How to Choose a Fuel Pressure Regulator For EFI or Carb

By: Jeff Smith | 09/25/2020 < Back to Motor Life Home

As the horsepower race continues to escalate this places demands on many other systems. This is no more true than with fuel delivery for either carbureted or EFI engines. It follows that if an engine makes more power, it’s going to need plenty of fuel to make that power.

While much of the attention centers around fuel pumps, once the delivery capacity is achieved with the pump, the next move is to accurately ensure all that fuel arrives at the carburetor or injectors with the appropriate pressure. To do that requires a high quality fuel pressure regulator and that’s what we’ll the focus on in this discussion.

We will break down the various types of regulators as they are not all the same nor do they operate the same way. There are regulators for low pressure carbureted as well as high pressure EFI applications. Then there are deadhead (sometimes called blocking regulators) which operate differently than return style versions. There are plenty of details and application-specific fine points that are important to understand in order to design a fuel system that not only flows the desired volume of fuel while maintaining pressure, but it does so with accuracy and precision. So let’s get started!

This is a typical Holley Billet return style regulator. Inlet (1) and outlet (2) are interchangeable while (3) must always be used as the dedicated return back to the tank. Pressure changes are accomplished by loosening the lock nut and turning the adjusting Allen stud (4). The small vacuum nipple (5) is used for either multipoint EFI applications and/or as a boost reference connection for blower applications. The small 1/8-pipe plug in the center of the regulator (6) can used as a pressure tap to monitor fuel pressure.

Perhaps we should start by stating that not all fuel pumps – either mechanical or electric - require a pressure regulator. Low pressure electric fuel pumps like Holley’s classic Red vertical pump are designed to produce sufficient fuel pressure to feed a mild carbureted application without the need for a regulator. Many, but not all, mechanical pumps are also designed to operate without the need for a regulator. For most high performance engines making 450 hp or more and especially all EFI applications, a regulator is a requirement to accurately set the desired fuel pressure.

In transportation such as railroads or trucking, a deadhead is a one-way trip. The same definition is true with deadhead or one-way fuel pressure regulators. The classic application for this style regulator is a single carburetor where the regulator is positioned between the fuel pump and the carburetor to limit fuel pressure.

Simplicity is the biggest advantage of a deadhead system as there is no requirement for return plumbing. This also means a deadhead style regulated system is less expensive to build and maintain because of the fewer components, connections, fittings, and lines. Fewer connections means fewer potential leak paths. The simplest deadhead regulator offers only two ports – one in and one out. However, many modern deadhead regulators offer three ports with one inlet and two outlets. These units might appear at first to be a return style regulator, which is why it’s important to ensure that the proper regulator is chosen and installed correctly.

This is a deadhead version regulator although it may appear to be similar in configuration to a bypass style. Note that the inlet is clearly marked on the bottom fitting as are the two outlet fittings. This is a common orientation for deadhead regulators.

A return style fuel pressure regulator operates much differently compared to a deadhead system. A return regulator can be quickly identified by a single port that will be marked as “Return”. Most of these regulators are designed with a pair of inlet/outlet (I/O) ports on opposite sides of the regulator with the return port on the bottom or opposite the inlet/outlets. In a typical installation, one horizontal port is plumbed as the inlet from the pump while the outlet feeds the carburetor or EFI system and the third is the return.

These two regulators also operate completely differently. A deadhead regulator is designed to be normally open, meaning fuel will flow through the regulator and be restricted once the pressure reaches its pre-set limit. This system places an additional load on the fuel pump since it now operates at full capacity and pressure against the regulator.

For a street car that operates most of the time at part throttle, this tends to pull more amperage from the charging system and places maximum load on the fuel pump, which can reduce its lifespan. Even worse, at low speeds the fuel is heated by the pump working against this head pressure. In hot weather this can cause vapor lock issues near the pump. In a drag car, during the launch, this instantaneous increase results in a momentary drop in fuel pressure until the pump can catch up to the increase in load.

