Quote:
Originally Posted by C5GTO
Ed: thanks for your insightful and detailed explanation. It makes the most sense to me for the symptoms I'm seeing.
The EFI pressure regulator is an integral part of the injector fuel rails on this EFI system. The fuel rail was provided in the EEC-IV EFI kit I bought from Ford Motorsports back in the middle 90's. The pressure regulator has a vacuum line running to it. As I recall from when I was first installing this EFI, it uses the vacuum signal to adjust fuel pressure in the rail/injectors in an effort to predict fuel squirt volume at the injector tip in varying vacuum conditions. I think the objective for the Ford EFI engineers/designers is to have higher fuel pressure at low vacuum and lower fuel pressure in high vacuum conditions such that a predictable amount of fuel will leave the injector tip even when the engine vacuum changes. If the pressure regulator were moved from it's present position, then I'm guessing the ECM programming would need to also change as it was optimized during dyno runs on the current setup.
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Joel,
You've been doing homework, haven't you?
The pressure regulator on the fuel rail is used essentially, as you suggest, with a few differences. Ford has a preferred fuel system pressure which they operate all their EFI systems at and rate all their injector flow rates at. That pressure is 39.15 psi, or in BAR terms (if you think that way), it is 2.7 BAR. This metric is essential in determining the pulse width the ECU wants to command the injector to operate at.
At idle, the engine will run with its max intake manifold vacuum. It will not be an absolute vacuum but a partial vacuum. Essentially what we are doing is choking off the engine's air supply. Today Ford uses fairly fancy Mass Air Flow (MAF) based systems to precisely measure airflow for emissions compliance reasons. Back in the day, when they designed your EFI system, they were 'evolving' to what they use today.
Your system also measures and approximates intake air mass by using Boyles Law (remember High School Chemistry?). To do this, they need to know the Intake Air Temperature (IAT) in the manifold and the air pressure or vacuum in the intake manifold. To get these numbers, they use a relatively straightforward air temp sensor and a Manifold Air Pressure (MAP) sensor — essentially an electronic barometer.
With that information, they can do a reasonably accurate calculation of the air mass in the cylinder at any given time. The calibration engineers have target Air Fuel Ratios (AFRs) they are shooting for to meet both emissions and driveability targets. They want to operate the injectors in a delivery model that will not command a pulse width so small the injector can not respond, nor so large the injector bangs up against a 100% duty cycle where the injector coil is on all the time. That magic pressure Ford has hung their hat on is 39.15 psi.
The 39.15 psi assumes the injector is firing into an air space at an ambient pressure of 29.92 inches of mercury. So what happens when the injector fires into a partial vacuum like the engine sees at various throttle positions and, in particular idle?
If the idle vacuum is 14 inches of mercury suddenly, the pressure across the injector nozzle looks ambient (29.92 in Hg) plus the vacuum in the intake manifold. The vacuum is literally sucking the fuel out of the fuel rails. The vacuum referenced pressure regulator adjusts its fuel rail pressure to maintain a constant 39.15 psi delta pressure across the injector nozzle to maintain a constant fuel pressure drop across the injector to maintain emissions compliance.
It turns out that this kind of control is needed not only for emissions compliance but, as luck would have it, engine control and vehicle driveability. To do this, Ford uses a manifold referenced fuel pressure regulator. The small rubber hose from the regulator to the manifold provides a manifold vacuum reference for the regulator so it can maintain a constant 39.15 psi delta pressure across the injector nozzle.
Lots of words, I know. The good news is that you can replace the OEM vacuum referenced fuel rail pressure regulator with an Aftermarket vacuum referenced equivalent and place it wherever you want. You should remove the OEM regulator from the system, or it may be a restriction. If removal is a plumbing problem, then you can gut the OEM regulator, so all it ends up being is a bulge in the fuel delivery lines.
So, where do you get these aftermarket regulators? Summit and Jegs are the easiest and most cost-effective. Don't forget you are not fueling a 1000 HP monster. You are probably in the 550, maybe 600 HP window. Most any of the aftermarket regulators on Jegs or Summit websites will work just fine for you — and co-incidentally better than the OEM unit.
When you install the aftermarket regulator, you set it with the vacuum line disconnected from the manifold and the manifold port line plugged. Start the engine and turn the adjusting screw on the regulator until a pressure gauge you have somewhere on your fuel rail reads 39.15 psi. BTW most fuel pressure gauges do not read that accurately, so you will adjust to a whisker over 39 psi. When you have, shut the motor off. Unplug the vacuum line and connect it to the vacuum barb on the pressure regulator — you're done!
With respect to fuel tank venting, it is an absolute necessity — but probably not for the reasons you think and possibly not the way you might suspect.
Again because of the federally mandated emission regulations, OEM fuel tanks are not allowed to vent unfiltered gasoline vapors into the atmosphere. Your system may use a charcoal filtering element to allow this outbound venting. Without an inbound vent, it will eventually become impossible to be draw gas from the fuel tank. The vacuum above the gas will easily overcome the drawing capability of any fuel pump you can use.
To fix this problem, the OEM venting model allows for atmospheric air to be drawn in, but unfiltered gas tank vapors must not be allowed to escape. The fix is accomplished typically with a valve that uses a spring-loaded check / shuttle or ball to be pulled down to allow air entry and pushed back up to prevent gasoline vapor escape to the atmosphere.
Because race cars are not subject to emission regulation, they do not have to worry about this. A racing application can vent the tank to the atmosphere to prevent the internal pressure build-up that creates the whooshing effect as you open the cap. If you are not subject to emissions testing, you can also.
Notwithstanding the fact you can, the whooshing is still signaling an overheating of the fuel, and all that nice $10+ per gallon 110 octane race gas is being reduced to the octane of whatever its base stock was. You're throwing good money away and potentially putting your engine at risk. Which brings us back to keeping the tank and gas inside cool, and not boiling off the light ends at the pressure regulator under the hood, returning them as gasses back to the fuel tank.
p.s. At the rate pump gas price is rising you may be able to buy $10/gallon pump gas shortly. Either way the vaporization of the light ends brings you back to base stock octane numbers. Thoughtful location of the pressure regulator and sensitivity to unnecessary heating of the gas tank contents will help in mitigating but not necessarily eliminating the problem.