Why is a vacuum line attached to a fuel pressure regulator on some port fuel injected engines?

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Location: Cleveland, OH

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Vacuum line to fuel press reg with ITB’s

Should I run a vacuum line to fuel press reg with ITB’s? If so is it a problem to run it from one cylinder?

Racer

Join Date: Oct 2010

Location: San Martin, CA

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I don’t run a vacuum line to my fuel pressure regulator. If you do, I would think using just one ITB would be quite bad. That one ITB vacuum line would have the correct vacuum only when the cylinder was firing. The rest of the time the vacuum would be incorrect.

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We build a simple vacuum manifold that has an inlet for each ITB and an output for the FPR and Brake Booster if so Equiped. It can also balance a signal to a MAP sensor if your ECU tuning choices require it.

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Most Electronic fuel injection systems with a fuel return have a pressure regulator connected to engine vacuum source. The purpose of the vacuum source attached to the regulator is to keep the differential pressure at the injector constant. This causes fuel pressure to be lower at idle than at full throttle. This is especially important with Turbo cars where fuel pressure increases with boost. Later vehicles that do not have a fuel return us different strategies to accomplish the same thing. Some have variable speed fuel pumps and some have the return at the pump (probably other ways as well). If you only had vacuum attached at one port you would have a very erratic signal as vacuum is only generated when that cylinder is on the intake stroke. You can create a more constant signal by doing what Jeff does and connect all the cylinders together into a manifold. If you are programing the injection system you could program around the change in differential pressure but you might not get a consistent spray pattern from the injector which can cause other issues.

john

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Ive been wondering this as well. If you are running a modern EFI, why would it be preferable to vary fuel pressure based on vacuum rather than in the ECU programming?

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If you have aftermarket EFI, the vacuum hose is likely not needed. GM went away from the vacuum referenced regulators years ago. Here's what they say about it: "The fuel pressure regulator attaches to the fuel return pipe on the fuel sender assembly. The fuel pressure regulator is a diaphragm-operated relief valve. A software bias compensates the injector on-time because the fuel pressure regulator is not referenced to manifold vacuum. The injector pulse width varies with the signal from the mass air flow (MAF)/intake air temperature (IAT) sensor. "

Even later model vehicles use a returnless fuel system with fuel pressure controlled by the PCM.

Racer

Join Date: Oct 2010

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My setup includes a MoTec M130 ECU and Injector Dynamics 1050x injectors with the fuel rail pressure set at 50 psi. This combination has no need of manifold pressure compensation for the fuel pressure regulator. The MoTec can easily control the injector pulse width as required and the injectors can accommodate the pulse width changes. Who wants to add all the vacuum fittings, manifold, and hoses to the top of the engine if you don’t need to?

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If you have programable ECU and a intake vacuum reference (MAP sensor) you could change the injector pulse width to compensate . Since the vacuum referenced pressure regulator only works when there is a vacuum which is idle and part throttle cruse it would not make much difference on track car. If you look at Porsche EFI systems that had vacuum referenced pressure regulators they often did not have a MAP sensor tied to intake as part of the injection system. Programming strategy depends on intake style and boost or no boost. With individual throttle bodies because the vacuum signal (MAP) off idle changes so quickly it is often not used as a reference in these transitions. ITB programming is typically Alpha-N (throttle position) or blended strategy. Map is still a good reference with for altitude compensation. With boosted applications a MAP sensor is almost always used as it allows you to properly fuel with changes in pressure. Boosted system also typically use a manifold referenced fuel pressure regulator. If you had a fixed fuel pressure of say 30LB and ran 30Lbs of boost you would have no fuel flow from the injectors. Return less fuel systems were not necessarily added because they were better. They were a response to emissions and fuel mileage. Recirculating fuel from the tank to a hot engine heats the fuel and causes more load on evaporative emissions system. Eliminating the return line and down sizing evap hardware saves a littler weight which aids fuel economy.

john

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You could eliminate the fuel return pipe with modern engine management. Depending on the system, you could add a fuel pressure sensor and use the EFI computer to control the fuel pressure via pulse width modulation, obviating the need for a boost referenced regulator.

John's right, they got rid of the return line to help with evap emissions. The heated fuel produces more fuel vapor which then must be recovered by the evap system.

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I'm glad I asked! We got a seminar. I just knew my Electromotive (now slightly old tech) system had a MAP sensor, and I built a block to merge vacuum signals from all six manifolds to it.

Racer

Join Date: Oct 2010

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Nope. No map sensor on my setup at all and you don’t need that or a vacuum line on the fuel pressure regulator to properly tune an ITB setup. Currently, my setup is alpha-n running closed loop with an O2 sensor on each bank. The car runs incredibly well (120hp per liter at the crank) and gets excellent fuel mileage.

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My setup also has fuel pressure and temperature sensors along with intake air pressure and intake air temperature sensors.

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HI Winders, I believe what you are calling "intake air pressure" is the same thing I am calling a MAP (manifold absolute pressure) sensor. Without a map sensor you have no altitude compensation your system will run rich at altitude as there is no way to tell the system there is less air entering the engine. May not be a problem were you live. I live in Denver and can go from 5000' to 14,0000' in less then a hour. FYI at 5000' there is approximately 20% less air, so our normally asperated cars make 20% less HP, so sad. Running rich is not usually a big problem. If you tuned your car in Denver and went to a lower altitude you would run lean, that is a problem.

john

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Injector flow rate is proportional to the fuel differential pressure not gauge pressure. For example, if the manifold pressure adjacent the injector tip is less than atmospheric the differential pressure is greater than the gauge pressure measured at the fuel rail. The opposite is true if there is boost in the manifold. For this reason a compensation is required in order to set the injector time for the desired fuel mass: Constant differential pressure using fuel pressure regulator. OR Measure both fuel pressure and manifold pressure, calculate differential pressure and set injector time proportionally.

Automotive manufacturers eliminated fuel return lines due to emissions requirements and cost advantage.

Racer

Join Date: Oct 2010

Location: San Martin, CA

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John, My setup uses a MAP sensor to measure intake air pressure. But the sensor is not measuring manifold air pressure...it only measures ambient air pressure in the air box before the ITB air filters.

The intake air pressure sensor along with the closed loop lambda system easily compensate for altitude changes.

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Last edited by winders; 12-27-2020 at 08:50 PM..

Racer

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Wow, robcf, if you are right my engine should just melt into the ground......

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Scott, Unfortunately I cannot comment on the intricacies related to your fuel system or engine but what I wrote above is factual and true, nevertheless I hope your engine doesn't melt into the ground. The low manifold pressure (resultant from the piston moving down in the cylinder) is why air flows into the cylinder, it is why fuel flows through a carburettor and will effect fuel flow through a fuel injector. There are a few reasons why compensation may not be required: Constant manifold pressure - there is little to no change across the engine operating range. The fuel injector is located in an area where the manifold pressure is close to ambient - i.e. fuel injector is located above the throttle plate. Finally, the hypothesis can be tested by logging MAP vs Lambda, the test should be done at constant engine speed and load to avoid transient fuel adders creating noise in the data. With no compensation lambda should rise and fall proportional to MAP. These effects will be most noticeable at low throttle angles where mass flow is typically low (and therefore fuel mass required) and MAP is low (greatest differential pressure). Thanks,

Rob

Last edited by robcf; 12-28-2020 at 01:01 AM..