The left regulator is machined to accept what are called o-ring boss (ORB) fittings and you can see the o-ring attached to the fitting. On the right is a regulator machined to accept a typical tapered pipe thread (NPT) Holley offers many regulators with separate part numbers for either NPT or ORB fittings.

Return style regulators operate exact opposite from a deadhead. A return regulator is normally closed until the pressure limit is reached, opening the bypass outlet to return the excess volume back to the tank. This places far less load on the pump for street-driven cars because the pump isn’t struggling against high pressure on the inlet side of the regulator. Excess fuel is returned to the tank, which generally reduces fuel temperature, although some heat is picked up from the pump as it circulates. This is especially true with inline versus vertical pumps.

We should emphasize that a return style regulator cannot be used as a dead-head regulator by blocking the return port. If the return is blocked, the fuel pressure will spike, which could cause all sorts of bad things to happen.

There are very few aftermarket high-pressure, deadhead regulators. Nearly all deadhead regulators are designed to be used in 5-7 psi, carbureted applications. Many more options exist on the return regulators side because carbureted engines can also enjoy the benefits by using a return style fuel delivery system.

This is a PN 12-846 return style regulator that has been disassembled to reveal the simplicity of operation. The adjustment screw works against the diaphragm and spring pressure to set the pressure. Note also the return hole in the center. We measured this particular regulator’s return at 0.138-inch.

It might be worthwhile to point out that all or nearly all current OEM fuel systems are designed to run as a dead-head system. The difference is that an accurate fuel pressure sensor monitors the pressure and controls it by pulse-width modulating (PWM) pump operation. We won’t get into all the details on that as there is plenty of technical data on PWM control systems should you be interested in learning more about how this system operates.

Most fuel pressure regulators are designed to operate either as a dedicated low-pressure device (roughly 3 to 16 psi) or as a high-pressure regulator (40 to 70 psi). Recently Holley has released a billet fuel pressure regulator that can do either job with a range of 4 to 65 psi (PN 12-879 or 12-880). The regulator comes configured as a high volume, low-pressure regulator but can be reconfigured to control high pressure just by changing a spring that is included with the regulator. This is designed for enthusiasts who are currently running a carbureted system that may at some point convert o EFI. This saves the expense of purchasing a separate regulator.

This illustration shows how a deadhead regulator must be installed for a carbureted application along with recommendations for pre- and post-pump filters.

One function of the regulator that deserves mention here is the small external fitting that you will see on many (but not all) return style regulators. This is actually used to perform two different yet related functions. The first application is for multi-point EFI applications where the injectors are located in the intake manifold and exposed to manifold vacuum.

With the engine idling for example, manifold vacuum creates a greater pressure difference between atmospheric pressure on the outside of the engine compared to the low-pressure situation inside the manifold. The net effect of this is the pressure in the fuel line is essentially greater than gauge pressure. This requires some explanation.

Choosing a return style regulator for a carbureted engine requires a dedicated return line to the tank. While more complex, this offers many advantages. While not shown here, it’s also possible to place the regulator downstream of the carburetor. Never block the return port in a return style regulator in an attempt to convert it to a deadhead regulator. This will result in excessive fuel pressure.

As an example, if the engine is idling at 18 “Hg of manifold vacuum, this is equal to a negative or -9 psi inside the manifold. With no pressure regulator compensation, this effectively raises the gauge pressure of 43 psi to the equivalent of 52 psi (43 + 9 = 52 psi). To compensate for this, manifold vacuum is applied to the back side of the regulator which effectively balances the pressures. At wide-open-throttle (WOT), manifold vacuum is very close to atmospheric and no pressure compensation is required.

Now let’s add a supercharger or turbocharger to this engine, still fitted with injectors located in the intake manifold. Under boost, now there is pressure above atmospheric in the manifold and the opposite situation occurs where the injectors are fighting against pressure in the manifold. As an example, let’s set the boost at 10 psi. This means there is 10 psi above atmospheric pressure in the manifold and now with 43 psi of line pressure, the actual fuel pressure pushing fuel through the injectors is only 33 psi because of the 10 psi of pressure in the manifold (43 – 10 = 33 psi).

