The ABC’s of EFI

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Illustration by Hector Cademartori
efi-illustration.pngby Karl Borum

Harley-Davidson introduced Electronic Fuel Injection (EFI) as a feature for Softail models in 2001. Obviously that proved successful, because the MoCo slowly rolled out EFI to all models. Today, almost all factory Harleys built in the US are fuel injected. So you ought to know a thing or two about EFI.

EFI has both pros and cons, but for street riders, EFI has mostly pros:

-Unlike carbureted models, EFI automatically adjusts for altitude
-EFI compensates for air temperature and is therefore more precise
-Better cold temperature starting
-No need for a choke
-More precise tuning is possible for a variety of engine modifications
-Fuel adjustments can be made for a wide range of engine modifications using an aftermarket EFI module

Precise tuning of the fuel flow by an aftermarket EFI module over a wide range of engine modifications is one of the more significant advantages to Electronic Fuel Injection. Precise tuning of the fuel flow is accomplished by adding (or subtracting) fuel using a fuel map. A stock Harley Davidson comes with a factory fuel map stored in the factory Electronic Control Module (ECM). An aftermarket EFI module “piggy backs” onto the factory ECM and adjusts the fuel map to better serve your particular needs.

A fuel map is simply a table of values that controls the fuel to the injectors at various throttle/ RPM settings. Fuel to the injectors is controlled by sending pulses to the fuel injectors to open them. Longer pulses mean more fuel. Shorter pulses mean less fuel. All fuel maps are determined by measuring the air to fuel ratio while the bike is under power, and making fuel adjustments to maintain a perfect (near perfect) air to fuel ratio.

An aftermarket EFI module intercepts the pulses coming from the factory ECM and makes them longer or shorter, depending on the aftermarket ECM’s fuel map. So a fuel map is simply a “text” file with fuel injector values that determine when and how long the fuel injector squirts gas into the cylinder.

Air flow (air filter system), fuel flow (aftermarket cams) and exhaust (mufflers and pipes) all work together to create the fuel flow–or air to fuel ratio, to be exact. When any of these components are changed in a fuel injected engine, the air fuel ratio is likely changed as well, resulting in a less than optimal fuel mixture, i.e., richer or leaner. Very lean conditions resulting from modifications can actually fry your engine.

In a carbureted engine, the engine tuner would keep changing out needle jets to arrive at the “best” setting for optimal air fuel ratio over the throttle and RPM range. With an aftermarket EFI module, the technician can change the fuel map in the aftermarket module by adding or subtracting values in the fuel map, with a laptop or PC. And a more precise air/fuel mixture will result, across the power band (throttle and RPM ranges).

Here is where the two main EFI cons come in. First, because the “E” in EFI is “electronic”, you can no longer push start your bike–ever. Don’t even try (don’t ask me how I know). For the ECM and EFI to work, you must have a charged battery (why do you think they call it electronic).

Second, an aftermarket EFI module is one more thing to trouble shoot when things go bad. A badly behaving EFI could render your ride into a boat. I know what you are thinking: “I’ll just unplug the module, toss it in the saddle bag and be home for dinner”. Unplug the module, yes, to see if the module is causing your problems. Ride without it? Absolutely not. Your EFI module modifies the factory fuel map, without it you could damage your engine, most likely by running too lean.

There is something you can do before hand to prevent engine damage if number two ever happens. A Harley-Davidson factory “performance map”, loaded into the factory ECM, which closely approximates your set up (air cleaner / cam / exhaust / displacement…) can save you. If you have a Harley performance map loaded at the dealer, you CAN remove a faulty aftermarket ECM, and be home for dinner.

Any dealer worth his Bar and Shield can approximate your set-up with a factory upload. Have one loaded today and it may save you a tow to the dealer down the road. And if your module goes bad and no one has one for 250 miles, a Harley performance map–already loaded–can save you in more ways than one. Remember, your after market module will have to be mapped differently with an HD performance upload than with the factory ECM map. This is because the ECM module adds to an existing map. Make certain that your EFI module is mapped to your upgraded HD fuel map.

Electronic fuel injection may have its cons, however, EFI has far more advantages than disadvantages. You can upgrade your ride, one system at a time, with the same EFI module by simply remapping your module. Many fuel maps are available from both the manufacturers and on the internet.

If you want help with any projects or need more info, check out the Electrical/Ignition/Tuner/ECM/Fuel Injection Forum!

Or check out more stellar HDForums stories:
Ape Hangers: How High is Too High and Other Monkey Business
Stirring the Pot: Top Five Reasons SOA is SOL!
Buying Harley Davidson Parts on eBay: A User’s Guide
Calgary’s Noise Snare: A Speed Camera for Loud Pipes

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