Fuel Pressure

[LGAFF]

Idle with FPR connected 50lbs
Idle with FPR disconnected....60lbs

Free Rev with FPR connected drops to 40lbs

Free Rev with FPR disconencted 60lbs


Does the lack of load account for the drop?

[XfireZ51]

The idea of an FPR in PFI is to maintain the same fuel
flow across all MAP levels.

[LGAFF]

So faulty FPR? I would think if it were the pump, it would drop when the FPR was disconnected, same for the filter being clogged...

[scottfab]

Idle with FPR connected 50lbs
Idle with FPR disconnected....60lbs

These numbers look great to me.
From memory mine were 49 and 55


Free Rev with FPR connected drops to 40lbs

Free Rev with FPR disconencted 60lbs


Does the lack of load account for the drop?

Really ? 40?
Mine hits 44 but sometimes 43.

[Marc Haibeck]

Hi Lee,

Yes, faulty fuel pressure regulator.

On an LT5 the fuel pressure should be 3 Bar or 43 psi at idle with the manifold vacuum on the regulator.

With the vacuum hose off or at WOT the fuel pressure should be 51 PSI +/- 1 psi.

In the range of 45 to 55 psi each psi of pressure changes the AFR about .1. At 60 PSI at WOT you could be running an AFR point rich.

[Schrade]

Ray Bohacz says that port fuel injection has capability of 'Acceleration Enrichment', and 'DE-celeration enLEANment' (the dashpot, in the carb), and how to check it, as Marc is saying here (5th page, 1st column, in PURPLE) :

(and when you know it all, like most gurus here, sometimes the simple stuff gets right by you)

(every time I read this article, something in it comes to me, that I didn't get before)

(and never mind my notes, just ideas I had as I read and re-read)

http://www.thirdgen.org/techboard/attachments/tpi/263231d1376239860-tpi-articlke-all-tpi-tpi1.jpg

http://www.thirdgen.org/techboard/attachments/tpi/263232d1376239860-tpi-articlke-all-tpi-tpi2.jpg

http://www.thirdgen.org/techboard/attachments/tech-general-engine/263225d1376227807-my-o2-sensor-tpi3.jpg

[Schrade]

(5th page, 1st column, in PURPLE)

http://www.thirdgen.org/techboard/attachments/tpi/263233d1376240107-tpi-articlke-all-tpi-tpi4.jpg

http://www.zr1.net/forum/attachment.php?attachmentid=2700&stc=1&d=1379194846

[Schrade]

and the last 2 pages

http://www.thirdgen.org/techboard/attachments/tpi/263235d1376240237-tpi-articlke-all-tpi-tpi6.jpg

http://www.thirdgen.org/techboard/attachments/tpi/263236d1376240237-tpi-articlke-all-tpi-tpi7.jpg

[XfireZ51]

AE is a pumpshot in carb terms. DE is not to be confused w DFCO
cuts off fuel not reduces it.

[XfireZ51]

At 51psi, 22.7# injectors will flow as 24.7#, at 60psi, they flow like 27#. So at least 10% richer all around. Your going from a hypothetical 12.8 to an 11.6AFR at WOT.

[LGAFF]

This is really what I am asking.....searched the net, some say thats normal, others its the sign of a defective FPR....


Free Rev with FPR connected drops to 40lbs


Does the lack of load account for the drop?

[Schrade]

AE is a pumpshot in carb terms. DE is not to be confused w DFCO
cuts off fuel not reduces it.

Not sure exactly what you're saying here Dominic... :confused:

Is the article wrong? Or it just doesn't apply to LT5?

What does RB say that doesn't apply here? (he DOES say that fuel pressure shuts down on de-celeration)

[XfireZ51]

Not sure exactly what you're saying here Dominic... :confused:

Is the article wrong? Or it just doesn't apply to LT5?

What does RB say that doesn't apply here? (he DOES say that fuel pressure shuts down on de-celeration)

What I'm saying is that there really is no mystery to ECM control. It just does the same things we did before, just many more times per second.

[XfireZ51]

Not sure exactly what you're saying here Dominic... :confused:

Is the article wrong? Or it just doesn't apply to LT5?

