Fuel Pressure

[Hog]

Originally Posted by 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.

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]

Originally Posted by Hog

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.

[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]

Originally Posted by 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....

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]

Originally Posted by Hog

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]

Originally Posted by XfireZ51

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.

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]

Originally Posted by XfireZ51

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.