The classic Harley-Davidson engines are two-cylinder, V-twin engines with the pistons mounted in a 45° "V". The crankshaft has a single pin, and both pistons are connected to this pin through their connecting rods.
This design causes the pistons to fire at uneven intervals, the consequence of an engineering tradeoff to create a large, powerful engine in a small space. This design choice is entirely vestigial from an engineering standpoint, but has been sustained because of the strong connection between the distinctive sound and the Harley-Davidson brand. This design, which is covered under several United States patents, gives the Harley-Davidson V-twin its unique choppy "potato-potato" sound. To simplify the engine and reduce costs, the V-twin ignition was designed to operate with a single set of points and no distributor, which is known as a dual fire ignition system, causing both spark plugs to fire regardless of which cylinder was on its compression stroke, with the other spark plug firing on its cylinder's exhaust stroke, effectively "wasting a spark." The exhaust note is basically a throaty growling sound with some popping.
The 45 degree design of the engine thus creates a plug firing sequencing as such: The first cylinder fires, the second (rear) cylinder fires 315° later, then there is a 405° gap until the first cylinder fires again, giving the engine its unique sound.
Harley Davidson has used various ignition systems throughout its history - be it the early points/condenser system, (Big Twin up to 1978 and Sportsters 1970 to 1978), magneto ignition system used on 1958 to 1969 Sportsters, early electronic with centrifugal mechanical advance weights, (all models 1978 and a half to 1979), or the late electronic with transistorized ignition control module, more familiarly known as the black box or the brain, (all models 1980 to present).
With the implementation of Electronic Fuel Injection (EFI), Harley Davidson uses a single fire ignition system on the models equipped with EFI, which now includes 100% of their product line with the introduction of the 2007 models.
In 1991, Harley-Davidson began to participate in the Sound Quality Working Group, founded by Orfield Labs, Bruel and Kjaer, TEAC, Yamaha, Sennheiser, SMS and Cortex. This was the nation's first group to share research on psychological acoustics. Later that year, Harley-Davidson participated in a series of sound quality studies at Orfield Labs, based on recordings taken at the Talladega Superspeedway, with the objective to lower the sound level for EU standards while analytically capturing the "Harley Sound." This research resulted in the bikes that were introduced in compliance with EU standards for 1998.
On 1 February 1994, the company filed a trademark application for the distinctive sound of a Harley-Davidson motorcycle engine: "The mark consists of the exhaust sound of applicant's motorcycles, produced by V-twin, common crankpin motorcycle engines when the goods are in use". Nine of Harley-Davidson's competitors filed comments opposing the application, arguing that cruiser-style motorcycles of various brands use a single-crankpin V-twin engine which produce a similar sound. These objections were followed by litigation. After six years, Harley-Davidson withdrew their trademark application.
When Honda first began making a motorcycle with a 45° V-2 design, the Honda Shadow, it used a more advanced engineering approach with an offset crank design featuring dual crankpins, which allows for even firing pulses and higher horsepower because of the reduced vibrational stresses on the engine. Responding to customer demands for a more traditional V-twin, in 1995 Honda rolled out the Shadow A.C.E., a single-crankpin design, with a more traditional exhaust note and feel, but more vibration and less maximum power.