Of Harley-Davidson's lines, only the Dynas and Sportsters don't have EFI as an option. Most tourers, Softails and all current Buells all offer fuel injection as original equipment or an option, and while the purists are crying foul, and talking about the "good old days" when they could repair their bike at the side of the road, we reckon it's essential to take a good look at new technology as it relates to the bikes we're interested in.

For the last 20 years, almost every change in technology to Harleys has been met with scepticism when it first came through, and it doesn't hurt to review them to put them into context: the Evo engine (remember the "Hear no Evo, Speak no Evo, See no Evo" t-shirts?); electronic ignition; belt drive; CV (constant velocity) carburettor; and there is still some dissent in the ranks about the Buell, the Twin Cam engine and the V-Rod, but it's early days yet.

Each of these developments has had the traditional riders of the time complaining about the changes and how The Motor Company had forgotten its roots and their long time customers, but much of the criticism was very harsh and has been shown to be unfounded as things have settled down. No company can remain static and stay in business, so Harley has been forced to move with the times. Government regulation, an ageing customer base and increased competition from the metric manufacturers forced Harley-Davidson to update their product line. So while Harley riders thrive on nostalgia, the new technology has eventually taken hold and has become 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 penknife and a matchbook, but turn that on its head and question whether perhaps older Harleys were more prone to breaking than the later generations of bikes - and its very simplicity proved too great a temptation for idle hands. Do we really want to go back to the days when people jibed that HD meant "hardly driveable". While we can argue about the changes in character of the engines, there is no doubt that the change in technology introduced by Harley-Davidson over the years has continually improved the breed. Maybe the "good old days" were not as good as we remember, and there is no reason not to expect EFI to improve the reliability and performance of the good ol' V-twin engine.

Many mechanics and most riders really don't have a good idea on how Electronic Fuel Injection works or how the primary components interact, so misconceptions about the technology 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 carburettor and EFI, and 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.

The Basics
In its simplest terms, an electronic fuel injection system is a computer-controlled fuel delivery system. An electronic control unit (ECU) reads various sensors located on the vehicle and determines how much fuel to allow the engine to have based on a preset table of values - the fuel map. Using the data from the sensors, and the settings of the fuel map, the computer will open and close an injector, delivering fuel into the airstream that is sucked into the motor through the injector's body. The various sensors measure engine speed, engine temperature, air temperature, throttle position, manifold pressure, crankshaft position, all of which provide the essential information on operating conditions and load on the engine. Overleaf, you will find a detailed drawing of a typical fuel injection system and the sensors that might be in place, and the EFI Components table, across the bottom of the next few pages, 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 they determine the engine's requirements, which can either be sensed by how far the rider has twisted the throttle, or by the MAP - the Manifold Absolute Pressure, and not to be confused with the fuel map. 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, while those that sense engine load by the use of a MAP sensor, and determine the engine load based on the intake manifold vacuum, are referred to as a Speed-Density system, and Harley-Davidson has used both types on its vehicles. The Magneti-Marelli system used on Evo and Twin Cam big twins until 2001 and Buell XL engines is an Alpha-N EFI system, coupled to the throttle. The newer Delphi EFI system used on current Twin Cam engines is a Speed Density system.

Many of the sensors are common to both types of EFI systems.

Why are there different types of EFI systems?
There are benefits to each of these types EFI system - which will form the base of this article, as we are only interested in V-twin engines here - and the choice of systems can be dependent upon the type of vehicle and its usage.

In the case of Harley-Davidson's 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 factory injection system because Speed Density 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 system was not the first choice for Harley-Davidson engines with long duration cams or individual intake runners, but despite what appears to be an insurmountable problem in creating steady vacuum readings, there are many 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 H-D intake, allowing it to determine more accurately 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 explain the process, in conjunction with the Glossary of Terms below - which should provide some useful information - there are some simple graphics to which will hopefully simplify it further.

Fuel System
This fuel system diagram shows a simplified view of how fuel flows within an EFI system. The fuel pump can be located within or external to the fuel tank itself, and provides a high-pressure feed to the fuel rail, which contains the fuel injectors. On the fuel rail there is a pressure regulator that limits the fuel pressure to 39-45psi and maintains a constant pressure. The pressure regulator has a vacuum line that connects it to the intake manifold. Any excess fuel leaves the pressure regulator and is returned to the fuel tank.