What does RB say that doesn't apply here? (he DOES say that fuel pressure shuts down on de-celeration)

There is a difference between DFCO and DE. That's what I was pointing out. Perhaps RB was generalizing and trying to show the contrast of adding fuel and deleting it.

[Hog]

At 51psi, 22.7# injectors will flow as 24.7#, at 60psi, they flow like 27#. So at least 10% richer all around. Your going from a hypothetical 12.8 to an 11.6AFR at WOT.
When setting up your initial IFR(Injector Flow Rate) you use the rail pressure that occurs when the the vaccuum reference is disconnected, correct?

On my setup, the rail pressure is 58psi vaccuum reference detached at idle with vaccuum reference attached the rail pressure drops down to 51-52 psi.

My injectors flow 25 lb/hr at 43.5psi/3BAR, the calculates out to 28.7 lb/ht at 58psi/4BAR.
The IFR I would use in my PCM is 28.7 lb/hr or 3.62 grams/second.

Bottom line, use the unreferenced regulator rail pressure for IFR calcs?

[Hog]

Not sure exactly what you're saying here Dominic... :confused:

Is the article wrong? Or it just doesn't apply to LT5?

What does RB say that doesn't apply here? (he DOES say that fuel pressure shuts down on de-celeration)
The part of the article that is circled in purple is wrong, well at least missleading..

The article is describing Acceleration Enruchment which is correct.
Then the article describes DE or Deceleration Enrichment incorrectly. The description given is for DFCO or Decel Fuel Cut Off, not DE.
DFCO actually shuts off injectors during periods of coastdown.

AE is used during short periods of time when the engine takes an extra "gulp" of air, exactly what happens when you stand on the throttle, AE is exactly like the carbs "pumpshot" that will prevent a lean stumble.
AE is a function of necessity, DFCO is purely an economy measure. DFCO is not necessary for proper engine function like AE is.

There is also PE mode where extra fuel is added when engine load is increased and extra power production is required.
There is also a Power Enleanment mode which is the opposite of Power Enrichment mode. Both of these are typically Open loop and do not use narrowband O2 sensor input.

[XfireZ51]

Hog,

The injector flow rate used in the calibration is the one you would have at idle with the FPR attached to a vacuum port. In a stock cal, its set to 22.7#. In older calibrations like the one I had for my 84 Xfire, you would calculate the BPC or Base Pulsewidth Constant. Here, the cal does it for you using the IFR.

[Schrade]

cuts off fuel not reduces it.
That's what the text says.


There is a difference between DFCO and DE. What is DE? (the author says, "Deceleration EnLEANment, which is injector shutdown. "
:confused:


...

Then the article describes DE or Deceleration Enrichment incorrectly.

...




I think EVERYone has mis-read the words in the author's text ( all the while, agreeing with the author).

The text describes Deceleration EnLEANment (WHICH HE SAYS IS INJECTOR SHUT-OFF) ... NOT DECELERATION ENRICHMENT.

Circled in purple.

Handheld digital 'smart' devices aren't good for reading comprehension. Seriously......................

[GOLDCYLON]

This is really what I am asking.....searched the net, some say thats normal, others its the sign of a defective FPR....


Id go with post #5's answer. He's prob forgotten more than most of us could ever know about the LT5

[XfireZ51]

Just to be clear, DE is deceleration enleanment meaning it leans out the AFR as you feather the throttle. Fuel delivery is reduced, not stopped. Lifting off the accelerator will invoke DFCO, Decel Fuel Cut Off, if it is enabled. There are RPM and MAP "windows" where these take place. They don't happen everywhere. Mostly for low end. Both of these are for emissions. For the most part, I have DFCO disabled and I believe Marc does this also in his cals, particularly for those that complain about motor stalling when freewheeling or rolling to a stop.

[LGAFF]

Id go with post #5's answer. He's prob forgotten more than most of us could ever know about the LT5


I generally agree, although the response focused on the high results with the vac line off.....was not sure if he noted the PSI drop note

As noted before, there are examples of this with other cars on the net doing the same; oddly no one posts their findings, just post the issue...many with new pumps or AM FPRs

[XfireZ51]

I generally agree, although the response focused on the high results with the vac line off.....was not sure if he noted the PSI drop note

As noted before, there are examples of this with other cars on the net doing the same; oddly no one posts their findings, just post the issue...many with new pumps or AM FPRs

I think Marc did note both vac ref and non fuel pressure.

