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The basics of Fuel Injection explained

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Harley-Davidson Fuel Injection basics
It is a sad day in Hogville.

The mighty Harley 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.

EFI Update for 2006 and 2007 HD

For 2006 HD Dyna model bikes and all 2007> later HD motorcyles, the Delphi fuel injection now operates in closed loop mode with a pair of narrow band O2 sensors. The NBO2 sensors control the fuel mixture when idling and cruising. Because the O2 sensor keeps the fuel mixture at an EPA friendly 14.7:1 AFR, the exhaust temperature tends to be quite warm. has a "fix" available that will eliminate the overheating, improve throttle response and increase power while the engine is operating in close loop mode. Contact us for more details on this upgrade

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.

bulletThe change from Shovelhead to EVO engines.
bulletThe change from points and condenser to electronic ignition.
bulletThe change of chain to belt drive.
bulletThe change from butterfly to CV (constant velocity) carburetor.
bulletThe introduction of the Buell.
bulletThe change from EVO to Twin Cam engines.
bulletThe 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 here to view EFI terminology

Fuel System Diagram

The fuel system diagram provides a simplified view on how fuel flows within an EFI system. The fuel tank supplies gasoline to the fuel pump. The fuel pump can be located in the fuel tank or external to the tank. The fuel pump provides a high pressure feed to the fuel rail that contains the fuel injectors. On the fuel rail there is a pressure regulator that limits the fuel pressure to 39-45 PSI 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 Diagram

The electronics and Sensors diagram is typical wiring diagram for an EFI system. Some EFI systems may have more or fewer sensors than this drawing. This represents a typical fuel injection system. The engine and air temperature sensors are simple devices that change in resistance as the temperature changes. The MAP and TPS sensors provide a 0-5 volt signal back to the EFI system depending upon the amount of pressure in the intake or the position of the throttle. The Oxygen Sensor provides a 0-1 volt signal back to the EFI system depending upon the air/fuel ratio at the exhaust. The injectors have a 12 volt power supply to them at all times. The ECU activates the injectors by switching the voltage to ground. The idle solenoid is activated by a 12-volt signal and allows additional air into the intake manifold to increase the idle RPMs. The main relay is activated by the ignition switch and provides power to all components in the EFI system. The fuel pump relay is activated by the EFI ECU when the engine is running. If the engine stalls, the ECU will deactivate the relay and shut the fuel pump off as a safety feature.

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?

Throttle Position








Idle or Deceleration


















Wide Open Throttle






Table 2 - Sample Fuel Map

Engine Temperature






Adjustment %






Table 3 - Sample Engine Temperature Adjustments

Air Temperature

-10 F

50 F

70 F

90 F

105 F

Adjustment %






Table 4 - Sample Intake Air Temperature Adjustments

In is time to try to explain what is happening in the ECU now that we have confused you with a lot of rhetoric, diagrams and tables. While the real details of what is happening is very complex, we are going to present a greatly simplified version of what the computer happens.

  1. The computer determines the current RPM of the engine based on input signals from the Crank Position Sensor.
  2. The computer determines the load on the engine by checking the Throttle Position Sensor or MAP sensor.
  3. The computer takes the RPM and Engine Load information to look in the Fuel Map table located in the ECU. While an actual fuel map has more values in it than the sample map in Table 2, a real table looks very similar to the simplified map shown. 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 can not find exact matches to the RPMs 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 at Wide-Open Throttle at 3900 RPMs, it calculates the fuel value to be 44 that is half way between 33 and 55.
  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. The computer 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 30 degrees F, the computer knows the engine is cold and needs more fuel, just like a choke on a carburetor. The computer looks up an engine temperature adjustment percentage that could be 20% (see table 3). If our previously looked up 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.
  6. The computer 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. Lets go back to our full throttle acceleration at 3900 RPMs and assume our engine has warmed up to normal operating temperature, but the outside air temperature is 90 degrees F. The computer looks up an adjustment percentage of -2% (see table 4). The computer actually understands that at high air temperatures, the engine requires less fuel 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. The computer has now determined that at Wide Open Throttle, at 3900 RPMs, normal engine temperature and a 90 degree air entering the intake that the engine needs to keep each fuel injector open 43% of the time.
  8. The computer 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 rough idea of the minimum amount of work the system actually has to do. The real activities are much more complex than explained, but you get the idea that your EFI ECU is pretty good at knowing how much fuel your engine really needs.

Why is EFI better than a carburetor?

Choosing a carburetor and tuning it is always a series of compromises. A carburetor is a demand device dependent upon the velocity of the air entering the venturi to create the air/fuel mixture that feeds the engine. A carburetor 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 RPM range 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 (Thunderjet) in an effort to improve the fuel curve. The interactions of these additional circuits further complicate the tuning efforts. A change in jetting to one fuel circuit can impact the required jetting in another circuit. Sound complicated? It certainly can be.

