Why Does Harley-Davidson's Milwaukee-Eight Engine Have Four Spark Plugs?

As arguably the most recognizable motorcycle brand in the world, Harley-Davidson is consistently at the forefront of innovative engineering. It's all about giving owners the most modern technology, resulting in a riding experience that's both powerful and efficient. So when the company introduced the Milwaukee-Eight engine in 2016, it's no surprise they did something new by installing four spark plugs instead of two.

The reason for this has to do with how the engine burns its air-fuel mixture during combustion. The engine has two cylinders, and two spark plugs per cylinder, which allows the air-fuel mixture to ignite more evenly. Because of the engine's large V-twin design, a dual-spark plug setup makes the most sense. By using this configuration, Harley-Davidson could improve efficiency and emissions in the Milwaukee-Eight engine, while also supporting its strong output.

Overall, Harley's Milwaukee-Eight engine was meant to be a broader modernization of their popular V-twin series. In addition to its dual spark plug design, the Milwaukee-Eight was engineered to deliver up to 11% more torque than Harley's Twin Cam V-twin engine, which did have some big problems like oil leaks and timing chain issues. Additionally, the Milwaukee-Eight had faster acceleration, improved heat management, and a redesigned exhaust system in comparison to its predecessors. All of these features, combined with other internal upgrades, meant that the Milwaukee-Eight was a significant step forward from the engines Harley-Davidson used before.

Harley-Davidson's Milwaukee-Eight engine is standard on many of the company's motorcycles. This includes the Touring class, which has featured the engine since model year 2017. The Milwaukee-Eight has also powered Harley's Softail bikes since 2018. There are three versions of the engine with different cubic inches available as of this writing: The 107, 114, and 117. If you've heard good things about the Milwaukee-Eight, and want to try it out for yourself, contact your local Harley-Davidson dealer.

But be aware that some owners have big problems with the Milwaukee-Eight 114 engine. A Harley-Davidson technical service bulletin, published through the National Highway Traffic Safety Administration (NHTSA), addressed an oil management condition in 2019 that affected certain 2017-2019 models. This condition, known as "sumping," occurs when engine oil accumulates in the crankcase instead of being fully cycled back to the oil tank. According to the bulletin, the condition could result in loss of power and increased engine braking under certain operating conditions.

If you're curious about whether or not a new or used Harley-Davidson you're thinking of buying has this issue, you can visit the NHTSA's website. Using the organization's Recalls page, you enter the bike's VIN or license plate number to get the information you need. You can also check the website for any service bulletins that may affect you; from there, you can make a more informed decision about what to do next.