Fuel Injection in Harley Davidson Bikes: The Path to Improved Performance and Lower Emissions

Iconic American brands come by the dozen, but nothing beats a Harley if you’re after pure riding pleasure. The Motor Company has been churning out fan favourites for well over a century, with cruisers and tourers catering to different tastes and needs. The major drawcard is the level of customisation in each bike. Styling, safety, and convenience additions are readily available for any bike, new or used, and they are provided directly from Harley-Davidson or the thousands of aftermarket brands. This also extends to different variants of the V-twin engines through the years.

Performance and tuning options abound, allowing riders to get the most out of their bikes. One of the biggest sellers is automated tuning modules for injected bikes which can calibrate the fueling and timing parameters at different RPMs, all at the press of a button. This customisation option allows riders to adjust performance characteristics in their bikes for varied riding conditions, without spending thousands for a pro tune.

From Carbs to Injectors

Harley Davidson was relatively late to the fuel injection scene. The gradual shift from carbureted to injected engines started in the mid-1990s, with old-school nostalgia as the main hindering factor. Nevertheless, every two and three-wheeler option today features a more efficient  Harley Davidson fuel injection system, providing more precision, improved reliability, lower emissions and fuel use, and near-endless tuning opportunities.

The simplicity of cable-operated carbureted systems lasted till the eighties, with throttle input from the rider essentially controlling air intake to open or close a valve and regulate fuel flow for optimised combustion. A downside was the lack of metering precision, inaccurate fueling for each cylinder at any given time, and the inability to adjust to changing air or fuel temperatures and atmospheric pressure. This resulted in wasted fuel, power loss and cold starts. The engines though were simple to maintain and fix.

Fuel injection was first offered in the 1995 Electra Glide Ultra Classic, partly due to rising emissions concerns. Carburetors gave way to throttle bodies, fuel injectors, sensors and electronics. Data from various sensors (such as the oxygen sensor lining the exhaust) supply the bike’s computer or ECM with temperature and pressure readouts in real-time, allowing the computer to adjust fuel spray from injectors based on the engine loads and the position of the valves in the throttle body set against optimised values stored in the ECU as ‘maps’. Deviations from optimal air-to-fuel ratios registered in the maps (and based on data from sensor readouts), meant easier adjustments in fuel injector pressure, no wasted fuel or excessive burns and enhanced performance with optimal power and torque curves at defined engine speeds.

Two sensor-based Harley Davidson fuel injection systems were developed based on registered engine loads. Early variants relied on data from throttle position sensors, effectively changing fuel injector pressure based on how far the throttle is open. Later systems determined fuel mixtures from MAP sensor data and vacuum in the intake manifold. Both offer several benefits over carbureted V-twins.

Benefits of Injected Engines

The reluctance to see carburetors go, especially among die-hard fans, can be attributed to the simplicity of repairing the engine if something goes wrong. Early fuel injection systems in bikes were also based on variants used in cars and had some performance hiccups and hunting to get the fueling right. Space was also a hindering factor.

Even so, the reliability and efficacy of today’s systems far surpass that of carbureted Harleys of old. Yes, there have also been serious advancements in carburetor designs to ensure optimal fueling at low, mid and high RPMs, but even these are dwarfed by rudimentary injected systems that get all the basics right.

To see why Harley Davidson joined the fuel injection bandwagon, consider the benefits these systems offer:

• Improved performance and higher fuel efficiency – fuel injection is all about precision, delivering the right amount of fuel at the right time. Translated or real-world riding, this means smoother throttle response, fewer engine vibrations, optimal fuel delivery at all RPMs (carburetors could struggle at idle or high engine speeds) and easier in-gear acceleration. Less wasted fuel from rich mixtures also means savings, higher mileage and avoiding common engine issues like fouled spark plugs, misfires or clogged exhausts.
• Reliability – while more complicated and consisting of more parts, a modern Harley Davidson fuel injection system is more reliable. They are less prone to common fueling problems such as vacuum leaks, evaporation deposits or clogging, and exhibit lower susceptibility to environmental factors, including altitude changes and temperature extremes.
• Lower Emissions – part of the push to go with electronics and fuel injection was the high emission of toxic gases. This is in parallel with higher fuel use, instances of black smoke on startup, backfires from unburnt fuel and more. Today’s bikes may come in bigger displacements but put out considerably less CO2 and NOx.
• Tunability – engines built around ECUs, sensors and electronics are simple to tune, with auto-tune modules loaded with preset maps that are easily embedded into the bike’s ECU. These can change fueling and engine timing and tweak power and torque curves. Want more low-end torque? Or more grunt at higher redlines? Tweaking how the bike reacts to throttle input, and how fuel is delivered without starving or flooding the engine at defined loads is simple. A plus is that there are several maps, with defined engine metrics to get the desired results.
• Simple diagnostics – related to fuel injection is that ECUs also allow for easier diagnostics. Anything wrong with the bike or engine is quickly discovered with diagnostic scanning tools, many of which also list solutions and steps for common issues. Scanners can also uncover problems before they spiral out of control, and cost you thousands in repair bills.

Why Consider Fuel Injection Tuning Modules?

The two available options to tune your injected Harley are getting it on a dyno and connecting the bike to a computer or using tuning modules. Both work in similar ways, by monitoring air/fuel ratios and power and torque numbers at set engine speeds. Tuning modules are simple plug-n-play options, connect to your bike and ECU via the PCV or diagnostic connector with a single cable, a 12-volt connection and in bikes without ‘weld-in’ O2 sensors, come with supplied 12 or 18mm bungs.

Modules can detect instances of off-air-fuel ratios from the stock narrow-band O2 sensors (with readings between 14.3 to 15.2.:1) and adjust fueling from data registered using supplied wide-band sensors that pick up lean or rich readings, for instance when idling, at lighter engine loads, or to ensure proper cruising performance. This is done by metering values in the exhaust and monitoring how the ECU compensates against set stock values.

Tuning modules can come as pre-programmed, with set engine metrics to get streamlined throttle in all riding, weather and road conditions, basically an auto-tune function that rewrites factory ECU settings. Or they let you fine-tune separate metrics, such as fueling in each cylinder separately, adjusting peak power, throttle opening at set RPMs (for instance for lower fuel use or higher torque) and even the fueling programmed for each gear.

More sophisticated units have easy-to-use color touchscreens enabling easy and quick adjustments, with the option to store several customised tuning settings (for example street vs highway cruising) as well as downloadable settings. All tuning kits also come with option to the revert to factory settings and maps or start over if you’re not happy with the results.

But they make the most sense with power additions and parts, such as when going with high-flow air cleaners, less restrictive exhausts or modifying the engine internals. And they carry all the Harley drawcards, but most importantly, a high level of customisation and making the bike your own.