[Hog]

Hog,

The injector flow rate used in the calibration is the one you would have at idle with the FPR attached to a vacuum port. In a stock cal, its set to 22.7#. In older calibrations like the one I had for my 84 Xfire, you would calculate the BPC or Base Pulsewidth Constant. Here, the cal does it for you using the IFR.
Just discussion here:
Why wouldnt you use an IFR that is without the vaccuum source plugged in?

The purpose of a vaccuum refernced FPR is to keep the pressure differential across the fuel injector as close to a constant as possible. As pressure increases inside the intake(vaccuum decrease) like when goiing to WOT, the VRFPR(vac reference fuel pressure regulator) will increase fuel pressure differential equal. Lower plenum pressure(high vaccuum-throttle closed) requires less rail pressure, and higher plenum pressure(low vaccuum-open throttle) requires less rail pressure.

So if you input an IFR based off of actual injector flow that is reduced because of plenum vaccuum, your WOT fueling would be off.

I am just used to entering IFR based off of unrefernced fuel rail pressure. Such as when getting rail pressure at KOEO(Key On Engine Off) which will be the same rail pressure with the engine running vaccuum refernced detached.

It seems like using IFR based on unrefernced rail pressure, then let the VRFPR do its work when idling or cruising when plenum pressure is low(vaccuum is high).


Using Marc's pressures 43 psi should be observed at idle vaccuum source attached, or 51 psi at KOEO or vaccuum source detached.

Yes, no? Hmmmm??

[Hog]

I think Marc did note both vac ref and non fuel pressure.

Marc's numbers
Yes, faulty fuel pressure regulator.

On an LT5 the fuel pressure should be 3 Bar or 43 psi at idle with the manifold vacuum on the regulator.

With the vacuum hose off or at WOT the fuel pressure should be 51 PSI +/- 1 psi.

In the range of 45 to 55 psi each psi of pressure changes the AFR about .1. At 60 PSI at WOT you could be running an AFR point rich.

VS.

LGAFF's numbers

Idle with FPR connected 50lbs
Idle with FPR disconnected....60lbs

Free Rev with FPR connected drops to 40lbs
Free Rev with FPR disconencted 60lbs
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxx

I find it concerning that a free rev causes you FP to drop down to 40psi, thats without load as well.

[LGAFF]

I find it concerning that a free rev causes you FP to drop down to 40psi, thats without load as well.


I do as well, I just wanted to make sure there is no anomaly that could cause it, just as a car cannot build boost without load, maybe there is a reason for this....I am retesting the car today to see if there is anything shutting off the 2ndry pump

[XfireZ51]

Just discussion here:
Why wouldnt you use an IFR that is without the vaccuum source plugged in?

The purpose of a vaccuum refernced FPR is to keep the pressure differential across the fuel injector as close to a constant as possible. As pressure increases inside the intake(vaccuum decrease) like when goiing to WOT, the VRFPR(vac reference fuel pressure regulator) will increase fuel pressure differential equal. Lower plenum pressure(high vaccuum-throttle closed) requires less rail pressure, and higher plenum pressure(low vaccuum-open throttle) requires less rail pressure.

So if you input an IFR based off of actual injector flow that is reduced because of plenum vaccuum, your WOT fueling would be off.

I am just used to entering IFR based off of unrefernced fuel rail pressure. Such as when getting rail pressure at KOEO(Key On Engine Off) which will be the same rail pressure with the engine running vaccuum refernced detached.

It seems like using IFR based on unrefernced rail pressure, then let the VRFPR do its work when idling or cruising when plenum pressure is low(vaccuum is high).


Using Marc's pressures 43 psi should be observed at idle vaccuum source attached, or 51 psi at KOEO or vaccuum source detached.

Yes, no? Hmmmm??

