Injector Flow Rate

[bobbycollier]

I have a dumb question... Given that a stock C5's fuel pressure regulator is manifold pressure/vacuum referenced, why do we need the injector flow rate table to be anything other than a constant value? If the fuel pressure is vacuum/boost referenced, the injectors should flow a constant value at any pressure / vacuum level. What am I missing? Is there anything different about the way the GM regulator maintains pressure versus an Aeromotive or Magnafuel boost-referenced fuel pressure regulator?

Thanks,
Bobby

[joecar]

Hi Bobby,

The very first C5 year model used a manifold-referenced FPR, and so required a flat IFR...

But, the later C5's and all the LS1 Camaro/Firebird's used an unreferenced FPR, so they required a sloped IFR
(the IFR actually goes as the squareroot of manvac).

More info: showthread.php?t=4821 (http://forum.efilive.com/showthread.php?t=4821)

[bobbycollier]

Thank you. I have a 1998 C5 with a '99 operating system. My shop advised that it has a manifold referenced regulator. So... based on your feedback, I should probably set the entire table to 1 value. Do you know if this regulator references appropriately both vacuum and boost? Would there be any need or advantage to change to an aftermarket regulator? I'm looking for ~60 PSI to be maintained under all operating conditions. Currently only running about 9lbs of boost. Thanks, Bobby

[joecar]

Take a look for yourself... look between the fuel rails, if you see a "regulator" (looks like a metal can) with an air hose/tube going to the intake manifold, and with supply AND return fuel line connections, then you have a manifold-referenced regulator... don't confuse this with the pulse dampner which has no reference hose/tube... take some pics and post here.

You can also tell by connecting a pressure gauge to the end of the fuel rail... as you open/close the throttle, if the gauge pressure goes up/down by the proper amount then you have a manifold-referenced regulator.

--> You won't get 60 psi under all conditions because the manifold-referenced FPR changes the fuel pressure according to MAP... as MAP goes up so will FP:

At WOT with 9 psi of boost FP will be 84 psi (60 psi base pressure + 15 psi baro pressure + 9 psi boost).
At WOT with no boost (or with reference hose disconnected) FP will be 75 psi (depending on BARO).
At idle FP will range from 65 psi (cam makes good vacuum) to 70 psi (cam makes poor vacuum).

The pressure difference across each injector (regulated pressure on top, MAP on bottom) will always be constant regardless of vacuum/boost --> this is why the IFR will be flat --> this is why manifold-referencing is used

What altitude are you located at, what is your BARO pressure...?

The stock referenced regulator can handle some boost, but an after market referenced regulator is probably more precise and more reliable.

[bobbycollier]

I'm in Houston, so baro is 101 kpa as we are at Sea Level. I will take some pics when I get home and post them. Thanks again for the advice.

[bobbycollier]

Attached is a picture of my fuel regulator. It appears that there is a vacuum line attached to the regulator. It is routed to the air intake path directly in front of the throttle body (vacuum line runs above my blue scribble). Why exactly would it be routed to the air intake path? Wouldn't that simply be BARO? If I wanted a 1:1 rise with boost, wouldn't this need to be referenced to the manifold such as a T-connector to the MAP sensor vacuum line?
5038

Thanks,
Bobby

[bobbycollier]

Here are a couple of close ups if that helps...

5040
5039

[joecar]

The reference has to be on the manifold side of the throttle body, otherwise (as you said) it will be BARO always (and it becomes unreferenced -> sloped IFR).

[ChipsByAl]

Also on your boosted application the software sloped IFR becomes stuck at 1 bar. Using a manifold referenced pressure regulator will allow you to keep one IFR value.
Al

[bobbycollier]

Thanks guys