Electronic Fuel Injection Basics

Electronic Fuel Injection Basics
In 1995, Harley-Davidson began offering electronic fuel injection (EFI) as either standard or optional equipment, and since 2007, all Harley models include EFI. That means if you own a mid-1990s or later Harley, chances are high that you have a fuel injected model. And like most Harley riders, you probably want to improve your engine's performance by installing a few performance goodies. Unlike a carbureted engine where you can turn a few screws or replace a jet in your garage to make tuning changes, an EFI engine is much more complex. Even the name, electronic fuel injection, implies some amount of complexity. With an EFI-based engine, you cannot easily tune the induction system by only twisting a screw or two. Instead, you typically need to download software fuel maps or install a new electronic "black box." And in most cases, you're also required to do some serious dyno tuning to dial in the engine's induction system. With the complexity of today's EFI, constantly changing EFI technology, and the high cost of EFI tuning equipment such as a dyno, it is usually not cost effective to tune your own EFI system. Instead, it is typically wiser to leave critical tuning to your engine builder or dyno tuner. However, having a basic understanding of EFI can save you costly surprises when making performance upgrades and help you make intelligent choices for EFI-based components. The following is an overview of the V-twin's EFI system.

Electronic Fuel Injection
In simple terms, EFI is a computer-controlled fuel delivery system. From 1995 to 2001, the factory used the Magneti-Marelli EFI system. Starting in 2001, a completely different system by Delphi Electronics system was phased into production. Today, all Harley V-twins are equipped with a Delphi system.

The factory's EFI system can be divided into four major components: electronic control module (ECM), throttle body, fuel injectors and sensors. The ECM is the brain of the system and gathers data from a handful of engine sensors. The ECM checks an information table that was programmed into its memory. The information table is often called the calibration map. Based on sensor data and the calibration map, the ECM then instructs the electro-mechanical fuel injectors when to start and how long to spray fuel into the intake tract. The ECM also controls ignition timing. The throttle body includes a butterfly valve (one for the Delphi and two for the Magneti-Marelli) that regulates airflow through the intake tract. The opening and closing of the butterfly(s) is controlled by the throttle mounted on the handlebar.

EFI Fuel Flow
Figure 1 shows a simplified view of the V-twin's EFI fuel flow. The fuel pump, which is located in the fuel tank, supplies a high-pressure flow of fuel to a fuel rail, which may be attached to the throttle body. In turn, the fuel rail provides fuel to the fuel injectors, which spray fuel into the intake tract. With the V-twin engine, the fuel injectors are normally attached to the intake manifold. A fuel pressure regulator maintains a constant fuel pressure while limiting fuel pressure to approximately 38-45 psi. The pressure regulator is connected to the intake manifold via a vacuum line. Excess fuel is returned to the fuel tank by the regulator.