How To Prepare Your Project For Conversion To Electronic Fuel Injection

10 min read

How To Prepare Your Project For Conversion To Electronic Fuel Injection

10 min read

From increased drivability to easily accommodating forced induction, there are numerous benefits to installing a modern electronic fuel injection system on your project. While the process is pretty straightforward for most hot rodders, the devil is in the details. The seemingly endless list of fuel system and ignition options can seem daunting, but we'll try and take the guesswork out of it in this article. We'll tackle the process of preparing your vehicle for fuel injection by covering fuel system choices, oxygen sensor installation, and ignition options.


What's the Maximum Horsepower?

Two major factors determine how much fuel your engine needs: horsepower and Brake-Specific Fuel Consumption (BSFC). BSFC is how much fuel your engine needs per horsepower. Naturally aspirated engines need less, while forced induction engines need more.

Tip: It's OK to overestimate a little bit, as it's better to have a safety cushion instead of leaning out the engine because the fuel pump can't keep up.

EFI conversion fuel pump flow

There's a formula you can use to determine your fuel flow requirements based upon typical BSFC and the weight of your fuel, but don't worry: we've done the math for you and made a handy chart so you can quickly determine how much fuel flow you need. Note that there are different lines for naturally aspirated, nitrous, and boosted engines running gasoline or E85. Some fuel pumps are rated in gallons per hour (GPH, the axis on left), while others are rated in liters per hour (LPH, the axis on right).

What's the Maximum Fuel Pressure?

As fuel pressure increases, fuel pump flow decreases. Some fuel pumps are rated at fee flow, or zero fuel pressure, so you need to know if a fuel pump can supply the needed flow at the maximum working pressure. Different fuel injection systems are designed for different fuel pressures. Most Holley fuel injection systems are designed to run at 43 PSI or 60 PSI. Check the instruction manual for your specific kit.

EFI conversion pump performance data

Most fuel pumps list their flow at a certain pressure. Bonus points for providing a graph, like this one for Holley's HP series of in-line electric fuel pumps. Note: If your engine has forced induction, don't forget to add maximum boost pressure on top of the nominal fuel pressure rating! For example, a fuel pump sized for 43 PSI fuel pressure with 15 PSI of maximum boost will need to flow the required fuel at 58 PSI. (43 PSI + 15 PSI = 58 PSI)

In-Tank or External Fuel Pump?

While carbureted engines usually have engine-mounted mechanical fuel pumps, EFI systems use electric fuel pumps mounted in the tank or in-line between the tank and the engine.

Tip: External pumps should be mounted as low as possible and close to the tank to assist in pump-priming and resist vapor lock.

Option 1: In-Tank Pump


Fuel cools pump

Pump noise muffled by fuel and tank

Less plumbing complexity

Some setups use an in-tank fuel pressure regulator and return to simplify plumbing


More difficult to service

EFI Conversion basic layout

A great option for converting a carbureted car to EFI is to use a Holley Sniper EFI Conversion Fuel tank. Not only is it a great time to replace your crusty old fuel tank, but Sniper EFI Conversion Fuel Tanks incorporate OEM-style fuel sending units so you don't sacrifice your fuel gauge. The internal 255 LPH fuel pump module includes a 100-micron filter sock.

EFI pump module

If you want to re-use your existing fuel tank or have a vehicle for which a Sniper EFI Conversion Fuel Tank isn't available, Holley offers In-Tank Retrofit Fuel Modules. These units replace your original fuel pickup with a 340- to 525 LPH fuel pump and include a Holley HydraMat fuel sock. They are incredibly easy to install and almost 100% universal based on their cam-lock design and 1in compressible gasket.

Option 2: External Fuel Pump


Easier to upgrade or service

Wide range of pump configurations and sizes


Can run hotter

Susceptible to heat from the exhaust or other components

Noise is less muffled than with an in-tank pump

External fuel pump layout

Holley EFI pump

External fuel pumps are available in a wide range of configurations and sizes. For example, Holley's three HP series of pumps free flow 65, 80, or 100 gallons per hour.

Holley EFI two-stage fuel pump

Holley's three Dominator series pumps pick up where the HP Series leaves off by moving 130, 160, or 200 gallons per hour. They feature a "two-stage" design: one pump is used for low demand situations (like idling and cruising) and the second pump can be activated when needed (when boost builds or nitrous is engaged).

