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playing around with my tune a little bit and i had a thought

i have a fairly mild cam, which creates slightly lower vacuum than stock, as well as an aluminum flywheel, which obviously creates a fair vacuum fluctuation at idle, as it fights to keep running.

everyone must have noticed that a cam creating lower vacuum tends to run quite a bit richer on the low end with the stock tune. part of this is obviously the MAP reading lower than usual, which tends to richen the mixture.

my idle is a bit rough, even for having an aluminum flywheel, despite the fact i'm running a 900 idle with an M6. i have corrected for it a little bit, by backing off the timing corrections at idle (so it's not fighting itself too much), but i've never been really happy with it.

is there another part to the equation?

i noticed that with a fuel pressure gauge connected at idle, pressure is fairly unstable, surging between 36 and 44 psi.

obviously with the regulator unplugged, idle gets a bit on the rich side, but it's so much smoother!

then i noticed applying a solid 30" of vacuum to the regulator actually smooths out idle a lot

further experimentation revealed that the stock vacuum hose for the regulator actually provides a slight pulse restricting effect, kind of like the small orfice on a vacuum gauge (because of the small size of the line). the larger silicone line that i replaced it with creates more vacuum flutter @ the regulator.

is it possible that the fuel pressure regulator is fighting with the inconsistent cam/flywheel surge, and making things worse? or am i overthinking it...

does anyone just unplug the damn regulator vac line and tune their idle with fixed fuel pressure? i can't really see any reason not to at this point..
 

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Think of a fuel injector as a nozzle the amount it flows with everthing constant is due to the pressure difference across it. The pressure regulator makes this differential constant with the. Changing vacuum in the intake . If u leave the vac reference off it or will run rich everywhere besides when the intake pressure is equal to barometric pressure
 

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I guess what I mean is, the pcm guesses a injector pulse with based on the air it calculates to be in each cylinder and if the injector is flowing a differnt amount of fuel for that pulse width bc of different pressures across the injector. It will make the delivered fuel wrong from what the pcm thinks , maybe o2 has the capacity to correct who knows
 

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well obviously the stock tables are calibrated to account for that fuel pressure difference at various manifold pressures..

the ecm does not have any magic knowledge of how fuel pressure works, it's all a correction factor that must be dealt with when tuning every fueling table that deals with map.

im just starting to think that letting engine vacuum have DIRECT control over AFR in any way is dumb, when you have low or unstable vacuum to deal with.

the ecm has much more precise control with map based fueling tables, why not give full control to the ecm?

can anyone else figure a reason why just running max fuel pressure with the stock regulator would be a bad idea? i dont even see a reason to buy an adjustable one if i can just basically cap the thing off...
 

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Discussion Starter #6
ok here's a question i'd love an answer to:

what is the benefit in variable, map driven fuel pressure in an engine design like this?
 

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Hmm what I'm trying to say is the pcm assumes a flowrate or slope of the injector bc of constant differential pressure besides inj volt offset and what not . I guess computer controlled map driven fuel pressure would be more adjustable/ accurate but that is what we have its just controlled mechanically by the fpr
 

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You would still have a bouncing map sensor reading as opposed to a an actual bouncing vacuum reference to the fpr
 

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ok here's a question i'd love an answer to:

what is the benefit in variable, map driven fuel pressure in an engine design like this?
Your fuel pressure rises with manifold pressure, least thats why I run a vac referenced one :D

Only real disadvantage I can seem to find is that at WOT or required full flow times your pressure at the rail will fall ever so slight. The article mentioned that the LS1 4th gens and vettes will lose ~6psi to achieve full flow at WOT. So a tune for this would def need to be done.

Granted you are not changing the system plumbing but IMO dead head setups like the LS1 are a step back from the semi decent return LT1 style fuel system.



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I don't know if this info will be of any help, but older Ford multi port fi systems had constant fuel injector on times and raised or lowered fuel pressure to change mixture going into cylinders.
 

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Since we are talking theory.. IMO.. Don't forget there are two factors to keep in mind, rail pressure and intake vacuum. Together they are the total pressure difference through the injector. At idle 20Hg of vac is about 10 PSI so you end up with 38 lbs pushing from the rails and 10 sucking from the intake. The MAP adjustable regulator helps smooth out that difference as the intake drops the rails climb to 43. I believe with a static pressure it could be tuned to work, but it seems the injector offsets and patterns would be more drastic. And rich at idle and lean at WOT would be amplified. Newer, faster, more sophisticated type ECM/ PCM setups can probably handle static fuel pressure injector requirements , where the old slow pre 286 , 64K chip stuff we are talking about was fairly limited.
 

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Yet one more reason to convert to a 24x system and run GM's newer 0411 PCM! The LS engines run a returnless fuel system which is a fixed pressure. All manifold pressure/fuel rail pressure compensations are done electronically.

Bobdec is exactly right that the vacuum referenced fuel pressure regulators are designed to keep fuel the fuel rail pressure to manifold pressure differential constant. As vacuum decreases (manifold pressure increases), fuel rail pressure needs to increase by an equivalent amount to keep the pressure difference across the fuel injectors constant. With the dramatically better computing power of the newer PCM's, this is no longer necessary as it's a simple process electronically to adjust the pulse width to compensate for changing manifold pressures.

On my 396 build, I'm setting mine up as a returnless system (only one line from the tank to the engine) set a 60 psi. My fuel rails are set up in parallel now and there is no more regulator at the fuel rail. From there, all manifold vacuum compensations can be fine tuned with my HP Tuners software.
 

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Out of curiosity, dosent the ls1 have a set pressure regulator that returns to the tank after the fuel filter? By set I mean it has no vacuum reference just a spring opperated relief type valve. I thought I rememberd seeing that on a c5. Or else how would the pcm know what pressure the suppy line is at. Are these pumps pulse width modulated also?
 

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Out of curiosity, dosent the ls1 have a set pressure regulator that returns to the tank after the fuel filter? By set I mean it has no vacuum reference just a spring opperated relief type valve. I thought I rememberd seeing that on a c5. Or else how would the pcm know what pressure the suppy line is at. Are these pumps pulse width modulated also?
No, the PCM just commands an "on" signal to the pump, and it just runs at full speed all the time. The PCM also does not know what the fuel rail or line pressure is, not fancy like a diesel.

The LS1 does have a set regulator, some of them are at the tank and some are at the fuel rails, both do not see or care what intake vacuum is.



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