VRFPR calculations

[hog]

I'm kind of confused after talking too a gentleman online. We are talking OBD1 LT5 DOHC 350's. He has gotten me all confused about a simple, simple topic. To clear my mind.

I have a PFI injector that flows 21 lb/hr @ 39psi
The LT5 has a KOEO(key on engine off) rail pressure of 55 psi.
Idle pressure is 43.5 psi

When calculating for the IFR to be entered into the calibration

Using an IFR calc: I use the 39psi as the old pressure
And 55 psi as the new pressure
And 21 lb/hr as the rating of the fuel injector

I punch these into this calc and get 24.94 lb/hr@55 psi. Isnt this the number that is entered into the calibration for IFR?

He is say ing that you use
39psi as the old pressure
And 43.5 as the new pressure(he says you use this vaccuum referenced pressure as the nominal pressure)
21 lb/hr as the rating of the injector
He gets gets approx 22 lb/hr.

here is the calc I used.


So when doing injector IFR calcs, do you use regulated pressure or unregulated pressure numbers in teh calculator.

thanks in advance

peace
Hog

[joecar]

I have a PFI injector that flows 21 lb/hr @ 39psi
The LT5 has a KOEO(key on engine off) rail pressure of 55 psi.
Idle pressure is 43.5 psi

This is ok (FPR sees BARO during KOEO, but make sure you cycle the key several times to build/stabilize pressure)

to be more consistent, do it like this (since fuel pump is running): with engine running, pull off the FPR reference hose, measure rail pressure.


To do the calc you first measure rail pressure while the FPR sees BARO on its reference port (i.e. open to atmosphere).

When calculating for the IFR to be entered into the calibration

Using an IFR calc: I use the 39psi as the old pressure
And 55 psi as the new pressure
And 21 lb/hr as the rating of the fuel injector

punch these into this calc and get 24.94 lb/hr@55 psi. Isnt this the number that is entered into the calibration for IFR?

Yes, correct (where 55 psi would be the measured rail pressure with FPR temporarily unreferenced, i.e. exposed to BARO)

[joecar]

He is say ing that you use
39psi as the old pressure
And 43.5 as the new pressure(he says you use this vaccuum referenced pressure as the nominal pressure)
21 lb/hr as the rating of the injector
He gets gets approx 22 lb/hr.

Nope, because vacuum varies as engine runs, causing rail pressure to vary.

That can't be right because because going from 39 psi to 55 psi the flowrate increases much more than only 1 lb/hr.

[hog]

OK thanks Joe, I thought so, but needed backup. Mucho appreciated.

peace
Hog

[joecar]

You could also do it at KOEO (engine not running) if you can make the fuel pump run continuously (there's a jumper wire near the relay).