Replacing the legacy fuel delivery system in your classic car or truck can unlock better fuel economy, simplify maintenance, and improve reliability and drivability. Installing electronic fuel injection (EFI) using a kit from Holley is accessible to any hobby mechanic with basic skills, but it’s wise to consider the steps needed to get the car’s electrical system ready for the switch. For expert input on setting yourself up for a successful installation, we spoke with Doug Flynn, Holley’s senior systems engineer for fuel injection.
First, the basics: Disconnect the battery power to prevent hot wires from causing a short circuit. Invest in some quality wire strippers and crimpers to prevent inconsistent connections that can cause problems down the road. And if you need to add connections beyond what’s included in the EFI kit, high-quality TXL or GXL wire from an online supplier is usually preferable (and cheaper) than off-the-shelf wire from an auto parts store.
Another good preparation step is to obtain a factory wiring diagram for your vehicle, which can be helpful when connecting to the ignition-switched power, especially on newer vehicles. A quick web search can often locate a free PDF download. “Making sure that switched power is supplied to the EFI is fundamental, but sometimes trips people up,” says Flynn. “Having a factory wiring schematic can assist with finding that connection.”
Switched power sources only provide power when the ignition is turned on, allowing for devices to operate while the engine is running but not when it's off. In a car's fuse box, switched power circuits are usually labeled as "ACC" (Accessory) or "IG" (Ignition).
Another area that may require interfacing with the factory wiring and hardware is the ignition system. “Some builders may be replacing the factory distributor and ignition system with parts that Holley and MSD offer, in which case, everything is plug and play,” he adds. “But if you want to retain the factory ignition system – which is possible in almost all cases – then you just need to know how to connect into it properly.”
The good news is that beyond the ignition, there is limited interaction between the legacy factory wiring and the new wiring for the fuel-injection system. Most of Holley’s harnesses are plug-and-play, which simplifies connecting to an LS swap engine, for example. However, on newer cars, the replacement EFI system won’t integrate with the factory body control modules without the use of other devices, leading to a loss of functionality for items like cruise control and dashboard gauges. One solution is to buy a Holley digital dash or replacement factory-style cluster from Classic Instruments.
Shop Holley EFI Kits and Components Now
Older, pre-CAN bus cars have simpler wiring, but the disadvantage is that it may be in a poor state of repair. To prevent legacy wiring issues from causing problems with the EFI, Flynn recommends completely separating any new wiring from the old.
“Whether it’s a street car or a Pro Mod race car, when you’re installing an EFI system, a nitrous system, a power distribution module, or something else, never reuse any of the wiring that’s already in the car, especially if it’s an older vehicle,” he cautions. “The wiring that you install should be stand-alone, including feeding power and ground directly off the battery, then running that to the distribution blocks. Don’t tie it into the car and build old problems into the new system.”
He also recommends separating ‘dirty’ power and ground wires from the ‘clean’ ones, for example, by not tying them together at the same stud. ‘Dirty’ in this case refers to anything involving a solenoid, such as a nitrous system, transbrake, alternator, or starter.
“All that electrical noise can feed back, so you should keep it separate from your ‘clean’ EFI power and ground runs,” he says. “Likewise, the power and ground runs for a digital dash or any other electronics should be brought off separately into their own distribution block or stud, and you should not stack noisy, dirty power or ground wires on the same stud or distribution block as everything else. Keeping the old and new separate helps to eliminate a lot of problems with the installation.”
Eliminating electrical noise from the system is also the reason why it’s best practice to run the main power and ground wires from the battery posts when installing an aftermarket EFI system.
“The battery acts as a big capacitor, so any noise in the electrical system is absorbed by it and won’t find its way into the EFI system,” Flynn explains. “Probably the number one issue that people have with EFI is ignition noise getting into the wiring and affecting performance.”
“If I’m converting a car, I like to get a battery with both top- and side-post terminals. I mount the EFI wiring and any other wires for aftermarket electronics to whichever terminal position the main car is not using, top or side, to keep them separate.”
If the battery is in the trunk, do not use the vehicle frame or body to ground the electronics. In this case, Flynn recommends running a ground cable from the battery to the engine block or head so that the ignition system energy can find its way back to the battery. Any other grounds should be connected to the battery with cable, not the frame, as well.
Additional preparation may be needed for a remote ECU like a Terminator X, HP, or Dominator. When possible, it’s better to mount a remote ECU in the vehicle interior, rather than in the heat of the engine compartment. Fishing the harness through the firewall will either mean finding an existing factory grommet that can be removed to route the wires, or drilling a new hole in the firewall and fitting an appropriate grommet to prevent wire damage.
With proper preparation and careful work, an enthusiast mechanic should be able to fit an aftermarket EFI system with no major issues. But if you run into trouble, there are resources to tap into. These include the Holley manuals and technical service support line, but also the Holley Facebook groups, whose members will often provide great answers to questions in just a few minutes.
“The most important thing is to be caring and meticulous in what you do,” sums up Flynn, who says a properly executed basic wiring installation in a street car should take around 15 hours. “That’s something everyone can do: you don’t need experience to be careful with your work.”
.
