like?
You all agree?

Gert

[jfpilla]

Just wanted to be sure about the injector flow rate table.

It shows injector flow (lbs/h) as a function of manifold vacuum (inHg)
As the vacuum increases the flow inceases because the total pressure
differential across the injector increases (58psi constant from the pump).
It seems that the total injector flow rate is the highest when at idle (23.5inHg) and the lowest at WOT (0 inHg), exactly the reverse of what I would like to have. Also, is 23.5 inHg the max vacuum pressure at closed throttle - that's what it looks like?
You all agree?

Gert


Isn't it because lower values increase flow rate?


Hope you don't mind if I ask a question on your thread?

The IFR tables I've see scale from 20 to 100 kPa vs g/s. Why 0 to 80 in EFI?
Another one I don't get. Why kPa or PSI, aren't the Injectors contolled by MAF?

[John Skiba]

At idle as you stated the vacuum is at it's highest. Under vacuum an injector delivers little bit more in the same time period (ms). On the flip side, under boost it's harder for the injector to deliver the same amount of volume in the given time period, thus you increase fuel pressure or compensate other ways.

I'm pretty sure that's it. ;) If not someone quickly correct me.

[XLR8NSS]

Lower numerical IFR values INCREASE flow rate. That is why at 0 inHg the values are the lowest in the IFR table. :)

**edit** John boy beat me to it. :D

[GMPX]

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.

[John Skiba]

corrected...

[jfpilla]

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.

If I get what you're saying, 100 kPa is 0 vacuum and therefore 0-80 would be correct.
So, would what XLR8NSS and I said about IFR values at higher kPa be right, because you need less inj. flow at higher vacuum?

[John Skiba]

100kpa would be 29.53 inHg

inHg = 0.295300 x kPa

80kPa is 23.624 inHg

errr... atmosphere

[XLR8NSS]

For anyone wanting to convert KPa to InHg go here. ;)

http://www.onlineconversion.com/pressure.htm

GMPX - The pressure regulator in Fbodys is not manifold referenced. If it was the IFR table would be one number all the way across. :)


Because the pressue reg is referenced to manifold vac

[XLR8NSS]

Just wanted to be sure about the injector flow rate table.

It shows injector flow (lbs/h) as a function of manifold vacuum (inHg)
As the vacuum increases the flow inceases because the total pressure
differential across the injector increases (58psi constant from the pump).
It seems that the total injector flow rate is the highest when at idle (23.5inHg) and the lowest at WOT (0 inHg),

Correct.


exactly the reverse of what I would like to have.

WRONG. Remember that as the numerical value of the IFR decreases the actual flow INCREASES. You want lower IFR values as you get closer to 0 vacuum in the intake manifold.

I think you have it all worked out except the fact that lower IFR values mean more flow and higher IFR values mean less flow. If you look at the tables with that in mind they make perfect since. :)

[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.

[joecar]

Michael,

If you connect the vacuum line to the regulator (MAP referencing),
then the regulator will vary fuel pressure as the vacuum changes such
that there'll always be a constant pressure difference across the injectors
(i.e. the injectors will have constant flow rate regardless of MAP value).
[MAP referencing cancels out any MAP influence on the pressure difference across an injector].

In this case, the IFR table must be flat instead of sloped
(you don't want the PCM to adjust the flow rate in this case).
:!:

Otherwise, if the regulator is not MAP referenced,
then the PCM uses the sloped IFR table to allow the correct
mass of fuel to be delivered under all vacuum/MAP conditions.

The IFR table tells the PCM how the injectors flow under the various vacuum conditions
(since the injector flow rate varies with the pressure difference across the injector,
and this pressure difference varies with vacuum/MAP).

Cheers,
Joe
:)

[joecar]

Hmmm...
It seems that the IFR chart has positve slope with increasing MAP
(rather than negative slope since at higher MAP the injector flow rate is less)...
:?: :? :?: :?

[joecar]

The horizontal axis goes upto 80kPa only....
The slope is positive...

:idea: This leads me to think that the horizontal axis is not MAP but rather BARO - MAP (where BARO is 1 ATM).

:?: Otherwise how do you interpret the positive slope...?

:?