Lets simplify matters and assume a carburetor with nothing more than an idle circuit and main circuit. To optimize low RPM performance a small diameter carburetor provides the best performance, but at high RPMs a large volume of air is required to feed the engine. This requires a much larger diameter carburetor. The ability of a carburetor to provide a good air/fuel mixture is very dependent upon the velocity of the air going through the carburetor. This "signal" must be present to maintain good throttle response. If the diameter of the carburetor is too large for the engine, low RPM performance can be very poor. Most HD mechanics and riders are aware that the "large carburetor" syndrome creates a poor running engine. This situation does not occur with an EFI engine.

With an electronic fuel injection system, the required fuel amount for each RPM and engine load condition is located in the fuel map located in the ECU. Once this primary fuel amount is known, then the ECU further adjusts the fuel mixture for the engine and air intake temperatures. In many cases, the mixture will even be adjusted for the barometric pressure and altitude. Based on the various sensor inputs to EFI ECU, there is only one fuel value that is generated. The correct fuel amount is fed into the engine at all times. Because airflow does not have to pass through a venturi to provide an air/fuel mixture, the throttle plate diameter can be quite large. This allows engine is allowed 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 airflow.

A word about exhausts
There are more performance questions 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 carburetor 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 resisting 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 airflow by pulling air into the cylinder or reduce airflow by pushing against the air coming from the intake. 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 exhaust system, 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 airflow, the proper air/fuel ratio will not be provided. Since none of the systems used by Harley-Davidson use airflow 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 generated by drag pipes actually reduce airflow into the engine at low/mid power ranges.

A carburetor and injected engine react to a change in pipes in an identical manner. The difference is in how you resolve the situation. In a carburetor engine, you alter the jetting to compensate for changes in airflow through the engine, and many people don't realize 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 airflow at various engine speeds. It sounds worse than it is because the carburetor'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

The authors home grown fuel injection system

It stands to reason that if you have read to this point, there is great interest 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 carburetor engine to EFI. We are going to keep you waiting a little longer for that information. A discussion of HD EFI upgrades, modifications and retrofitting EFI to carburetor bikes will be in "Part 2" of this article.

Web links for EFI.

Below you will find an extensive resource list on general EFI topics, aftermarket EFI systems and Harley-Davidson EFI upgrades. The author may not agree with information contained in some of these WEB sites, nor endorse the products listed. The information is provided as a service to readers to assist them in their performance projects. (
Taylor Marine MAD EFI (
MegaSquirt DIY-EFI (
MegaSquirt Discussion Group (
Simple Digital Systems (
Techlusion TFI (
ThunderMax ECU by Zippers Performance (
Dyno Jet PowerCommander (
S&S Cycle (
Daytona Twin-Tec (
Horsepower Inc (
RB Racing (
Dakota Kid (
Gerolamy Company (
Delphi Ion Sensing Knock Detection (

EFI Sensor Diagrams Copyright Lance Gardiner
MegaSquirt Copyright Bruce Bowling and Al Grippo

EFI article Copyright Stephen P. Mullen


Table of Contents
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Modification, Installation, Maintenance and Tuning Index  will help you find most of the information you want on one page.

How to get Professional Tuning Results at home
Testing the Innovate Motorsport LM-1 portable air fuel meter

Profession Tuning on a Shade Tree Budget

Veypor VR2 Data Logger and Instrument Panel
Video Installation and Demo
Purchase VR2

Engine Performance
How to Build a
TC96 2007 Engines
TC88 70HP Stage1  
TC95 128HP Stage 3
TC95 100HP Street
TC96 2007 Stage 1/2
EVO 64 HP Stage 1
EVO 74 HP Stage 2
EVO 82 HP Stage 3
EVO 95 HP Stage 3
883 to 1200 Upgrade
Shovelhead Modifications

New EFI for EVO and TC

Performance Gallery
Horsepower Gallery
Evolution 80
Twin Cam 88/95
Evolution Unlimited
Sportster Unlimited
Drag Strip Gallery
Land Speed Racing Gallery
CV Carburetor
Modifying the CV carb
Tuning a CV carb
Selecting a cam
Install a TC 88/95 cam
Install a Big Twin cam
Install Sportster cams

Camshaft Specifications
Twin Cam

Exhaust Systems
EVO Exhaust Testing
TC Exhaust Testing
Khrome Werks AR100 test
Making Drag Pipes Work

Shop Manual
Carburetor Troubleshooting
Finding Manifold Leaks
Cylinder Heads
Pistons and Cylinders
Belt Drive
Shop Manual Appendix
$20 Bike Lift
Plug Wires
Spark Plugs
Engine Tuning
Nitrous Oxide
Motor Oil
Stutter Box
General Information
WEB Links
Buy Books and Manuals
Performance Calculations
Estimate Horsepower
Estimate 1/4 Mile Time
Estimate Top Speed

Engine Displacement
Exhaust Length
Gear Ratios
Air Density

The Nightrider Diaries
The ramblings of a genius a, a madman and something in between.

Where is Sifton Cams?

Autocom Active-7 tested

Harley-Davidson EFI
-EFI basics explained
-EFI modifications explained

183 HP, 2 carbs, 2680cc

Copyright 1997-2006  Stephen Mullen, Oldsmar, FL -+-