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Return style fuel system, use fixed fuel pressure or use vacuum referenced???
Well i just got my fuel system all switched over to return style. Currently i don't have the vacuum reference line hooked up so i am running 58-60psi all the time and i left my IFR table the same in the tune.... Basically i made zero tune changes and I noticed my tune stayed the exact same, meaning my LTFTs were +/- 3% as usual. So i am wondering what benefit would I have in hooking up the vacuum reference??
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From what I've read about the vacuum line, it opens up the diaphram to allow more fuel to go to the rails under wot, keeping the pressure at 58-60psi. So if you didn't hook up the line, you run the risk of going lean under wot.
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Ok. I did not realize that. Honestly i am kinda FPR stupid, i have never owned a return style setup before.
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I'm running a vett fuel filter to regulate my fuel. I see a constant 56-57psi. The tune needed to be adjusted for the non referenced fpr.
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It helps at vacuum by effectively lowering fuel pressure so your injectors act "smaller" than they really are. Exactly the same as raising fuel pressure will make injectors flow more, lowering the pressure make them flow less. Because injectors are electric solenoids with a real delay time, they can be unpredictable at low pulse widths (and highly non-linear), so by lowering the rail pressure you force them to have a desirable pulse width. And similarly at boost pressure it keeps the fuel pressure across the injector constant by raising rail pressure. Keep in mind its the goal of the FPR to keep the pressure difference across the injector the same at all times. Even though the rail pressure will be less, the pressure across the injector is the same because the engine is pulling a vacuum on the bottom of the injector which is exactly the same as pushing harder from the top.
If you do not want to run a reference line to the regulator you need to change the IFR table to reflect the increase in flow at engine vacuum. However, if your wideband shows its fine, the idle and fuel trims are probably absorbing this difference in real and expected flow. But I recommend either setting the IFR table correctly, or leave it constant and hook up the reference line. |
Originally Posted by Atomic
(Post 4956769)
It helps at vacuum by effectively lowering fuel pressure so your injectors act "smaller" than they really are. Exactly the same as raising fuel pressure will make injectors flow more, lowering the pressure make them flow less. Because injectors are electric solenoids with a real delay time, they can be unpredictable at low pulse widths (and highly non-linear), so by lowering the rail pressure you force them to have a desirable pulse width. And similarly at boost pressure it keeps the fuel pressure across the injector constant by raising rail pressure. Keep in mind its the goal of the FPR to keep the pressure difference across the injector the same at all times. Even though the rail pressure will be less, the pressure across the injector is the same because the engine is pulling a vacuum on the bottom of the injector which is exactly the same as pushing harder from the top.
If you do not want to run a reference line to the regulator you need to change the IFR table to reflect the increase in flow at engine vacuum. However, if your wideband shows its fine, the idle and fuel trims are probably absorbing this difference in real and expected flow. But I recommend either setting the IFR table correctly, or leave it constant and hook up the reference line. X2. If you hook the line up, just open table B4001 and copy the 0 cell value and paste it in all the other cells. The tune "should" remain the same. |
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