reference or unrefernced pressure used for calculating base IFR

I am embarassed to post this, but I need assurance as I have been discussing with 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 the calculator.
thanks guys.

peace
Hog

The old and new pressure readings are generally taken with no vacuum applied to the regulator, key on and engine off. If you use a vacuum referenced pressure there is no promise that the two readings are taken with the same vacuum conditions.

I calculate 25 lb/hr injectors at 55 psi.

depends on the application. if you have an engine with a vacuum referenced FPR(and injectors that fire after a throttle plate), then the flowrate of the injectors at any point between 0 vacuum and full vacuum doesn't change since the delta between the fuel pressure and manifold pressure never changes.

if no vacuum reference and injectors shoot above the throttle, same situation, delta shouldn't change(unless you experience large arometric pressure variations).

if shooting under the throttle and regulator isn't vacuum referenced.... well, injector flowrate will be different for every MAP/vacuum situation and you'll have a table to tailor to this, as opposed to only needing to change a single value.

Obvioulsy we are talking about a vaccuum referenced system because the question is whether top use regulated pressure or un regulated pressure for the base IFR calcs.

Thanks for the input fellas, just needed to back up what I thought I knew. He was convincing in his arguements.

peace
Hog