Installing Thunderheader’s 2-into-1 on a 2018 Road King

Install • By Wendell Christopher
There are three sensations that define baggers: visual, audio, and performance. With that in mind, there are very few products that will enhance all three. But in that elite group there are some standouts, one of which is the Thunderheader exhaust system. Over the years Thunderheader has earned a well-deserved reputation. Many a Twin Cam-powered bagger stormed into the real riding world with a set of Thunderheader pipes. That ticket is now available for riders of Harley’s Milwaukee-Eight-powered baggers.

Harley really improved the handling on its 2014 bagger models. Then in 2017 with the addition of the new 107″ Milwaukee-Eight motor, H-D gave a ton more resources to build on. But as usual with a mass-produced motorcycle, there are a few performance drawbacks. In its new bikes H-D started with the factory exhaust system; its 2-into-1-into-2 system doesn’t really allow these 107″ and larger motors to develop their complete potential. Accompanying the incomplete power package is a system lacking a strong, high-quality sound.

Nary a new Harley slips by the crew at Thunderheader, and the M-8 -equipped baggers didn’t escape they orbit. Thunderheader developed a true 2-into-1 high-performance exhaust system that lets all of the Milwaukee-Eight’s inches breathe. But wait. When you say breathe, you’re talking about an exhaust system, not an air filter or throttle body. I know, but an internal combustion motor’s breathing starts by filling an empty combustion chamber, and it requires an exhaust system that really empties the motor’s combustion chamber of all of its burned gases. If the introduction of the Milwaukee-Eight motor lit the proverbial match under H-D’s baggers, now Thunderheader’s new 2-into-1 exhaust system is fanning those flames into a firestorm of power and sound.

I installed Thunderheader’s 2-into-1 exhaust systems on several hot Harleys, and they all performed very well. The Thunderheader design features an expansion chamber type of muffler that produces suction inside the exhaust system’s headpipe. This actually pulls the spent exhaust gasses out of the motor’s exhaust ports and down the pipes until they reach the reverse cone at the muffler’s exit end. Because the new Milwaukee-Eight motor has a different exhaust port layout, Thunderheader had to design a completely new system. The system incorporates new, larger diameter headpipes, a higher capacity expansion chamber type of muffler, and new heat shields. The muffler’s inlet is a larger diameter and the muffler has greater capacity. A very cool feature is that the headpipes’ cylinder head-mounting flanges do not use snap rings to secure them to the pipes’ inlets; they are secured with steel collars, which are welded to the end of the headpipe. For bagger riders that wish to have a left-side exhaust system that balances the bike’s side-to-side appearance, Thunderheader offers a false left-side pipe and muffler system, which includes a partial pipe, an empty muffler, and their mounting brackets. For this article, we ordered Thunderheader’s black system and matching left-side false system, part number 1075XFB, MSRP $575.

Installing the Thunderheader wasn’t a hard job, but because I stopped the technicians, Memphis and Lucas, to snap pictures, it took about four hours. The system’s three shipping boxes not only contained the pipes and mufflers, but every nut, bolt, and bracket that was needed; the instructions were also complete. The install only required hand tools. Not a hole was drilled nor a piece of the bike modified to fit the pipes. There are only three joints in the Thunderheader system, two between the headpipes’ inlet and the exhaust outlets in the motor’s cylinder heads and one at the end of the headpipe. The headpipe is made from two seamless tube sections that are welded together at the front of the muffler. Each of the three form-fitting heat shields was an exact fit. The hardware kit included two brackets to support the system. One of the system’s big features is the bracket that mounts to the transmission; Thunderheader also included new clamps for the heat shields. The elimination of the two snap rings for flanges at the cylinder head exhaust port is well done. Installing the false left-side pipe and muffler was a no-brainer.

Complemented with a free-breathing Screamin’ Eagle air filter, the completed exhaust worked great. The 2018 Road King now has fantastic sound quality, much deeper and mellower than the factory pipes. But the real deal was the bike’s overall performance improvement; it was like a brand-new day in a more powerful world. The Thunderheader is also a great visual complement to what I think is one of Harley’s best looking and performing bikes. This is not the end of our Milwaukee-Eight’s performance road, but the opening to a new and much wider four-lane highway.

Sources: Thunderheader, Black 2-into-1 system
#1073XB, $1,364 Matching left-side false system, #1075XFB, $575 Thunderheader.net

For step-by-step instructions and to read the full install story, pick up Issue 119 of Garage Build RIGHT NOW at GreaseRag.com.

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