Fuel Pressure

[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]

Originally Posted by 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...

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]

Originally Posted by XfireZ51

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

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]

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.

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]

Originally Posted by 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. 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.