Fuel Pressure for Vehicle Swaps

[Cringer]

What is the best way to deal with a non-GM fuel system with different fuel pressures on a vehicle swap and no FPCM? I am guessing these two fields are needed:

[ECM] 13541 - Minimum Rail Pressure
[ECM] 13542 - Maximum Rail Pressure

Then hook a physical gauge to the rail to get pressure at idle for max and go another reading at WOT for max?

[TheMechanic]

Different fuel pressures typically I just run the injector through the online calculator to change [ECM] 12307 - Injector Flow Rate vs. Pressure Delta:

https://www.rcfuelinjection.com/technical

[Cringer]

Different fuel pressures typically I just run the injector through the online calculator to change [ECM] 12307 - Injector Flow Rate vs. Pressure Delta:

https://www.rcfuelinjection.com/technical

OK interesting, so if I have an LS3 injector (42 lb/hr @ 58 PSI) and it is now operating at 30 PSI, the new flow rate (according to that calc) is 30.21 lb/hr...
Would I want to multiply the entire table by 0.719 (which is 30.21 / 42)?

58 psi to 30 psi.jpg

[blindsquirrel]

You can take real injector flow rate (and Offset, too, which is HARD to interpolate for a different fuel pressure independently!) from a tune file that used those injectors and has all the tables fully populated with real data (not placeholder data, like in some of the 2006-2007 files).

screenshot.02-02-2024 15.40.31.png

[Cringer]

You can take real injector flow rate (and Offset, too, which is HARD to interpolate for a different fuel pressure independently!) from a tune file that used those injectors and has all the tables fully populated with real data (not placeholder data, like in some of the 2006-2007 files).

screenshot.02-02-2024 15.40.31.png

Not sure I am following you...are you saying there were factory '06-'07 cars with LS3 injectors running on 30 PSI fuel pumps and I can copy that data? I thought all Gen4 stuff was nominal 58 PSI?

[blindsquirrel]

The ECM doesn't know what the rail pressure is, without a FPCM it will assume it's 58 (400 kPa). And it will only use the cells in the tables that go from 400 (or 408) to 480 (or 488). I don't think it will adjust to using the correct cells by just changing the numbers for the Min/Max rail pressure.

What ECM are you working with? Early with 128-640 range or late with 128-788?

[Cringer]

The ECM doesn't know what the rail pressure is, without a FPCM it will assume it's 58 (400 kPa). And it will only use the cells in the tables that go from 400 (or 408) to 480 (or 488). I don't think it will adjust to using the correct cells by just changing the numbers for the Min/Max rail pressure.

What ECM are you working with? Early with 128-640 range or late with 128-788?

Later 128-788 style. I was trying to fix the 58 psi assumption by altering the pressure min/max fields...but I guess that is not possible?

[hjtrbo]

With no fpcm your easiest route is a manifold referenced reg. Flatline the ifr appropriately, use the ls3 data for the remaining inj tables like blindsquirell said and go hooning.

[blindsquirrel]

It'll work fine returnless at 30 PSI* as long as the data is taken from the right pressure Delta range. For 30 PSI returnless that would be the columns from 208 to 288 kPa, pasted into the columns that go from 408 to 488. It's just shifting the data over into the columns that will actually be used.

* assuming the spray pattern doesn't go to shit when run at half the pressure they were designed for

[Cringer]

The other side to this problem is that the fuel pressure is flatlined at 399 kpa / 57.9 psi all the time at idle, CL, and PE. It never moves. What controls the fuel pressure estimation (E67 btw)?

Here is a log that is 34 minutes of driving. Perfect pressure!
399 kpa.jpg

[blindsquirrel]

It's a fake value, it's just assumed to be that and all the pulsewidth calcs are based on that number.

[blindsquirrel]

And I'm curious, what kind of fuel system runs at only 30 PSI but still has enough volume to support whatever HP this is?

[smokeshow]

If you have user defined parameters, you can hunt down the fixed rail pressure calibration. And injector cal would be handled just like blindsquirrel said.

[Cringer]

It's a fake value, it's just assumed to be that and all the pulsewidth calcs are based on that number.

Right. Exactly.

In the real world pulse width (aka flow) increases and pressure decreases. So why would this ECM assume constant 400 kPa pressure even at WOT?

And once I jiggle the handle on the IFR v Pressure...the ECM still won't calculate pulse width correctly because the fuel pressure is assumed to be flat lined.

[blindsquirrel]

Fuel pressure is constant, the pressure delta across the injector is not! Delta changes constantly with manifold pressure, therefore flow rate (& Offset) does too.

edit: And the ECM knows the Delta at whatever point in time, because it has a MAP sensor.

[hjtrbo]

The secret sauce. I understand the method now blind squirrel. Thanks for explaining all that.

[GHuggins]

If you have user defined parameters, you can hunt down the fixed rail pressure calibration

Seems like you would still need to do this part for the ecm to fully know wouldn't you? Of course I guess as long as you shifted everything it wouldn't matter? Or would it still apply the short pulse adder based off of the higher pressure reasoning?

[blindsquirrel]

SPA is PW-based, only IFR & Offset reference the pressure.

[GHuggins]

Yeah and I guess as long as your correcting those they in turn correlate the other. I was just wondering if there was a background table playing into the injectors correction based off of calibrated pressure. You're correcting it either way using Blind's method, so don't guess it really matters.

[hjtrbo]

If you have user defined parameters, you can hunt down the fixed rail pressure calibration. And injector cal would be handled just like blindsquirrel said.

Found it.

There are 7 switches in total. Default, hot, crank, high flow, cold, low flow, normal. They are defaulted to off. When switched on, the ecm will use the respective fuel pressures in [ECM] 7030 through 7035