Thread: Fuel Pressure
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Old 09-11-2010, 5:06pm   #2
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Originally Posted by Munch View Post
I believe I read somewhere that the C5 LS1 has a fuel rail pressure of 58psi, please correct me if I'm wrong.
That is correct.
I'm wondering what would be considered an unsafe drop in pressure. 56, 50, 45psi?
I'd be concerned if dropping below 50 psi.

I am more concerned with WOT, high RPM situations but all conditions interest me.
That will be the only time you may ever notice a drop in pressure, I'll better explain why below.
Lastly, are there any components of the system other than O2 sensor readings such as the injectors that alert the PCM to low pressure conditions?
The PCM doesn't monitor fuel pressure in any way, shape, or form. Even the 02 sensors are of no help.

This will be more of a long but simple tuning explanation, but it will hopefully help you better understand what's going on in all aspects of fuel delivery.

The 02 sensors are only "in the loop" during normal street cruising or "light throttle" conditions. The feedback from them are helping the PCM control fuel to the engine to keep it operating at stoichiometric mixture (~14.7 AFR). We call this closed loop mode, because that is the time the PCM is using the 02 feedback data to constantly adjust fuel delivery to the engine to maintain stoich, for best economy and emissions.

The stock narrowband 02 sensors are only accurate at, or very near stoich (+/- 1). Once you get outside of that narrow AFR window, they become rather useless.
Because of this, when you step on the big pedal and go WOT, the PCM switches to open loop mode.
The feedback loop is open, because the 02 sensors are now being ignored. At this point, we also introduce PE - Power Enrichment mode. This is what triggers the PCM to add extra fuel and richen up the mixture, usually to around 13:1 or richer during WOT to keep chamber temps down and hopefully not detonate or fry a piston.

This is all done using several different fueling tables within the PCM tune. These tables are all of fixed value AFR (or AFR modifier values) set by the tuner. At any given combination of operating conditions and tables referenced, the PCM is ultimately "told" to fuel the engine to a desired AFR (ie: 12.9:1).

So how does the PCM know it's achieving that desired AFR? It doesn't. Not stock, not modified.
This is why we tune, and why we use widebands, so we can more accurately determine what's actually happening and then set those fuel tables accordingly, based on scanning, measuring, logging actual running (dyno or street). The goal being to match desired AFR to actual measured AFR. If the tuner can do that, all is well, even if fuel pressure is declining up top (an unknown w/o measuring it too).

And speaking of fixed values, another one of those fuel tables is the IFR table. Injector Flow Rate. This table is how the PCM is told how much fuel is flowing through the injectors in the whole spectrum of operating conditions.

The IFR table is real simple, here is mine as an example:

Injector Flow Rate (Lbs/Hour) (right column)
Manifold Vacuum in kPa (left column)

0      43.899431
5      44.147450
10    44.395470
15    44.705494
20    44.953513
25    45.201533
30    45.511557
35    45.759576
40    46.007596
45    46.255615
50    46.565639
55    46.813659
60    47.061678
65    47.309697
70    47.557717
75    47.805736
80    48.053756
Now, here's where I kinda get to answering your question. I have 38# injectors, so why does that table start at 44# and go up to 48#?

The first reason is because of fuel pressure. Understand non-GM and aftermarket injectors are usually flow rated at 43 psi of fuel pressure. But since our cars run at 58psi, that additional pressure causes them to flow more fuel than their advertised flow value. Munch, since you stated you have the 42# (SVO green tops?), you'd probably find your IFR table starts somewhere around 48-49# and goes up from there. That's what your 42s actually flow in your engine, at your fuel pressure.

The other thing that alters injector flow, is manifold vacuum or pressure. This is the other column of the above table. Notice the table above starts at 0 vacuum (the inverse of MAP). That would represent a WOT condition.
During idle or cruise, there's more manifold vacuum present, and that works just like added fuel pressure, it is helping to draw the fuel out of the injectors during the intake charge. So as manifold vacuum increases (MAP drops), actual flow through the injectors increases.

(That was the simple N/A explanation. Boost is a whole 'nother topic, but suffice it to say fuel flow isn't as easy for their injectors when faced with many lbs of intake pressure).

The bottom line is the PCM absolutely needs to "know" this flow rate to correctly control the injector's pulse width (open time), and squirt the correct amount of fuel being called for to reach that desired AFR.
The IFR table is a key player, one of the very first things to set when tuning, because any change made to it, directly affects every other fueling table setting too, and when in open loop mode, that's all the PCM is relying on to prescribe fuel delivery to the engine. There is no feedback, other than perhaps the knock sensors.

So now that you have an idea how the IFR table works, and how the PCM uses it, realize any change in actual fuel pressure directly changes actual IFR too, and if the PCM doesn't know about that change (no tuning change done to reflect it), everything else in the tune instantly becomes inaccurate as a result.

If the fuel pressure is falling off up top, it CAN still be tuned to compensate for that change (even without knowing or monitoring fuel pressure). Seeing fuel pressure fall off does not necessarily mean you're running lean, so long as you're able to control and maintain desired AFR with injector duty cycles not exceeding 100% when tuning it.

ps, The cliff notes: Bolt stuff on, take car to Ed. Enjoy.
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