Last month we began a two-part series on the engine's induction system. We covered induction system basics along with several key performance issues related to carbureted engines. For this installment, we'll look at electronic fuel injection (EFI). If you haven't already done so, you might want to check out last month's installment for a review of the basics.
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 selfcontained 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.
1. Installing a larger TB on an EFI engine is similar to installing a bigger carb on a carbureted engine. One key difference to note is that a carb controls both airflow and fuel. In contrast, a TB only controls airflow while fuel is regulated by a combination of the ECM and fuel injectors. TB size should be matched to the engine's potential horsepower level.
1. Installing a larger TB on an EFI engine is similar to installing a bigger carb on a car
2. The inlet to the TB should be radiused to smooth the airflow into the body's throat. A wide free-flowing air cleaner element will help the air turn into the body's throat.
2. The inlet to the TB should be radiused to smooth the airflow into the body's throat. A
3. Shown is a Kryakyn 57mm TB on a 120ci engine. The large TB tucks in close to the engine similar to stock while flowing more air. Tempe, Arizona-based Short Block Charlie's used stock injectors and a Screamin' Eagle Race Tuner to dial in the ECM's fuel map.
3. Shown is a Kryakyn 57mm TB on a 120ci engine. The large TB tucks in close to the engine