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C5 Open Discussion General and technical C5 Corvette discussion - ownership, maintenance, repairs, modifications, tuning |
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09-11-2010, 5:43am | #1 | ||||||
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Fuel Pressure
I believe I read somewhere that the C5 LS1 has a fuel rail pressure of 58psi, please correct me if I'm wrong.
I'm wondering what would be considered an unsafe drop in pressure. 56, 50, 45psi? I am more concerned with WOT, high RPM situations but all conditions interest me. 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? Thanks
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09-11-2010, 5:06pm | #2 | ||||||||||
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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: Code:
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 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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09-11-2010, 5:54pm | #3 | ||||||
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That was the best explanation I have read to date, thanks. The only thing I didn't get is Fuel Injector duty cycle, but that may be because you didn't cover that, right?
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09-11-2010, 6:48pm | #4 | |||||||
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Duty cycle is basically the % of engine revolution the injector is held open. It is the ratio of IPW (Injector pulse width-open time), and engine RPM, the time allowed before the next injector cycle begins). In the quest to satisfy desired AFR, as engine load and RPM go up, and because IFR is fixed, the PCM commands more fuel by increasing the Injector Pulse Width (open time) to feed more fuel. But obviously the injector can only be held open for so long, before the engine spins around, and that injector is called to fire again for the next cycle. The higher the engine RPM, the less time the injector can remain open, the higher the duty cycle becomes. At ~100% IDC, the injector becomes static and never closes. Injectors should ideally be sized to achieve about 80% IDC at peak RPM. Exceeding that is ok, but 100% is generally considered the absolute limit. Over 100% means the PCM is calling for more fuel than the injector can possibly deliver, and again, although measured AFR may still be good, seeing IDC near/at/over 100% is clear indication you need more injector and should definitely upsize them. |
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09-11-2010, 7:02pm | #5 | ||||||
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Looks like you were right. Bolt stuff on, take car to Ed. Enjoy.
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09-11-2010, 7:36pm | #6 | ||||||
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Barn Stall Owner #7 Barn Raising I,II,IV NCM Supporter '13,'14
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Honestly, you would be a good candidate for buying your own tuning software.
You are asking the right questions, you are interested in understanding what's going on, and you are buying the tools you need to measure these things. The only thing you are lacking now are the tuning tools to change and correct things yourself. I'm no pro tuner by any means and will never claim to be one, but just having the ability to tune my car is some of the best money I ever spent. I've learned a LOT just because of that, and i think you would too. |
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09-11-2010, 8:13pm | #7 | ||||||
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It would be nice to be able to make minor adjustments in instances like this one. Taking it to Ed is not cheap and it eats up a lot of time as well.
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