Regardless of whether your engine is carbureted or fuel injected, the principles remain the same and performance gains can be had by moving more air through the engine. The engine’s induction system starts with the air cleaner and ends at the intake valve in the port. Sandwiched between the air cleaner and intake valve of an EFI engine are a throttle body (TB) fuel injector, and sensors. Collectively, the induction system’s parts for moving air and fuel are called the intake tract.

Electronic Fuel Injected Engines
The Factory initially offered Electronic Fuel Injection (EFI) as an option in 1995, and it remained an option on various models until 2007. However, starting in 2008, EFI became standard on all models. Both Magneti-Marelli and Delphi EFI systems have been used since 1995. Although the two systems differ in the way they work, they accomplish the same thing and include similar components.

Essentially, EFI is an electronically controlled fuel delivery system. In other words, it works digitally instead of analogically. Unlike a carbureted engine, where the cylinder’s air/fuel mixture is metered mechanically with jets and tuning is accomplished using replaceable jets and simple screwdrivers, electronic fuel injected engines rely on an Electronic Control Module (ECM) to determine the amount of fuel to be supplied and the exact moment at which the fuel will be discharged into the intake tract. As a result, the ECM must be recalibrated to make tuning adjustments for any changes made to the engine parts combination.

Whereas a carbureted system uses gravity to feed gasoline from the fuel tank to the carburetor float bowl, EFI systems uses a fuel pump, fuel filter, and fuel regulator to deliver fuel in the proper amount and pressure to electro-mechanical injectors fitted into the intake manifold. To regulate airflow through the intake tract, a butterfly valve on the TB is mounted at the outboard end of the intake tract. The TB only controls airflow based on the position of the handlebar throttle while fuel is regulated by a combination of the ECM and the fuel injectors.

Although a larger carburetor and matching manifold are commonly installed on a carbureted engine to increase airflow, in the case of an injected engine, a larger TB is used to increase airflow. One major difference to note, however, is that a throttle body only flows air while fuel is supplied downstream by the injectors. One benefit of this design is that fuel has less time to separate from the air in the induction tract. That can optimize the air/fuel mixture while improving combustion.

Similar to modifying a carbureted engine, modifying an EFI-based engine with less-restrictive intake and exhaust systems will require additional fuel, and increasing displacement and compression will require additional fuel and ignition timing adjustments. EFI systems allow fuel and ignition timing changes to be made electronically, instead of mechanically changing jetting. For tuning EFI, two options are available: A self-contained control unit requiring no computer interface, or a computer-based software program that can modify the instructions stored in the stock ECM. The self contained non-computer interface units allow fuel flow to be quickly adjusted on mildly modified engines. Software-based tuning systems are more comprehensive and intended for both mildly and heavily modified EFI engines. Software systems allow both fuel flow and ignition timing changes to be made to the ECM’s tuning tables using a personal computer. These procedures are commonly referred to as replacing the ECM’s fuel map.

EFI-based engines are most commonly tuned using a dynamometer; however, they can also be tuned through trial and error testing on the street and racetrack. To simplify tuning and reduce testing time, Harley-Davidson Screamin’ Eagle and aftermarket companies offer a variety of pre-tuned software calibration modules for various engine combinations. The calibration modules are software tables containing new fuel flow and ignition timing parameters, which are loaded into the ECM. Similar to a carbureted engine requiring jetting changes to maintain proper air/fuel mixture when modifications are made, an EFI engine requires ECM changes to alter fuel flow and ignition timing for a modified parts combination.

To obtain maximum performance from free-flowing exhaust systems, performance cams, high-flow heads, and big bore kits, more air and fuel is required. As with carbureted engines, the stock EFI air cleaner should be replaced with a high-flowing unit. A variety of performance air cleaners are available for injected engines. Be sure to match a new air cleaner to your year and model of injected engine.

Once you have installed a high-flow air cleaner, a larger throttle body can be considered to further increase airflow through the intake tract. However, you cannot treat the intake tract as a bunch of individual parts. Instead, you should look at it as a complete system of interdependent components, from the air cleaner and TB to the manifold and cylinder heads, which must satisfy the airflow requirements of the engine for a given application. Here’s the point: Upgrading parts separately or in a physically sequential order may not provide the best results. For example, it may be wiser to upgrade the cylinder heads before the TB or doing both at the same time. As one bottleneck in the system is eliminated, another impasse becomes the next restriction. In other words, you end up moving the bottleneck somewhere down the line. But the new performance limitation isn’t necessarily located immediately upstream from the old impasse. The true bottleneck must always be identified.

Anyway, getting back to the TB – it is the EFI equivalent to the carburetor, with one major difference: A TB only flows air. Injectors downstream of the TB supply fuel to the engine. TB design varies depending on the year and model of the EFI system. Magneti-Marelli systems were used from 1995 through 2001 and Delphi systems from 2001 to present. However, periodic updates were made to these systems, so make sure that a new high-flow TB fits your engine and EFI electronics.

The stock Delphi TB throat is about 46mm. Performance throttle bodies ranging between 48mm and 60mm and even larger are available. Since a fuel injected system is only flowing air from the air cleaner inlet to about the intake port (where the injector is located), TB sizing is not as critical as carburetor sizing, but it is still important because it can affect idle quality and acceleration. Ideally, you should match throttle body size to the engine’s horsepower level and not displacement, because horsepower is in part determined by airflow. However, since many engine builders think in terms of engine displacement, here are few very general TB sizing guidelines, with emphasis on the word “general”: 48mm TBs work well for mildly modified engines, 50mm TBs do well on 88-ci to 95-ci engines, 54mm TBs for 103ci to 113ci engines, and 54mm and larger TBs for 113ci and larger engines. Consult with your engine builder and tuner for more specific information. Additionally, make sure your original equipment EFI parts and electronics are fully compatible with a new TB.