With the vacuum port disconnected, the FP will be as it would be at WOT or 100kPa. As stated earlier, the fuel pressure regulator changes pressure in a 1:1 ratio in order to maintain the same pressure differential across the injector. This in turn allows the injector to continue to flow at the same rate. So PFI uses a 1:1 ratio VAFPR to increase fuel pressure in order to MAINTAIN the same fuel flow regardless of MAP. Whether at 50 or 100kPa, fuel flow will remain at ~23#/hr per injector. Raise the nominal FP, and the flow rate scales upward.
It doesn't work like this for carbs or TBI systems where a VAFPR causes the injectors to flow more by increasing FP.
Accels, for example, are rated 21# at 39psi FP. Since our nominal FP is 43psi, the injector flow rate is ~23#.
All of this is critical in calculating what the Base Pulsewidth is. Based on the BPC, the ECM increases or decreases pulsewidth to meter fuel as needed. An inaccurate BPC affects the rest of the fueling.

[LGAFF]

Ok, here is what I have found;

*Key on, not running both pumps produce good pressure;

*Using the Tech 1, I can turn on the 2ndry pump....it primes and the car idles, however the tech 1 is showing only .2 volts....plug in the primary pump....13.4

Rev the engine in the test mode, fuel pressure drops, dies


So if all of this holds true a static test of pumps is basically worthless and only good to find an absolute failure...

[Schrade]

You ARE saying FPR VACUUM dis-connected, just to be absolutely clear... right?

Idle with FPR connected 50lbs
Idle with FPR disconnected....60lbs

Free Rev with FPR connected drops to 40lbs

Free Rev with FPR disconencted 60lbs


Does the lack of load account for the drop?

Does FSM have spec tests? With vacuum ON, and OFF of FPR?

And if FPR isn't responding properly to vacuum [presence, or absence], could it be a vac leak?

What was pressure leakdown time? Or is it holding static pressure? (sorry if you posted that and I missed it...)

'''''''''''''''''''''''''''''''''''''''''
Is this FSM material?
Ok, here is what I have found;

*Key on, not running both pumps produce good pressure;

*Using the Tech 1, I can turn on the 2ndry pump....it primes and the car idles, however the tech 1 is showing only .2 volts....plug in the primary pump....13.4

Rev the engine in the test mode, fuel pressure drops, dies


So if all of this holds true a static test of pumps is basically worthless and only good to find an absolute failure...

[LGAFF]

No thats something I was testing, going to replace the secondary pump tomorrow....my only concern is that maybe the .2 is a reference and not the 2ndry pump voltage....

[LGAFF]

In the words of Frankenstein:

FUEL PUMP BAD!

Now it goes to 55+ when at WOT.


Old Pump read 1.9 ohms, new 63 ohms, old Callaway pump I had laying around 58.....those who know more about electronics might have a thought on this...

[Franke]

I think maybe your tech 1 tool was pointing that out earlier. You should have a better voltage reading now. The motor winding is shorted probably due to fuel breaking down the insulation in the windings or a mechanical breakdown internally. I've seen motors with bad bearings cause internal winding damage. I'm wondering why it didn't blow the fuse?

[XfireZ51]

In the words of Frankenstein:

FUEL PUMP BAD!

Now it goes to 55+ when at WOT.


Old Pump read 1.9 ohms, new 63 ohms, old Callaway pump I had laying around 58.....those who know more about electronics might have a thought on this...

Is this on the 90 ZR? And if so, is FUEL PUMP BAD the cause of your stumble?

[LGAFF]

The stumble for the most part went away with the harness and O2 change, however there was still a "warm up" issue periodically when I started the car. I checked the fuel pressure when I started having a stumble when the car was warm, but would have issues periodically stumbling from a start....this is the 90

I tested the car "free revving" and fuel pressure now goes up once you get into the the throttle hard......so I will see if there is a change in driving.

The TECH 1 test turned out to be inconclusive as it still shows .2 on the secondary pump; but I am not sure it was designed to perform in that manner.

[LGAFF]

I was out with the tech 1 today; there is a test under "output test" "secondaries" that shows the primary fuel pump volts and the secondary Pump...might have been more useful, although when I pull the 2ndry pump fuse I still see 13.2 volts/no change

[Hog]

With the vacuum port disconnected, the FP will be as it would be at WOT or 100kPa. As stated earlier, the fuel pressure regulator changes pressure in a 1:1 ratio in order to maintain the same pressure differential across the injector. This in turn allows the injector to continue to flow at the same rate. So PFI uses a 1:1 ratio VAFPR to increase fuel pressure in order to MAINTAIN the same fuel flow regardless of MAP. Whether at 50 or 100kPa, fuel flow will remain at ~23#/hr per injector. Raise the nominal FP, and the flow rate scales upward.

