In 1995, Harley-Davidson introduced electronic fuel injection (EFI) to its bagger models. At first glance, the faithful were leery of the emerging technology that replaced the venerable, easy-to-tune carburetor. The hog-riding public-as well as the Factory-isn’t always quick to embrace cutting-edge technology on its steel sleds. However, the Motor Company, always looking further down the road than just the next model year, saw the writing on the wall. In order to comply with ever-tightening emissions standards and future environmental concerns, EFI was more than just an exercise in “what if” engineering. Carbs worked pretty well for close to a century, and the straightforward accessibility and diagnosis of fuel-related problems was well within the reach of the average H-D Luddite. We embrace retro, whether it’s looks or using less-than-cutting-edge pushrod motors-but that’s a complex ideology best left for another article.
Whether you love it or hate it, the EFI revolution is here to stay, and, as with other advances in the V-Twin industry, it’s only going to get better and more refined. As of now, the Sportster is the only model in H-D’s lineup to not have fuel injection as an option. The ’01 model year saw the introduction of EFI to the Softail line, while in ’04 the Dynas had the optional computer-controlled gas mixer. Magneti Marelli (MM) designed Harley-Davidson’s first incarnation of fuel injection. That system was used on all the Evo EFI bikes, as well as the ’99-’01 Twin-Cam touring bikes. Beginning in 2001, H-D switched to a Delphi-engineered fuel-injection system on the Softail, with all subsequent models equipped with EFI utilizing the Delphi system. If you want an ’06 Dyna, EFI is the only choice-albeit with a sophisticated closed-loop feedback system that uses exhaust-gas analysis to make air/fuel changes on the fly.
Unfortunately, the early MM EFI had some shortcomings that affected performance, not just on stock bikes but particularly on modified motors. Some of the problems reported with the MM EFI were hard starting, erratic idle, and difficulty in tuning. For performance motors, the dual-plenum throttle body, with independent runners feeding each cylinder, was too small to pass the required air into the engine. Another major shortcoming was the flawed placement of the intake air temperature (IAT) sensor within the throttle body. The ECM uses intake temperature to make adjustments to the fuel mixture. The problem with the sensor in the MM system is that the measured air temperature is not an accurate reading of what the intake temp really is. When the air enters the venturi of the throttle body, it speeds up, changing its temperature. In addition, the throttle body sits right above the hot motor, compounding the problem of accurate air-temperature measurement. Another way the MM unit adjusts the fuel mixture is by the position of the throttle and rpm, a so-called “Alpha-N” system. A major advantage of throttle-based control is that there’s no dependence on intake-manifold pressure, which benefits motors with radical cams. For comparison, the Delphi EFI utilizes a MAP sensor in the manifold to determine the load on the motor and report back to the ECM for fuel changes. This method is called speed-density control, calculating airflow based on engine speed, manifold pressure, and temperature.
If you have an MM EFI-equipped bike and have been looking for better performance, Zipper’s Performance Products has the answer for you. The Zipper’s ThunderMax Marelli EFI Conversion Kit (MSRP: $1,849) was designed to make the bike run the way it should have from the Factory. Zipper’s addressed two of the major shortcomings of the early H-D EFI by using a larger, single-runner throttle body and moving the IAT sensor to outside the throttle body for better temperature measurement. In addition, the idle air controller (IAC) has been upgraded to the better Delphi style. A sophisticated ThunderMax EFI control unit, combined with the SmarTune software, allows the mechanic or home-wrench to tune the bike for different performance packages. The kits are available for ’96-’98 Evos as well as MM-equipped ’99-’01 Twin-Cams with two sizes of throttle bodies: 50mm for 80-103ci motors or 54mm for 107ci and up. Another nice feature is the ability to retain the original wiring harness and gas tank.
The ThunderMax Marelli EFI Conversion Kit contains the ThunderJet EFI throttle body and manifold, ThunderMax EFI control unit, pressure-regulator manifold, ThunderJet air-cleaner backing plate with manifold air temp (MAT) sensor and Hi-Flow air cleaner and cover, ThunderMax tuning software with cable and miscellaneous wires, relay, clamps, and hardware.
Follow along as we take you through the installation on an ’01 Road King.
Zipper’s SmartLink software allows the user to tune and modify the EFI and spark maps to accommodate changes in performance parts or to determine engine problems related to spark and fuel delivery. When you order the ThunderMax MM kit, an optimized base map that matches the components you’ve added to the motor is loaded into the ECM. This ensures optimum ridability and performance.