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Harley-Davidson Fuel Injection

The mighty Harley-Davidson is slowly loosing its carburetor. For those of you who haven't noticed or are stuck back in the 80's, a large number of the bikes coming from Milwaukee have fuel injection installed. International environmental concerns have slowly forced the change from carburetor to electronic fuel injection. Harley purists are mourning the loss of the venerable carburetor. You can be sure that it will not be many years before all V-twins have fuel injection as standard equipment.


Even Harley-Davidson's Sportster motorcycles have EFI as of 2007. FLH's, Softails, Dyna’s and Buell's all offer fuel injection as original equipment since 2007 or an option on older bikes since 1996. The purist's are crying foul and talk about the "good old days" when they would repair their bike on the side of the road. It might be nice to take retrospective view of the introduction of new technology to Harley-Davidson motorcycles and what current technology is available and what the future may bring.

For the last 20 years, almost every change in technology to Harleys has been met with skepticism. What were some of the improvements and changes? Here are a few items of note.

* The change from Shovelhead to EVO engines
* The change from points and condenser to electronic ignition
* The change of chain to belt drive
* The change from butterfly to CV (constant velocity) carburetor.
* The introduction of the Buell.
* The change from EVO to Twin Cam engines
* The introduction of the V-Rod.


All of these improvements and changes had the HD purists of the period complaining about the changes and how the motor company had forgotten its roots and their long time customers. Much of the criticism was very harsh and unfounded. No company can remain static and stay in business, so Harley has been forced to move with the times. Government regulation, an aging customer base and increased competition from the metric manufacturers forced Harley-Davidson to update their product line. Yet Harley riders thrive on nostalgia. Eventually, the new technology has taken hold and becomes the new standard for all cruisers. We have all heard the stories from the Old Rider reminiscing about how many times he fixed his Shovelhead on the side the road with his pocketknife and a matchbook. Yes, older Harley's were more prone to breaking than the later generations of bikes. Do we really want to go back to the days when HD could have easily meant "hardly driveable"? The change in technology introduced by HD over the years has continually improved the breed. Maybe the "good old days" were not as good as we remember. There is no reason not to expect EFI improving the reliability and performance of the venerable V-Twin engine.

Now that you have a brief history lesson, now we are going to give you a short lesson on fuel injection technology. Many mechanics and most riders really don't have a good idea on how EFI works and how the primary components interact. Misconceptions about EFI abound and the amount of bad information being passed around is high. By the time you have finished this article, you should be thoroughly confused or have a new understanding and appreciation for EFI. We will do our best to minimize the technical jargon and simplify the explanations, but there is still going to be a fair amount of buzzword bingo and acronyms that are required to explain the technology. We will explain the acronyms in simple terms. Starting with the basics on EFI, we will then move into the types of EFI available for V-twins, the differences between carburetor and EFI, then finally what modifications and upgrades are available for your bike. You will not be an expert on fuel injection at the end of this article, but your understanding and appreciation for the subtleties and complexities of these systems should improve dramatically. There is a good chance you will understand how to improve the performance of your EFI bike when you are done.

EFI Basics

In its simplest terms, an electronic fuel injection (EFI) system is a computer controlled fuel delivery system. This electronic control unit (ECU) reads various sensors located on the vehicle and makes the determination of how much fuel to allow the engine to have based on this information. The computer will open and close the injectors allowing gasoline into the engine based on the sensor inputs and the fuel map programmed into the computer. The various sensors (RPM, engine temperature, air temperature, throttle position, manifold pressure, crankshaft position) provide information on operating conditions and load on the engine. Figure 1 is a detailed drawing of a typical fuel injection system and the sensors that might be in place. The EFI Components table provides a detailed description of each of the major components.

We will limit our discussion to the types of fuel injection systems that are likely to be encountered on motorcycles. One of the major differences between fuel injection systems is how the sense the amount of load that is placed on the engine. The load can be sensed by how far the rider has twisted the throttle and MAP (manifold absolute pressure). EFI systems that sense engine load by the use of the throttle or TPS (throttle position sensor) are referred to as an Alpha-N system. EFI systems that sense engine load by the use of a MAP sensor are referred to as a Speed-Density system. Harley-Davidson has used both types of EFI systems on its vehicles. While many of the sensors can be the same in both types of EFI systems, there is a significant difference in the way that each system determines what the load on the engine actually is. The Magneti-Marelli system used on EVO and Twin Cam FLH's until 2001 and Buell XL engines is an Alpha-N EFI system. On these bikes, load sensing is determined by the throttle position. The newer Delphi EFI system used on current Twin Cam engines is a Speed Density system. Speed Density EFI systems determine the engine load based on the intake manifold vacuum.

Why are there different types of EFI systems?

There are benefits to each of these types EFI system. We aren't even going to consider some of the other types of EFI systems currently available since we are only discussing V-twin engines. The choice of systems can be dependent upon the type of vehicle and its usage.

In the case of the HD Magneti-Marelli EFI system, the simplicity of using the throttle position sensor (Alpha-N) for determining engine load was probably the determining factor in using this for the first OEM EFI system.

A Speed Density EFI systems need a steady vacuum signal from the MAP sensor for the ECU to accurately determine the fuel mixture. If you have ever tried to read a vacuum gauge connected to a v-twin engine and watched the needle bounce around, you might begin to understand why a Speed Density EFI system was not the first choice for Harley-Davidson, engines with long duration cams or individual intake runners. Despite what appears to be an insurmountable problem with creating steady vacuum readings, there are a multitude of mechanical and electronic ways to "calm" the MAP reading for the ECU to use. The Delphi EMS system is able to use the manifold pressure from the HD intake, allowing it to more accurately determine the load on the engine based on the actual vacuum in the manifold rather than the position of the throttle. The Delphi unit also offers a level of sophistication and an ability to customize fuel maps in the field.

EFI Components Descriptions

To help you understand EFI, here is a table of technical terms commonly used and an explanation of the term along with some useful information. Along with the definitions, there are some simple graphics to further explain fuel injection systems. Click Harley-Davidson Fuel Injection basics to view  EFI Diagram and Harley-Davidson EFI Review