It doesn't work like this for carbs or TBI systems where a VAFPR causes the injectors to flow more by increasing FP.
Accels, for example, are rated 21# at 39psi FP. Since our nominal FP is 43psi, the injector flow rate is ~23#.
All of this is critical in calculating what the Base Pulsewidth is. Based on the BPC, the ECM increases or decreases pulsewidth to meter fuel as needed. An inaccurate BPC affects the rest of the fueling.
What we are in disagreement about is exactly the number you would enter as your base IFR, more specifically what you call "nominal fuel pressure".

Using your numbers with Accel squirters they are rated at 21 lbs/hr@39 psi.
The nominal LT5 FP is 55 psi, not 43.5 psi(this is the regulated pressure).

So use any basic IFR calculator, here's 1 [http://www.csgnetwork.com/fiflowcalc.html

Original fuel pressure= the pressure the injectors were tested at=39psi
New pressure=pressure at KOEO or VR disconnected=55psi
Rating of fuel injector= injector flow rate@ its testpressure=21lb/hr@39psi (testpressure=21lb/hr@39psi)

The new IFR is 24.94lb/hr not 23 lb/hr as you stated.

Since the VRFPR brings down when exposed to low manifold pressure(high vaccuum), you want to use the unregulated fuel pressure, not the pressure that has already been infuelnced by the VRFPR. If you go to WOT, the rail pressure jumps UP to 55 psi and the PE mode fueling is calculated, but because the VRFPR always brings pressure DOWN, IFR MUST be calculated from naked, unreferenced fuel rail pressure.

The difference is yours and my definition of nominal fuel pressure, you say its 43.5, I say its 55 psi.
For all PFI systems that use a VRFPR, GM always uses the unreferenced fuel pressure for IFR calculation.
The once the proper IFR is enetered into the calibration, the ECM can do its calcs for fueling (pulsewidth etc)

This is why it is important when calibrating any PFI engine to figure out EXACTLY what the fuel rail pressure is without VRFPR influence as this can vary from car to car, from regulator to regulator.

To find the new IFR of an injector that is being used at a rail pressure that is DIFFERENT than the injector's test pressure, you need :
1)the unrefernced fuel pressure or WOT or VRFPR disconnected pressure,
2)the pressure at which the injector was tested
3)the actual flow of said injector at that test pressure

then wash it through an Injector Flow Rate worksheet
http://www.csgnetwork.com/fiflowcalc.html

to get the base IFR spec that you enter into the ECM/PCM. Then the VRFPR once seeing engine vaccuum, brings the pressure down in order to keep the pressure differential across the injectors pintle valves the same regardless of engine vacuum. When manifold pressure is high(low vaccuum), like at WOT it takes more pressure in the rail to force the fuel against the higher manifold pressure(less vaccuum), when manifold pressure is low like when the throttle is closed when idling or cruising, it requires LESS rail pressure to introduce that fuel into the ports, hence the VRFPR effectivley lower the fuel pressure the injectors use.

For the LT5, DO NOT use the 43 psi as the nominal pressure, you DO USE the unreferenced fuel pressure which for an LT5 is 55 psi. (But its best to calculate using the exact pressure each car outputs with the vaccuum reference UNPLUGGED with the pump running. You have it backwards I think.

Example: Obviously different numbers than the LT5 but gives an accurate representation of what we are doing when we find Injector Flow Rates at fuel rail pressures OTHER than what the injector manufacturer used to rate the injector.

Injector Flow Rates At Different Pressures
Once an injector is manufactured, the flow rate can not be altered. Flow rates can only be changed on a limited basis by raising or lowering the system’s fuel pressure set point (the measured pressure with no vacuum or pressure on the regulator to manifold port). By raising fuel pressure, more fuel is forced out the metering orifice per pulse width. Thus, someone who has performed modifications to their vehicle (i.e. low restriction exhaust, improved air induction, increased boost, etc.) may be near the flow limits of their stock fuel injectors. By increasing fuel pressure, a few more lbs/hr of fuel flow may be squeezed through the injectors to meet engine demand.
The following formula can help you determine the injector’s flow rate with a change in the fuel pressure’s set point:

Q2 = {Square Root (P2/P1)} x Q1

Q1 = Original injector flow rate (lbs/hr)
Q2 = Injector flow rate at modified pressure (lbs/hr)
P1 = Original fuel pressure set point (psi)
P2 = Adjusted fuel pressure set point (psi)

For example, a Ford Mustang 5.01 uses a fuel injector rated at 19lbs/hr at a fuel pressure set point of 39psi. What would the flow rate be at 50 psi?