When activated by the ECM, the fuel injectors spray fuel into the intake tract. High-flow fuel injectors are available for modified engines and are rated based on the pounds of fuel per hour flowed. Although installing high-flow injectors can increase the maximum power potential for an EFI system, it does not guarantee that power will increase. Usually, injectors are not maxed-out until pushed beyond their 80-percent duty cycle. Additionally, fuel pressure can have a major effect on injector operation. Installing high-flow injectors when they are not required can cause tuning problems, so consider starting with stock injectors until it is determined that high-flow injectors are needed. Still, some tuners use high-flow injectors as a tuning aid, but often there are less costly ways to achieve the same results.

EFI Tuning Aids
Several EFI tuning aids are available to the engine builder and tuner to help eliminate popping, stumbling, surging, pinging, flat spots and overheating, which are sometimes encountered with modified EFI engines. EFI tuning aids can be divided into three major categories:
* Downloadable fuel maps (called ECM calibration by Harley)
* Add-on modules
* Map-based reprogrammers (an add-on module may or may not be used)

Downloadable Fuel Maps
Fuel maps are loaded into the stock EFI ECM to optimize the air/fuel mixture and ignition timing for a specific engine parts combination. Both the Factory and aftermarket manufacturers offer downloadable fuel maps. Harley calls their downloads “ECM calibration” and has several different calibrations for various stages of Screamin’ Eagle performance kits. Harley’s ECM recalibrations are specifically designed for engines equipped with Harley supplied EFI systems (Delphi or Magneti- Marelli). The way the process works is that you first purchase the fuel map from a Harley dealer, and then you pay them to downloaded it into your ECM. Several aftermarket EFI parts manufacturers offer downloaded fuel maps from their website for given engine combinations. Generic downloadable fuel maps are the lowest cost method for eliminating the guess work of tuning an EFI engine with performance modifications.

Add-On Modules
Add-on modules connect in series between the stock ECM and fuel injectors and modify the ECM’s output signals before the signals reach the injectors. Most units are adjusted by using a screwdriver to adjust the “pots,” which adjust rpm transition points (not the ability to increase the rev limit) and air/fuel mixture. Other add-on modules require entering numbers using a display/button device. Certain add-on modules can only instruct the injectors to add fuel or richen the air/ fuel mixture, while others include the ability to both add (richen) and remove (lean) fuel. Pot-based devices are relatively low cost but have certain limitations as to the range of performance engine modifications they can tune.

Map-Based Reprogrammers
Once your engine enters the 100-plus cubic inch range, or you have made major engine modifications, pot-like devices cannot satisfy the engine’s air/fuel requirements. That is where map-based reprogrammers, such as the Harley- Davidson SE Race Tuner, come into play because they have the ability to make virtually unlimited fuel map changes to the stock ECM.

Dynojet’s Power Commander is a mapbased reprogrammer that connects in series between the ECM and fuel injectors. The add-on computer stores maps that can be reprogrammed with a personal computer (PC) for a wide range of engine combinations. Fuel, ignition, and rpm values can be changed for both front and rear cylinders.

Harley-Davidson’s Screamin’ Eagle Race Tuner is a software map-based reprogrammer that requires no add-on module because it has the ability to reprogram the stock ECM. The Race Tuner software allows connecting a PC to the stock ECM. Once the Race Tuner uploads a new map into the stock ECM, the PC and required hardware key are removed and no additional parts remain on the bike. ECM fuel, ignition, start-up, warm-up, and rev limit values can be modified. The Race Tuner also includes a function for setting injector size for greater tuning flexibility. Several operating modes are available, from basic to advanced and data monitoring. The Race Tuner and Power Commander are similar in that fuel map cells represent the percentage of change, either plus or minus.

Aftermarket EFI Systems
Several aftermarket companies offer complete replacement EFI systems for the stock fuel injection system. These systems include their own ECM and software for controlling the ECM. Additionally, aftermarket EFI systems offer carbureted bike owners the ability to upgrade to fuel injection while offering owners of early stock Magneti- Marelli EFI systems an upgrade path that may better accommodate major engine performance modifications.

Final Thoughts
Regardless of whether you are modifying a carbureted or electronic fuel injected engine, the induction principles remain the same, only the way you go about making modifications changes. Remember that the objectives are to increase airflow while adding fuel through either jetting or fuel mapping changes to maintain an optimum fuel ratio for maximum power. For ’06 Dynas and all ’07 bikes, the Delphi EFI system operates in a closed-loop mode, requiring O2 sensors in the exhaust. Be sure any new exhaust system includes bungs for the sensors. Some ’06 and up Twin Cam EFI models run very hot. The problem is related to the EFI closed-loop operation. Re-calibrating the ECM and installing an engine oil cooler are temporary measures that can help minimize the problem, but first check with your dealer for any warranty implications.

If you have a carbureted engine, you still have the simplicity of tuning the induction manually by changing jets, needles and orifices. As for EFI owners, despite the complexity of fuel injection, you’ll find yourself much more comfortable working with them once you learn the basics. But in spite of the type of induction system you have, carbureteor or EFI, remember to upgrade and tune it as a complete system of interdependent parts and always identify the true performance bottleneck before making haphazard modifications.