EFI Electronics and Sensors
This electronics and Sensors diagram is typical wiring diagram for an EFI system. Some systems may have more or fewer sensors than this drawing, but this represents a typical example.

The main relay is activated by the ignition switch (in association with the kill switch), and provides power to all components in the EFI system.

The injectors have a 12v power supply to them at all times and are activated by the ECU by switching the voltage to ground.

The idle solenoid is activated by a 12v signal and allows additional air into the intake manifold to increase the idle revs when required.

The fuel pump relay is activated by the ECU when the engine is running, and the ECU will deactivate the relay if the engine stalls, shutting the pump off as a safety feature.
The engine and air temperature sensors are simple devices that change in resistance, which can be detected by the ECU, as the temperature changes.

The MAP and TPS sensors provide a 0-5v signal back to the ECU system depending upon the amount of pressure in the intake or the position of the throttle.

An Oxygen Sensor provides a 0-1v signal back to the Injection system, depending upon the air/fuel ratio at the exhaust.

In many cases, the mixture is adjusted for the barometric pressure and altitude.

In this drawing the coil or tachometer signal represents the EFI "trigger" signal. In the case of a sequential injection system like the Harley-Davidson EFI systems, this would be Crank Position Sensor.

How does an EFI really work?
Having confused you with a lot of rhetoric, diagrams and tables, it's time to try to explain what is happening in the ECU. While the real details of what is happening is very complex, we are going to present a greatly simplified version of what the computer does.

1. The computer determines the current speed of the engine, based on input signals from the Crank Position Sensor.

2. It then determines the load on the engine by checking the Throttle Position Sensor or MAP sensor.

3. Next, it takes the engine's speed and load information, and looks up the fuel requirement in the ECU's fuel map table. While an actual fuel map has many more values in it than our simple map (Table 2), it looks very similar. In our example, the number in the table represents the duty cycle of the injector, or what percentage of the time is the injector actually open and allowing fuel to flow. If the computer cannot find exact matches to the revs and Engine Load, it is able to estimate the required value by looking at the values just higher and lower to calculate the needed value. For example if the engine is running with the throttle wide-open at 3,900rpm, it calculates the fuel value to be 44: halfway between 33 and 55, as 3,900 is halfway between 3,300 and 4,500rpm.

4. Now that the computer has the main fuel value, it needs to determine if any adjustments to the fuel map value are needed.

5. It then determines the engine temperature. If the engine is cold, then the fuel number is adjusted up by a pre-determined percentage. For example when the engine temperature is a 30ºF, the computer knows the engine is cold and needs more fuel, just like a choke on a carburettor. The computer looks up an engine temperature adjustment percentage that could be 20% (see table 3). If the original fuel value was 44, the computer now adds 20% to that number, resulting in a new fuel value of 53. As the engine warms up, these adjustment percentages become smaller and at normal operating temperature the value is zero - you'll notice it increases again on a hot motor, as fuel can be used to cool an engine.

6. It determines the ambient temperature of the air entering the intake by reading the Intake Air Temperature sensor. As with the engine temperature adjustment, the computer will look up the percentage adjustment. Let's go back to our full throttle acceleration at 3,900rpm and assume our engine has warmed up to normal operating temperature, but the outside air temperature is 90ºF. The computer looks up an adjustment percentage of -2% (see table 4). The computer understands that the engine requires less fuel at high air temperatures, so it leans the fuel value down from 44 to 43. If the air temperature had been cold, the fuel mixture would have been made slightly richer.

7. It can now determine that at Wide Open Throttle, at 3,900rpm, normal engine temperature and air at 90ºF entering the intake, the engine needs to keep each fuel injector open 43% of the time.

8. The ECU now checks to see if it is time for the fuel injector to be activated. When it is time for the fuel injector to open, the computer allows current to flow through the injector for the required amount of time. The opening time can be controlled very precisely, down to the millisecond.

As you can see, the computer in a fuel injection system is doing a lot of things very quickly. While some EFI systems do more than listed above, the description above is a good idea of the minimum amount of work the system actually has to do. The real activities are much more complex than explained, but you'll hopefully get the idea that your EFI's ECU is pretty good at knowing how much fuel your engine really needs.

