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Selecting a fuel pressure regulator to ensure that your boosted vehicle has the correct amount of fuel pressure doesn't have to be a daunting task. The selection process requires an understanding of what the regulator is doing when the engine operates properly — and why.
A naturally aspirated EFI application typically uses fuel pressure set at the regulator around 43.5 psi, while fuel pressure in a boosted application base fuel pressure can be significantly higher, and rises with boost pressure, typically in a 1:1 equation. For every pound of boost pressure increase, the fuel pressure increases one pound as well, and there's a particular reason for that requirement.
"A fuel injector is just a valve — you open and close it fast or slow depending upon how much fuel you need, and you hold it open for longer or shorter periods. You have the pressure above the injector. When you open the injector, it sprays into the manifold, which, on a naturally aspirated engine, is either in vacuum or at atmospheric pressure (14.7 psi)," says Ryan Witte, Electronics Systems Specialist at Holley Performance Products.
For lower-horsepower combinations (think mild street builds and EFI conversions), the 12-882 die-cast regulator works well. It has 3/8-inch NPT inlet, outlet, and return ports and references boost 1:1 through the side port.
As a theoretical aside, let's consider what would happen if you had a fuel injector trying to spray 43.5 psi of pressure into an intake manifold with 60 psi of pressure. Instead of the injector spraying fuel, it would ingest air as the pressure is higher on the manifold side than on the fueling side. Make sense? This concept plays a part when trying to fuel a boosted engine properly.
"When you take that to an engine situation, you end up somewhere in the middle with 60 psi of fuel pressure and 30 psi of boost. Now you have half of the pressure going into the engine than you used to have because you have 30 psi [of boost pressure] pushing the other way," he says.
The 12-848 Dominator regulator matches up with Holley Dominator in-line fuel pumps and, as with all Billet EFI regulators, is constructed from 6061-T6 billet aluminum to provide durability. Each of the Holley Billet EFI regulators also works correctly with other pumps that are appropriate for the application. This regulator would also work well with a brushed dual-pump application.
This pressure change is the sole reason that boost referenced regulators are used: as boost pressure rises, the differential between the fuel pressure and the engine's internal pressure stays the same, so the injector doesn't have to work harder to supply the fuel to the engine. The term to describe this is called Delta Pressure, which is the pressure across the injector. Put simply, you are ensuring that the pressure increases in the engine and the injector cancels each other out, so the injector continues to behave as if it were at the base pressure.
"Ideally, you want the Delta between these two pressures to be constant. The engine management system takes in the different inputs from its sensors. It knows the injector size, the fuel pressure, and how much air the engine is consuming. From that, it calculates an injector opening time in milliseconds and checks its calculations using the oxygen sensor. Then it uses the closed-loop control to modify the injector opening time to achieve the targeted air/fuel ratio," says Holley Project Engineer Matthew Sosa.
The differential pressure must be consistent, so the engine management system can depend on its calculations to be accurate.
Setting the regulator up to perform correctly requires boost pressure to be applied correctly so the regulator can do its job. Every bypass regulator has a spring and diaphragm; the set screw is used to set base pressure. The user selects base pressure by screwing in the set screw while watching the gauge; the screw applies pressure to the diaphragm.
Fuel enters the regulator's internal cavity and meets the diaphragm, which creates pressure. Once that pressure overcomes the spring pressure set by the user and unveils the return port, fuel bleeds off and returns to the tank. The fuel flow only occurs in the bottom half of the regulator; the top half contains the spring and the reference port, which is where boost enters the scenario.
"You need to have a line that connects from the barb on the cap and plumb it to an appropriate manifold vacuum/boost port," says Sosa.
The VR Series 12-851/12-864 fuel pressure regulators are available in 2 and 4-port configurations to support more than 3,000 horsepower. Use this regulator with Holley's VR Series fuel pumps or other similar units, including belt-drive pumps. The 2-port regulator features -10AN O-ring inlet, outlet, and return, while the 4-port version has one -10AN O-ring inlet and four -08AN O-ring outlets for ultimate fuel flow. These are adjustable from 40-100 psi of base pressure. Regardless of spring length, it performs the same function — to set and maintain the base pressure. The diaphragm is constructed of nitrile material for long life.
Adding boost pressure to the diaphragm closes the valve to the return and provides higher pressure to the injectors and rail when necessary. When boost pressure is low (or vacuum is present), the valve is open, and all excess fuel above the set base pressure can easily return to the tank through the regulator's bottom port. In other words, when higher base pressure is set, the incoming fuel volume needs to work harder to press upward on the diaphragm to uncover the hole at the return port and begin to return to the tank.
Conversely, boost pressure applied to the diaphragm combines with the previously set base pressure to force the diaphragm downward and close off the return hole. This action increases pressure in the circuit to deliver the appropriate fuel volume and pressure to the fuel rail as necessary.
"Constant pressure is maintained by bleeding off the fuel that's unnecessary to make the appropriate pressure. More flow in than flow being consumed means pressure goes up. The regulator bleeds off any extra fuel from the pump that isn't required to support the engine's current performance," says Sosa.
This ties in with the explanation regarding how the engine management system monitors engine parameters and makes its real-time adjustments. We can think of the regulator as a constantly-variable orifice that ensures the engine always has the fuel supply it needs.
Along with the 12-851 "tall" regulator, the 12-846 and 12-848 regulators make up Holley's Billet EFI bypass regulator lineup. This side-by-side comparison shows the main difference between the 12-846 and 12-848 units. 12-846 utilizes dual -08 AN O-ring inlets and a -6AN O-ring return port, while 12-848 regulator features -10AN O-ring inlets and a -8AN O-ring return.
There are two options for plumbing a bypass-style regulator. The first has the fuel pump feeding the regulator, then traveling on to the fuel rails and injectors. The second has the regulator mounted post-rail, and after the injectors, so the rails and injectors get the pump's full flow without any restriction beforehand.
In the first scenario, the regulator uses both outlets, and in the second scenario, only two regulator ports are used as there is a Y or crossover used to feed the rails. The user can choose to feed one or both rails and then the second through a crossover, or feed both rails simultaneously and then Y back together into the regulator's inlet and use a traditional return line.
"For a street application, you can plumb the system either way. I would recommend a flow-through rail setup with the regulator post-rail in a racing application with large injectors. This ensures that the pressure is the same through the rails — there is fuel continually flowing through the rails at all times, from the pump through the rails to the regulator and back," says Sosa.
It is preferred for the boost reference to be plumbed directly to an intake manifold source or connected to a boost manifold plumbed directly to the intake source. The vacuum/boost reference is the most critical portion of ensuring that the regulator works properly when boost pressure rises at the engine.
When using forced induction, it is critical for the fuel system to be up to the task of supplying the engine with enough fuel volume to support the increased power level. Referencing the fuel pressure to the boost level helps ensure that the fuel supply is adequate when demand increases at the engine.