Holley VR2 Fuel Pump

The ultimate fuel pump for mega-horsepower vehicles is Holley's VR1 and VR2 fuel pumps. While the pump's brushless technology runs quieter, cooler, and draws less current than conventional electric pumps, they can maintain pressures of 130 PSI and up to 335 GPH flow! The dual pump design of the VR2 allows options for staged engagement, shared duty cycle, or primary and reserve plumbing setups.

What Type of Fuel Pressure Regulator?

Most EFI systems require a regulator to provide a steady fuel pressure across the injectors. One of the biggest challenges when converting a carbureted car is handling the additional plumbing of an external regulator and return line. There are a few options.

Option 1: Use an In-Tank Retrofit Returnless Fuel Module System

Using an internal fuel pump with an integral regulator eliminates the hassle of plumbing an external regulator and return line. It's a direct replacement for your factory fuel pickup, so the remainder of your fuel system can remain intact. It's a great solution for installing EFI on a carbureted vehicle for which no other fuel tank options are available, or where mounting an external regulator and return line isn't possible. Fuel pressure cannot be adjusted, and since the fuel isn't circulating beyond the tank, this system is more susceptible to vapor lock. Also, actual fuel pressure at the fuel injectors can vary, as the column of fuel from the tank to the engine has some inertia and frictional losses and the fuel pressure is regulated back in the tank.

Retrofit Fuel Pump Module diagram

Retrofit Fuel Pump Module

In cases where it's not possible or convenient to mount an external regulator and return line, Holley offers "returnless" fuel pump modules with an integral fuel pressure regulator. The extra fuel pressure is bypassed even before it leaves the tank.

Option 2: Mount an External Regulator Between the Fuel Pump and the EFI System

If space and the vehicle allows, a better method is to plumb a regulator externally, between the fuel pump and the engine. Installing an external fuel pressure regulator allows fuel to circulate in and out of the fuel tank to keep the fuel cool and resist vapor lock. Servicing and adjusting the fuel pressure regulator is easier since it's externally mounted. However, the fuel from the regulator to the engine doesn't cycle, so vapor lock at the engine is still possible. The fuel pressure can still vary at the engine, though the loss is a function of the distance between the regulator and the injectors. If you're using your existing carbureted fuel tank pickup, you'll need to find a way to return the fuel to the tank.

Tip: The closer the regulator is to the engine, the greater the portion of fuel that is circulated and less prone to vapor lock. Placing the regulator closer to the engine also reduces fuel pressure variability at the injectors.

EFI Conversion External Fuel Pressure Regulator

Holley Filter

A convenient solution to plumbing a vehicle is using a Holley Fuel Filter Regulator. It combines a 10-micron fuel filter and a 60-PSI regulator in one convenient package. Since fuel pressure cannot be referenced to manifold pressure, regulators like this are best suited to carburetor-replacement, or "throttle body injection" conversions.

Option 3: Plumb an External Regulator Between the Injectors and the Return Line

The ideal solution is to mount the fuel pressure regulator (such as Holley's 12-880) after the engine and in-line with the return back to the tank. With this setup, all the fuel continually circulates between the tank and the engine, so fresh, cool fuel is always flowing to the fuel injectors–greatly reducing the chances of vapor lock. As with any external regulator, if you're using your existing carbureted fuel tank pickup, you'll need to find a way to return the fuel to the tank.

Tip: Don't take any "advertised" or "pre-set" fuel pressure ratings for granted. Install a gauge to set and verify the fuel pressure!

External Regulator After Engine In Return Line

Holley 12-845 regulator

Holley offers adjustable fuel pressure regulators that not only allow the fuel pressure to be changed but include a vacuum/boost reference port so the fuel pressure across the fuel injectors stays relative to manifold pressure. This is recommended for "port-injected" setups where the injectors squirt fuel down the intake runners.

Filters are Essential!

Fuel tank crud kills fuel pumps and clogs injectors, so it's vital to use a 100-micron filter before the pump and a 10-micron filter between the pump and the injectors.

Tip: Make sure the filters you choose can flow 50% or 100% more (double) the flow of your fuel pump, otherwise they'll restrict fuel flow as they clog, and all the money you spent on your high-flow fuel pump is wasted.