Q2 = {Square Root (50psi/39psi)} x 19lbs/hr
= Square Root (1.28) x 19 lbs/hr
= 1.13 x 19 lbs/hr
Q2 = 21.5 lbs/hr



You are correct that everything ECM fueling related is based off these base IFR entries as the ECM bases all its calculations off of it. Precise Injector Flow Rates calculated using unreferenced fuel rail pressure is what the ECM needs for proper fueling calculations done for correct injectior pulsewidths.

Whew!

[XfireZ51]

Look at any stock LT-5 cal and the number in the box is 22.7#. The Accels work out to 23# at the same FP the stock injectors are rated at. I use the calculator from ThirdGen.
If you use some other injector, then you need to modify the cal. Again, the VAFPR in a MPFI system is to maintain the fuel flow, not increase it.
Go ahead and use the flow rate you get with 55# of FP, and burn a cal w that then tell me what happens. My guess is the motor will be lean.

[tpepmeie]

Stock LT5 injectors flow around 20.5 lbs at 43.5 psi. The fuel pressure specification is actually 350 kpa, or 50.76 psi. There is likely some production variation, but 55 would seem really high.

So the conversion factor from 50.76 to 43.5 psi, is 1.080272, making the stock injectors flow 22.1 lbs/hour.

The calibration is actually specified in "seconds per gram" of flow, but most consumer software converts this to lbs/hour for convenience.

0.3484 sec/gm is the stock value. 22.78 lbs/hr or so.

[XfireZ51]

Todd,

Why don't you bring that 427 to the Corvette Challenge next weekend?

[scottfab]

Stock LT5 injectors flow around 20.5 lbs at 43.5 psi. The fuel pressure specification is actually 350 kpa, or 50.76 psi. There is likely some production variation, but 55 would seem really high.

So the conversion factor from 50.76 to 43.5 psi, is 1.080272, making the stock injectors flow 22.1 lbs/hour.

The calibration is actually specified in "seconds per gram" of flow, but most consumer software converts this to lbs/hour for convenience.

0.3484 sec/gm is the stock value. 22.78 lbs/hr or so.

That matches up fairly well to the empirical evidence Hib came up with in 1998 when he wrote that article on flow variance between vendors. The stock ones flowed 19.25lb/hr (~200cc) .
I bought RC injectors that are rated at 21.2lb/hr (~220cc).

[Hog]

Look at any stock LT-5 cal and the number in the box is 22.7#. The Accels work out to 23# at the same FP the stock injectors are rated at. I use the calculator from ThirdGen.
If you use some other injector, then you need to modify the cal. Agreed 100%
Again, the VAFPR in a MPFI system is to maintain the fuel flow, not increase it.
Its not increasing the fuel pressure, its bring it down from unregulated pressure. If you have 55 psi(and some here said they had 55+psi) with the VRFPR disconnected, at idle when you connect the vaccuumrefernce, fuel pressure will come down. The purpose is to keep the the pressure differential the same no matter intake vaccuum. Then at WOT, the FPR allows full rail pressure.
Go ahead and use the flow rate you get with 55# of FP, and burn a cal w that then tell me what happens. My guess is the motor will be lean.I'm not saying to blindly use the 55psi spec. As Ive stated, you have to measure the fuel rail pressure without vaccuum refernce and then use the resultant pressure in your calcs. You were implying that you take the referenced pressure of 43.5 psi and use that in the calcs, and that is simply incorrect as that is a floating number based off of engine vaccuum, the unreferenced pressure is a constant, which is why you base the cals off of it.
My method is sound and is used by GM on all ECM/PCM's that use a fuel pressure regulator and 3 prominent ECM/PCM tuners that I have contacted, just to be sure I have been doing it correctly.