The combination of a filter and regulator is becoming popular so Holley has created this unit. It comes in two part numbers for either NPT or ORB fittings. The aluminum housing fits a 10 micron filter and the internal regulator is set a 59.5 psi. This particular version uses -8 ORB fittings but a second version uses 3/8-inch NPT connections. The part numbers are listed in the accompanying parts list.

To compensate for boost pressure, we plumb a hose from the intake manifold to the small fitting in the pressure regulator that will boost reference the fuel pressure at a 1:1 relationship to maintain our original fuel pressure. This compensation would also be necessary for a blow-through carbureted application because the carburetor vents are exposed to boost pressure. In that case, it’s best to plumb the boost reference line to the carburetor hat to ensure that the fuel pressure into the carburetor is boost referenced.

With both regulator design and pressure issues met, there are often questions regarding where to place the regulator. With a deadhead regulator the only way is to mount the regulator between the pump and the carburetor, preferably near the carburetor. Some racers will place the regulator actually on the engine next to the carburetor. For street-driven cars it’s best to move the regulator to a spot near the engine but in a location where the regulator can be isolated from excess heat.

There is far more mounting freedom when it comes to return style regulators. The regulator can be placed either upstream or downstream of either the carburetor or EFI injectors. The downstream location is what Holley engineers prefer to optimize regulator performance, but either location is acceptable. In either case, the regulator should be located near the engine itself so that the regulator is near where the pressure changes occur so it can more quickly react to these changes.

Occasionally a regulator may leak by dribbling fuel out of the pressure tap nipple. This means the diaphragm has cracked or split. This is an easy fix with a repair kit like this one with a new diaphragm and spring.

The return side of a full-flow system should also attract a bit of your attention. Low pressure carbureted applications are especially vulnerable if the return side line is undersized. For example, using a high capacity electric fuel pump to feed a carburetor with 4 psi of fuel pressure is not a bad idea. However, if the return line is undersized, (like a ¼-inch diameter line) or suffers a restriction like a kink in the line, then pressure can build up on the back side of the regulator and cause fuel pressure to drop. For example, with only 4 psi of line pressure, a 1-pound backpressure in the return line represents a pressure loss of 25 percent! A one-pound pressure in the return line on a 60 psi system is not cause for alarm as it represents barely a 1.6 percent change in pressure.

While regulators are relatively simple devices, they do require proper installation and some attention to detail. But perform a professional installation and that regulator will do a very accurate job for a long time and allow you to concentrate on figuring out how to make even more horsepower!

For this blow-through carbureted LS engine, we used a 12-846 Billet Holley fuel pressure regulator and plumbed a boost-reference hose from the nipple outlet on the pressure regulator (arrow) to the blower hat. This ensured fuel pressure would match blower boost. In this case, we were running 10psi of boost.

On return style EFI systems, you can place the regulator in the traditional location upstream of the injectors. Or, another option as show in this illustration is to place the regulator downstream of the injectors. This option offers a slight advantage in response time if that is of importance.

In this installation, we’ve mounted the fuel pressure regulator on the passenger side inner fender. Fuel from the in-tank pump is plumbed directly to the front of the engine with the braided steel line where it T’s into both fuel rails. The fuel then exits the rear of the fuel rails where the -6 lines are brought together with a Y using a single -6 line that enters the Holley billet fuel pressure regulator. Fuel returns from the regulator out the bottom and is plumbed back to the tank. This places the fuel pressure regulator downstream of the injectors as opposed to upstream.

A popular and inexpensive combination is Holley’s carbureted blue fuel pump sold with its matched deadhead regulator. When using a non-return delivery system, the pump must be equipped with a pressure relief valve to bleed off the pressure on the pump side of the regulator. The blue pump features a 15 psi internal pressure relief valve for this purpose.

A return style fuel delivery system is often advantageous for high horsepower, carbureted drag cars that experience a huge load dump on the starting line. A dead head system will generally experience a temporary pressure drop that is nearly eliminated when using a return system.

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