There is a basic truth about internal combustion engines; they need to breath! Restrictions to the intake and exhaust system keep engines from reaching their power output potential. Out of the box, a Harley Davidson motorcycle is ‘dumbed down’ to meet federal and or state emissions guidelines. Fuel systems run lean, air cleaners are restrictive, and exhaust systems act like a clogged drain on the entire engine. Fortunately there are factory and aftermarket parts for your motorcycle to help you beat the tax man. They can be installed separately for smaller performance improvements, or together for optimal power increase. Whichever way your budget allows you to approach paying the Harley Tax, have a plan for what the final improvements will be and how the parts will work together.
Replacing the Factory Muffler with Free Flowing Exhaust
Exhaust output is usually the most limiting aspect of any engine configuration. Take a deep breath and then squeeze your nostrils half shut and try exhaling, and you’ll have idea of what a Harley Davidson engine struggles with. Mufflers and emission devices such as catalytic converters create bottle necks that restrict exhaust flow. This restriction creates back pressure on the combustion cycle, making your engine work harder to produce a given amount of power. The crossover tube on your Harley is designed to help balance the exhaust flow between heads and improve performance, but the restrictive nature of the factory exhaust strangles the engine. Known as high-flow, performance or tuned exhaust, after market exhaust systems that help your motorcycle breath (and usually make it sound better) are a worthwhile upgrade. In my opinion, this should be the first part of the Harley Tax that you pay if you can’t do all three together.
Increasing the Fuel Flow
Leaning out an engine will improve fuel economy and reduce emissions, but it also will rob power. Harley Davidson’s default settings from the factory try to find that balance between emission control and performance, but they leave power potential on the table. Changing the fuel flow of a fuel injected motorcycle, usually starts with reprogramming the computer with different variable sets that control dwell time / fuel flow of the injectors. The good news is that most performance exhaust systems for fuel injected Harley’s include a computer upgrade and instructions for setting it up. For carburetor Harley’s, it will mean replacing the carburetor jets to increase fuel flow. This will entail backing the idle mixture screw out a little to increase flow at idle, and going up a couple of sizes on the slow and main jets. For most tax obligations, going up a couple of sizes will due the trick, but ultimately you should re-size based on your overall plan as discussed in the first paragraph. In my opinion, this should be the second part of the Harley Tax you should pay if you can’t do all three together. Adding a high flow air cleaner without enriching the mixture will only make the mixture more lean.
Replacing the Factory Air Filter with High Volume Filter
Making the fuel mixture richer without enough air to help it burn may help to lower cylinder head temperatures, but it won’t help you get the most bang for your power cycle buck. There are a two basic considerations to air filter design. One is to reduce the restriction / turbulence created as the air passes through the filter. Less turbulent air will flow more readily into the carburetor / intake, increasing the volume of air available to be mixed with the fuel. The second consideration is to increase the volume of air entering the intake by design characteristics. This may be accomplished by any combination of a wider air opening, a housing design conducive to air flow or a housing design that “puts its nose in the wind”, creating a type of “ram” effect to increase air volume. In my opinion, this should be the last part of the Harley Tax you should pay if you can’t do all three together.