[Hog]

Stock LT5 injectors flow around 20.5 lbs at 43.5 psi. The fuel pressure specification is actually 350 kpa, or 50.76 psi. There is likely some production variation, but 55 would seem really high.

So the conversion factor from 50.76 to 43.5 psi, is 1.080272, making the stock injectors flow 22.1 lbs/hour.

The calibration is actually specified in "seconds per gram" of flow, but most consumer software converts this to lbs/hour for convenience.

0.3484 sec/gm is the stock value. 22.78 lbs/hr or so.
Running your numbers through this calc
http://www.csgnetwork.com/fiflowcalc.html
Gets exactly the same numbers you get.
Stock flow=20.5
Fuel pressure used for rating the injector=43.5(3BAR)
Actual rail pressure(unreferenced) 50.76 psi

Gives me 22.1 b/hr of fuel at the fuel pressure the injector will actually be used at.

22.1 lb/hr converts to 2.78 g/sec
22.78lb/hr converts to 2.87 g/sec

Never seen seconds per grams before, but it all converts to the same nomatter the unit.

[XfireZ51]

You were implying that you take the referenced pressure of 43.5 psi and use that in the calcs, and that is simply incorrect as that is a floating number based off of engine vaccuum, the unreferenced pressure is a constant, which is why you base the cals off of it.
My method is sound and is used by GM on all ECM/PCM's that use a fuel pressure regulator and 3 prominent ECM/PCM tuners that I have contacted, just to be sure I have been doing it correctly.

If I understand correctly, I believe you are trying to account for FP variations that happens w different pumps and VAFPR and so to be more accurate you are calc'ing actual flow for greater accuracy. I'm not doing that. I am using what I find in the cal because its good enough IMO.
I would focus more on the correct injector offsets for non-stock injectors like the Accels as an example.
What you may want to try, for grins, is to note what kPa your motor idles at, say between 50-55kPa or ~ 15.5" Hg. Throw on a fuel pressure gauge,
attach a Mighty Vac with a gauge onto the fuel regulator, turn ignition on, and the apply 15.5" Hg. See what your FP is. Vary the vacuum and see what happens. I had to do that w my Xfire while using a VAFPR to manage fuel flow based on load. I had to calculate what the BPC was for each 5kPa increment and fill a table so the ECM was using the correct BPC based on load. With a modded TBI system, it was the only way to have enough fuel flow at WOT, while still being able to control idle.

Below is a screen shot from TPRT of the fuel flow found in the AYWT
bin for a 92 LT-5.

http://i187.photobucket.com/albums/x198/Z51Xfire/LT-5stockfuelflow_zps3e96b481.jpg (http://s187.photobucket.com/user/Z51Xfire/media/LT-5stockfuelflow_zps3e96b481.jpg.html)

[tpepmeie]

It is *differential* pressure that the FPR attempts to maintain. Differential meaning the pressure on the inlet side of the injector versus the outlet (manifold) pressure.

As you get closer to WOT, manifold vacuum approaches zero, thus the injector sees ~51psi differential pressure from the regulator. As seen with the vacuum reference line disconnected.

Less than WOT, the FPR reduces fuel pressure on the inlet to the injector because you now have a vacuum on the outlet side. Say you are idling at 8 psi vacuum. To achieve 51psi differential pressure, the FPR reduces the supply to 51-8=43psi. The injector behaves (flows) the same as it does with 51psi rail pressure at WOT.

At least that's how I understand it....

[XfireZ51]

Todd,

Think of it this way. At high vacuum, ie idle, fuel is being literally sucked out of injector. At WOT or very close to 100kPa, getting the fuel into the airflow takes more pressure therefore a 1:1 VAFPR to maintain the same amount of fuel flowing whether #/hr or gms/sec. In TBI systems the injector is always exposed to atmospheric pressure. Getting fuel into the airflow is a question of "shearing" the fuel off the Venturi walls as the air rushes by in order to atomize it. But TBI systems run at maybe 10-14psi. Atomization is where direct injection really shines due to the much higher pressures.

[Hog]

It is *differential* pressure that the FPR attempts to maintain. Differential meaning the pressure on the inlet side of the injector versus the outlet (manifold) pressure.