Why is EFI better than a carburettor?
Choosing a carburettor and tuning it is always a series of compromises. A carb is a demand device and fuel is drawn up from the float bowl dependent upon the velocity of the air entering the venturi to create the air/fuel mixture that feeds the engine. A carburettor maintains a series of fuel circuits to help maintain the best possible fuel mixture. The idle circuit, intermediate and main jetting circuits are used to tune the mixture across the operating speed of the engine. These different fuel circuits can interact with each other creating rich and lean spots in the fuel curve. Some riders will go as far as to add one or more additional fuel circuits - the ThunderJet springs to mind - in an effort to improve the fuel curve, but the interactions of these additional circuits can further complicate the tuning efforts. A change in jetting to one fuel circuit can impact the required jetting in another circuit. Sounds complicated? It certainly can be.

Let's simplify matters and assume a carburettor with nothing more than an idle circuit and main circuit. To optimise performance at low engine speeds, a small diameter carburettor provides the best performance, but at higher revs a large volume of air is required to feed the engine, and that requires a much larger diameter carburettor. The ability of a carb to provide a good air/fuel mixture is very dependent upon the velocity of the air going through the throat of the carburettor's body. This "signal" must be present to maintain good throttle response. If the diameter of the carburettor is too large for the engine, low speed performance can be very poor. Most H-D mechanics and riders are aware that the "large carburettor" syndrome creates a poor running engine. This situation does not occur with an EFI engine.

With EFI, the correct fuel amount is fed into the engine at all times, and because air flow does not have to pass through a venturi to provide an air/fuel mixture, the throttle plate diameter can be quite large. This allows the engine to draw all the air it wants. Since more air equates to higher horsepower potential, all the EFI has to do is provide the correct amount of fuel for the increase in air flow.

A word about exhausts.
There are more emails into American-V about the implications of switching exhausts on injected engines than any other fuel-related topics, and with good reason. There is plenty of confusion surrounding what happens even with a carb and it's all about back pressure.

When we talk about a lack of back pressure, we are actually talking about the management of reverse pressure waves echoing up the exhaust system. These pressure waves reflect up and down the pipe both reisisting and pulling exhaust gases from the engine but you need to know that any back pressure, or restriction in the exhaust reduces gas flow through an engine at some point in its cycle. The physical barriers in the exhaust system generate a variety of pressure waves and the time at which these pressure waves arrive at the exhaust valve can improve air flow by pulling air into the cylinder or reduce air flow by pushing against the air coming from the intake, and this effect changes with the engine speed. A well designed exhaust system either minimizes these pressure waves, or tries to control their timing to improve power by pulling air into the engine.

The fuel injection system is not directly affected by the silencer, but the amount of air getting into the engine is. More air needs more fuel, but if the system cannot sense the change in the air flow, the proper air/fuel ratio will not be provided. Since none of the systems used by Harley-Davidson use air-flow sensing to establish the fuel mix, the fuel map needs to be altered to ensure a proper mix.

When an EFI bike is modified with an improved airflow filter and a performance exhaust system, the EFI fuel map needs to be modified to compensate for the increased air getting into the engine. Slip on mufflers, conventional 2-2 and 2-1 exhaust systems all allow more air into the engine by reducing the exhaust restriction but they do not significantly alter the pressure waves in the exhaust system - unlike when drag pipes are used, because the restrictions in the exhaust system are decreased and the management of the pressure waves is altered dramatically. The pressure waves that are generate by drag pipes actually reduce air flow into the engine at low/mid power ranges.

A carburetted and injected engine react to a change in pipes in an identical manner. The difference is in how you resolve the situation. In a carburetted engine, you alter the jetting to compensate for changes in air flow through the engine, and many people don't realise that there are multiple fuel circuits that must be re-jetted in order to compensate for changes in pipes.
With a fuel injection system, the fuel map must be altered to reflect changes in air flow at various engine speeds. It sounds worse than it is because the carburettor's jetting is a compromise to match the jet to the engine speed and load, but when remapping fuel injection, the fuel is altered for a very specific speed/load condition allowing much better power across the entire range of the engine.

EFI for Performance
It stands to reason that if you have read this far, you're interested in the performance aspects of fuel injection systems. There are many products that offer the opportunity to modify or upgrade Harley-Davidson EFI systems. There are also products that allow you to upgrade a carburettor engine to EFI but we're going to keep you waiting a little longer for that. A look at Harley-Davidson EFI upgrades, modifications and retro-fitting EFI to carburetted bikes will be in "Part 2" of this article.