Written by Menswear Style
in
Cars
on the
27th January 2025
You can tweak and tune Harleys in dozens of ways and by using hundreds of original equipment and aftermarket parts. Most riders want more push when on the throttle start with changes to the engine. The V-twins are all known for their huge low-end torque. What’s missing though is mid-to-high rev power, meaning most Harleys, regardless of displacement, max out at 120hp. And compared to other big bikes can feel out of puff at highway speeds.
To extend power bands, and get more power higher up, consider 3 simple additions all connected with optimizing airflow – velocity stacks, air cleaners and aftermarket exhausts. The parts are relatively easy to install, cheap to buy and help boost power output when torque maxes out.
Velocity StacksÂ
Velocity stacks, trumpets or air horns are elongated tubes attached to the engine’s air intake. Their main purpose is to maximise airflow into the intake. They date back to the beginning of the internal combustion engine, allowing carburettors to adjust air-fuel mixtures as engine loads change. Their specific design means air is eased into the carb, and flows smoothly to the engine. Over time, different designs have been optimised to work with newer tech and today you’ll find Harley Davidson velocity stacks for both carburetted and injected engines, meaning everything from the Knucklehead to the Milwaukee Eight.
Velocity stacks are tasked with getting more air into the engine. More air means more fuel and bigger bangs. The way they do this is by accelerating air into the intake manifold, with the smooth and tapered opening also helping to reduce turbulence. The result is cleaner and more efficient combustion, and by default, more power.
Smoother airflow increases air volume and more air ready to be mixed with fuel. The curved, trumpet designs in velocity stacks help in this respect. Moreover, by changing stack tubing lengths, bike owners can also tune power outputs at different RPMs. In essence, shorter velocity stacks tune power and torque at higher revs, while choosing longer types gives more low-to-mid-end push. Something you’ll appreciate when getting up to speed or when overtaking.
The science at the core of this (called resonance supercharging) is all about tuning the pulses of air rushing through the intake. Choosing the right length helps prevent built-up air from hitting intake valves the moment they are closed and the subsequent effect of pulsating air between the intake runners and valves. With the valves open, air is pushed into the cylinder at higher pressure, resulting in a supercharging effect and improved throttle response.
Added benefits are the louder engine rumble in Harley V-twins and the machined looks bound to turn heads. The parts are supplied with included backing plates, breathers and front screens for filtering and included mounting hardware for easy installation.
Air Cleaner KitsÂ
One of the major gripes with Harley Davidson velocity stacks is that you’ll need extensive knowledge of how airflow works and choosing stacks in the right length tailored to the engine specifics and your tuning goals. Another is the limited filtering capacity from the attached screens. To allow for unobstructed air delivered in the right volume and right speeds at set engine speeds, consider going with aftermarket air cleaner kits. These are assembled filters with matching piping that smooth out and direct airflow to the carb or intake manifolds, depending on the engine in your hog.
Aftermarket cleaners also address the shortfalls of factory Harley air filters, made of low-quality paper and with lower efficiency, by removing common problems like dust and water that can hurt your engine. Look for cleaner kits with reusable and washable cotton cleaners that remove all particles to deliver oxygen-rich air in for combustion, and more of it. Again, the main prerequisite for engine power is how much air it pushes. Aftermarket cleaners are visibly bigger so draw more air in, and most include proprietary tech like machined openings and bolts that optimise airflow for both injected and carburetted Harleys.
Harley owners can choose OE replacement filters to supply more air from the same space, or complete kits with reusable filters and machined hardware that also looks good.
ExhaustsÂ
More air in means more out. If you’ve increased airflow using either Harley velocity stacks or a bespoke air cleaner kit, the benefits end there if you don’t also re-jet the carbs or do a fuel tune and change the stock exhaust piping. Stock exhausts are the major restrictors of spent gases. This is partially down to strict emissions and noise regulations that are effectively choking the engine and possibly one of the reasons Harley Davidson is increasing engine sizes in subsequent model years.
Like velocity stacks and air cleaners, exhausts work effectively by increasing airflow. This means less backpressure from parts that control noise levels like factory baffles and aid exhaust velocity, so the engine can take its next breath faster. Aftermarket parts can also include redesigned headers to help with the pulsating effect of scavenging or the vacuum that draws spent gases out of the cylinders faster. This is achieved with wider and/or straighter piping with carefully chosen lengths.
There are different exhaust options for your bike. The simplest way to get more horsepower to the rear wheel is to use a less restrictive aftermarket slip-on muffler. Besides increasing exhaust velocity and helping the engine breathe, slip-ons can add more substance to the exhaust note and also shed some weight. And as the name implies they’re easy to get on.
For gains that you can actually feel, choose 2-into-1 full exhausts to increase scavenging, especially at high and peak torque (around 3000 RPMs in most Harleys). This additionally means a livelier throttle response. Or go with 2-into-2 systems for improved scavenging and slight overlap that helps in the low to mid power band. A newer hybrid take on both types is the 2-1-2 exhaust, offering the best of both worlds.
Choose exhausts and exhaust parts that are compatible with your Harley, have included mounting hardware to shorten installation times, and are built with quality materials (and street-legal sound deadeners) to ensure they last.