As you get closer to WOT, manifold vacuum approaches zero, thus the injector sees ~51psi differential pressure from the regulator. As seen with the vacuum reference line disconnected.

Less than WOT, the FPR reduces fuel pressure on the inlet to the injector because you now have a vacuum on the outlet side. Say you are idling at 8 psi vacuum. To achieve 51psi differential pressure, the FPR reduces the supply to 51-8=43psi. The injector behaves (flows) the same as it does with 51psi rail pressure at WOT.

At least that's how I understand it....

You have it exactly Todd, 100%.

In newer MPFI systems that dont use any sort of FPR, this is all the pressure differential across the injector is calculated by the PCM at differnt MAP points.

[XfireZ51]

You have it exactly Todd, 100%.

In newer MPFI systems that dont use any sort of FPR, this is all the pressure differential across the injector is calculated by the PCM at differnt MAP points.

I recall hearing that the ZR no hyphen 1 varied its fuel flow elctronically in 3 stages. That way they could use 8 injectors to support 638hp, but still have a pulsewidth that was controllable for idle and emissions. Are current MPFI systems now Speed Density, MAF, or a combination?

[VetteVet]

Dom,

The LS stuff I'm fooling with now is blended MAF/Speed Density. The PCM uses both measured airflow from the MAF and speed density calculated airflow. The PCM blends these in different proportions based on load. It also does cross checks and will set a code if MAF and speed density airflows don't correlate within a given percentage.

Jep

[Schrade]

If I understand correctly, I believe you are trying to account for FP variations that happens w different pumps and VAFPR and so to be more accurate you are calc'ing actual flow for greater accuracy. I'm not doing that. I am using what I find in the cal because its good enough IMO.
I would focus more on the correct injector offsets for non-stock injectors like the Accels as an example.
What you may want to try, for grins, is to note what kPa your motor idles at, say between 50-55kPa or ~ 15.5" Hg. Throw on a fuel pressure gauge,
attach a Mighty Vac with a gauge onto the fuel regulator, turn ignition on, and the apply 15.5" Hg. See what your FP is. Vary the vacuum and see what happens. I had to do that w my Xfire while using a VAFPR to manage fuel flow based on load. I had to calculate what the BPC was for each 5kPa increment and fill a table so the ECM was using the correct BPC based on load. With a modded TBI system, it was the only way to have enough fuel flow at WOT, while still being able to control idle.

Below is a screen shot from TPRT of the fuel flow found in the AYWT
bin for a 92 LT-5.

http://i187.photobucket.com/albums/x198/Z51Xfire/LT-5stockfuelflow_zps3e96b481.jpg (http://s187.photobucket.com/user/Z51Xfire/media/LT-5stockfuelflow_zps3e96b481.jpg.html)

It WOULD be good enough (I think), if the fuel calculation was LINEAR. But the calculation for pressure P2 is exponential.

Seems then that the P1 value, as Hog posted, is CRITICAL, not only in theory, but in PRACTICE, because the P2 ADJUSTED pressure is dependent on a SPEC ZERO'ed reading.

Q2 = {Square Root (P2/P1)} x Q1

Q1 = Original injector flow rate (lbs/hr)
Q2 = Injector flow rate at modified pressure (lbs/hr)
P1 = Original fuel pressure set point (psi)
P2 = Adjusted fuel pressure set point (psi)

So a deficient P1 value will lead to a deficiency on a logarithmic scale, as the adjustment P2 is calculated.

Isn't this correct?

[Hog]

I recall hearing that the ZR no hyphen 1 varied its fuel flow elctronically in 3 stages. That way they could use 8 injectors to support 638hp, but still have a pulsewidth that was controllable for idle and emissions. Are current MPFI systems now Speed Density, MAF, or a combination?
The actual LS9 setup does use a variable speed fuel pump, on a returnless system-the pump is ECM controlled. There are seperate Low, Normal and High pump pressures based on pump duty cycles. Just as you describe it can use short pulsewidths to fuel idle and low load/rpm areas then crank up the fuel pressure and fuel all 638hp all in the same system. Cool setup.

A card style MAF is used along with MAP inputs. This would be a combination style setup.

I just went through a modern 2012 calibration and man its heavy, literally thousand and thousands of parameters.