Holley HydraMat

If you're using an in-tank fuel pump, the included pump sock or Holley's HydraMat are excellent 100-micron pre-filters.

100 Micro Flow Inline Filter

For external fuel pump setups, Holley offers a wide variety of 100-micron and 10-micron in-line fuel filters in varying flow rates and fitting sizes.

What Fuel Line Size?

It's important to match the fuel line size to the flow rate of the fuel pump as to not restrict flow.

Tip: A good rule of thumb is to match the fuel line size to the outlet size of the pump. For example, if the pump has a -6 AN outlet, the fuel lines should be at least that size.

Note: When using an external regulator, it's OK for the return line to be slightly smaller, as when fuel demand is highest, less fuel is returned to the tank.)

Fuel Line Size Guide

Here's a handy chart to identify fuel lines that are big enough for your application.


How do I Install the Oxygen Sensor?

The wide-band oxygen (O2) sensor is one of the most important parts of a fuel injection system. Without the O2 sensor providing feedback, the EFI system can't tune itself.

Some EFI systems use two sensors (one for each cylinder bank), while others use one.

Tips: The sensor(s) should be mounted more than twelve inches from the open end of the exhaust pipe, and close to the engine. It's also critical to ensure there are no exhaust leaks in the system as they can skew the critical readings being sent from the O2 sensor.

Option 1: Use a Weld-In Oxygen Sensor Bung

O2 Sensor Bung Weld In

The most common way to mount the oxygen sensor in the exhaust system is to drill an appropriate hole and weld a bung in the exhaust pipe.

Clamp-On O2 Bung

Option 2: Use a Clamp-On Oxygen Sensor Bung. If you don't have a welder handy, Holley has this nifty clamp-on oxygen sensor bung kit. Kits are available to fit the contours of 2-1/4", 2-1/2", and 3" exhaust pipes.

O2 Sensor Clamp-On External View

After drilling a 3/4" hole, the Clamp-On Oxygen Sensor bung saves you the hassle of welding in an oxygen sensor bung.

O2 Sensor proper layout diagram

So condensation doesn't collect in the oxygen sensor, it's important to angle the oxygen sensor installation up at least ten degrees from horizontal.


What Ignition System Will Work with EFI?

Getting your ignition system ready for EFI can be simple or sophisticated: it all depends upon what ignition system your vehicle has, and if you want the EFI system to control ignition timing.

Option 1: Leave it Alone

If you don't want to mess with success, you're welcome to leave your existing ignition system intact. The EFI system will need some type of signal from the ignition system or tachometer to run, so check your EFI system's instructions on how to hook it up.

Holley EFI Ignition 1

Option 2: Have the EFI System Control Timing

Giving the EFI system control over timing opens up more options in tuning. Not only are timing adjustments easier to make, but the EFI system can use ignition timing to stabilize the idle. On forced induction and nitrous applications, the EFI system can retard timing in response to boost or nitrous injection. To have the EFI system control the ignition timing, you'll need a distributor designed for EFI. Most Holley EFI systems interface with factory GM and Ford electronic distributors (either directly or via an adapter harness), or you can purchase an EFI-compatible aftermarket distributor.

Holley EFI Ignition System Diagram

Plug And Play Distributor

If you're not using a factory-style EFI distributor, MSD and Holley offer a broad range of distributors that plug directly (or via adapter harnesses) into aftermarket EFI systems. These not only provide an RPM signal but also allow the EFI system to control ignition timing.

Do I Need a Holley Dual-Sync Distributor?

A dual-sync distributor provides two streams of information to an EFI system: 1) RPM signal, and 2) when cylinder #1 should fire. While every EFI system needs an RPM signal, only sequential-fire port injection systems (like Holley's HP and Dominator series ECUs) need to know the camshaft position for true sequential injection.

In other words, if you have a carburetor-replacement throttle body EFI system (like a Holley Sniper) or a Terminator X EFI system, a dual-sync distributor is not required. EFI systems that can synchronize their injection with the firing of each cylinder (such as Holley's HP and Dominator EFI systems) need a camshaft position signal as well as an RPM signal. That's where a dual-sync distributor comes in. If you're installing a throttle body injection system (TBI) or using a Holley Terminator ECU, then a dual-sync distributor isn't necessary.


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