[dfe1]

Before you can fully understand injector flow rate, you have to be certain whether you're referencing it to MAP or manifold vacuum. Since the calibrations wthin the PCM are all MAP-based, I've found it easiest to use that and eliminate the need for conversions. If you think of 100 kPa as no vacuum within the manifold, and 0 kPa as maximum vacuum, it's pretty easy to see which way the flow graph should be sloped-- at max vacuum, (0 kPa) the engine is literally pulling (I hate to say sucking) fuel out of the injectors. Consequently, injector on time doesn't have to be as long as it would with no vacuum in the manifold (100 kPa). The easiest way to reduce injector on time is to tell the PCM that the injector is "larger" than its actual flow rating. Similarly, if you want to increase on time, you tell the PCM that the injector is "smaller" than its actual flow rating. If you look at the slope of the IFR graph, it should become obvious whether the reference axis is in manifold vacuum or MAP (manifold pressure). Hope this helps.

[joecar]

dfe1 wrote:

at max vacuum, (0 kPa) the engine is literally pulling (I hate to say sucking) fuel out of the injectors. Consequently, injector on time doesn't have to be as long as it would with no vacuum in the manifold (100 kPa). The easiest way to reduce injector on time is to tell the PCM that the injector is "larger" than its actual flow rating.

So...
at 0kPa MAP (max VAC) the IFR value is larger to tell the PCM to open the injector for less time,
at 100kPa MAP (zero VAC, WOT) the IFR value is smaller to tell the PCM to open the injector for more time,
or in other words: IFR table has negative slope with increasing MAP.

The slope displayed by the Tune tool is positive, so this means that the horizontal scale is vacuum
(vacuum VAC equals BARO minus MAP);
this also explains why the horizontal scale only goes up to 80 kPa VAC.

So then, 100 kPa MAP (WOT) is on the left (0 kPa VAC), am I right...?

If this is so, the IFR table description in the Tuning Tool needs to say this
(edit: that the horizontal scale is VAC = BARO - MAP).
:?

[Bruce Melton]

I think this expresses it pretty clearly:

http://sohat.us/IFR.jpg[/img]

[joecar]

It doesn't say if the horizontal axis is MAP (kPa) or VAC (kPa). :?:

If someone already knows that positive slope means "VAC referenced" and that negative slope means "MAP referenced", then it is pretty clear.

:shock: I'm just trying to understand it better...

[joecar]

This is what I found on another forum...


IFR (g/s)
5.80 5.81 5.83 5.83 5.85 5.95 5.95 5.95 5.95 5.95 5.97 5.98 5.99 6.00 6.02 6.04 6.06
0Vac 80Vac
100kPa 20kPa
WOT Idle

[AllCammedUp]

I've always read over on ls1tech that the IFR should also stay 'linear' in the sense that you should not edit one specifc cell or a group of cells to correct, for example, a lean idle. In other words, the every cell in the IFR should always stay scaled by the same value to keep it in a straight line.

I'm not sure that I really understand why this is not recommended. Anyone care to elaborate? Thank you!

[dfe1]

I think that a lot of people don't truly understand the IFR table, so they avoid using it. In the pre-LS1 systems, IFR is a single cell entry, not a table, because the pressure regulator is referenced to Manifold Pressure or vacuum modulated. The IFR table is simply the electronic equivalent of a vacuum line running between the intake manifold and the pressure regulator. The problem with altering the value in only one cell, is that the system interpolates values between cells. The smoother the graph, the smoother the transitions between cells.

[joecar]

Yes, exactly, I'm trying to understand it better.
:)

[dfe1]

Think about an engine's fuel requirements and pulse width times. If injector flow rate was not referenced to manifold pressure, it would be necessary to have extremely short pulse widths to properly control fuel flow at idle and light throttle settings. That means transitions from light to heavy throttle would require a more dramatic change in pulse widths, which could cause some transitional issues. This problem becomes more acute with larger injectors (as an example 30 pounds per hour versus 24 pounds per hour). By reducing fuel system pressure (by referencing the fuel pressure regulator to manifold pressure-- or vacuum, depending on your point of view) wider pulse widths can be used because system pressure is reduced. When a pressure regulator isn't referenced to MAP, injector size is scaled electronically to accomplish the same fuel flow control.