Harley’s All-New Milwaukee-Eight Big Twin!

Bigger, better, stronger, cleaner, more efficient. What’s not to like?

Harley-Davidson has become a master of hiding technology. Step up to one of the current Rushmore-era touring bikes and stare awhile. Unless you were told, you'd have no idea the bike is built on an engine that breathes through ride-by-wire technology, that has liquid cooling and emissions-scrubbing catalysts, or was fitted with a fully modern Bosch ABS with electronic proportioning. While we were all staring at the chrome and visible amenities, Harley has been quietly adding technology to help the powertrains run better and, crucially, meet worldwide emissions standards. All the while hewing to the call of "the faithful," who have repeatedly proclaimed that their only interest is in a visibly air-cooled, large-displacement, 45-degree V-twin. No matter the performance, if it doesn't look like a Harley, it isn't a Harley.

The engineering team in Milwaukee recognized that there was only so much it could do with the existing Twin Cam platform to move it forward, especially with incredibly restrictive Euro 4 emissions regs on the horizon. No amount of fine tuning would get a big-inch, two-valve-per-cylinder engine where it needed to go, even with the liquid cooling that debuted on some of the touring models. The bikes are getting more sophisticated, and heavier, so the need for more power is ever present. (Especially in the popular trike models.) A fundamental shift was needed.

Milwaukee-Eight cam cover
The Milwaukee-Eight engine carries a single four-lobe camshaft driven by a hydraulically tensioned chain.©Motorcyclist

The answer is the Milwaukee-Eight, an all-new power unit, the first total redesign since the Twin Cam 88, which debuted in late 1998 for the 1999 model year. Yes, it’s been that long. You’ll be excused for not seeing the huge differences that truly do exist; Harley styled the engine to retain the Big Twin heritage. It’s even a little retro, with the pointier cam case that recalls the single-cam Evolution engine that debuted for 1984 to much acclaim by everyone but Panhead lovers. Now you’re seeing how the pushrods come together at the cam case, and are not separated as in all the Twin Cam models. Ah-ha. Something’s up.

Milwaukee-Eight head and valves
You're seeing it right: That's a four-valve head. Harley's change to quad valves brings a more modern combustion chamber for better power and heat rejection.©Motorcyclist

Yes, and it’s all the way up top. For the first time in a Big Twin, the valve count is eight total. According to Harley’s chief powertrain engineer, Alex Bozmoski, several needs pushed Harley away from the familiar two-valve, quasi-hemispherical combustion chamber. First is power; a four-valve head has greater power “headroom.” Next is the ability to run such an engine more aggressively lean for emissions without risking damage or giving up power; the more modern combustion chamber along with dual spark plugs per head make this possible. And then there’s packaging: By doubling the number of valves, total valve area can be increased without having to resort to a tall combustion chamber, necessary to accommodate two relatively large valves. (Harley makes the job a little harder by sticking to long-stroke engines, so fitting two huge, free-flowing valves into a comparatively narrow bore forces a lot of compromises.) The resulting combustion chamber in the Milwaukee-Eight is much flatter than the Twin Cam’s, which makes it more efficient, cleaner burning, and less prone to destructive detonation. With a greater detonation margin, Harley can run more aggressive compression ratios—as much as 10.5:1 in the higher-performance versions, almost a full point up on the typical Twin Cam 103.

Milwaukee-Eight rockers
Twin rocker arms translate a single pushrod's motion to both paired valves (intakes and exhausts) without a means of adjusting clearances between them. Harley says the clearances are set at the factory and good for life.©Motorcyclist

Increasing compression ratio makes more power and improves efficiency all by itself, but Harley’s gone further by upping displacement. The rank and file engine will be 107ci from a 100mm bore and 111.3mm stroke (the same as on the TC103). The CVO models will get a 114ci version from a 102mm bore and a 114.3mm stroke. How much more power? Harley doesn’t release horsepower figures, but says the 107ci Milwaukee-Eight makes 10 percent more torque than the 103. And along with that boost in torque, H-D says we can also expect to see an increase in fuel efficiency as well.

Milwaukee-Eight cut-away
The blue passages are where oil circulates in the new Milwaukee-Eight head. All versions of this engine are technically liquid cooled; some with coolant and some, such as the Road King, with oil.©Motorcyclist

Power brings heat, and Harley’s met this problem head on. All Milwaukee-Eight engines will in some way be liquid cooled. The larger touring models (Road Glide Ultra, Electra Glide Ultra, Tri Glide trike) will keep versions of the Twin Cooled liquid-cooling system introduced on the Rushmore update three years ago. The rest will have dedicated oil passages in the head near the exhaust valves to put some of that thermal stress into the oil, which now circulates with an extra half quart of capacity (now 4.5 quarts) and a small cooler at the bottom of the forward frame downtubes.

Milwaukee-Eight rocker arms
Here's a more detailed view of the upper valve train. While there are no clearance adjustments up top, Harley has retained a roller-tipped hydraulic lifter at the cam interface.©Motorcyclist

There are many clever design details in the valve train alone, which vaguely resembles a four-valve Moto Guzzi’s in the sense of there being a single pushrod for each pair of intake and exhaust valves per cylinder reaching from the cam case up to a robust-looking rocker-arm assembly that operates each valve with a separate arm. As before, there’s a hydraulic lash adjuster (lifter) between the cam and the lower end of the pushrod. And you might go looking inside the rocker box for some way to adjust the valves relative to each other, intake to intake, exhaust to exhaust. After all, it’s got to be really difficult to get the components of the valve stems and rocker arms to wear evenly so that there’s no clearance dissimilarity between each pair of valves, right? Not this time. Harley wouldn’t say how it was accomplished, but we’re guessing that a combination of metallurgy, valve train geometry, and carefully chosen cam profiles have made it so there’s no need to compensate for wear between each valve pairing; it’s set at the factory and set for life. So they tell us.

Milwaukee-Eight cam
A four-lobe cam resides in the normal place for a Harley Big Twin but is spun by a quiet chain. The oil pump is just below the cam.©Motorcyclist

Cam drive reverts to a single, four-lobe cam, like on the Evolution engine, for the primary purpose of reducing complexity and noise. The cam is not gear driven, as on the Evo, but powered by a hydraulically tensioned chain. Noise reduction was among the engineers’ top priorities with the Milwaukee-Eight for the simple reason that every little bit of piston slap, gear whine, or chain rattle takes away what internal-combustion music Harley can legally leave behind. A quiet engine allows for a more throaty intake and exhaust, which the M8 models definitely possess.

So, the engines are bigger, and have all-new heads with four valves per cylinder. But we’re just getting started. More power means greater bottom-end stress, so the connecting rods are stronger with larger bearings; because it has retained a knife-and-fork connecting rod arrangement with a built-up crank, these are still roller-element bearings. The massive crank itself weighs the same as the old 103 but has greater inertia because more of the mass is near the outside diameter of the crank. Housing the new bottom end is an entirely new case set, which has been optimized, or “shrink wrapped” around the moving parts and given a new breather system to reduce internal losses. Even at a modest redline of 5,500 rpm, the 4-plus-inch stroke moves a lot of air around the crankcases.

Milwaukee-Eight counter-balancer
The gear at the top of this photo drives the counterbalancer low and forward in the engine cases. While the new flywheel set weighs the same as on the Twin Cam 103, it has more inertia because the mass is concentrated further away from the center.©Motorcyclist

Look more closely at the bottom end and you’ll spy a gear-driven counterbalancer low and forward. Every Milwaukee-Eight engine will have a counterbalancer, whose purpose is to reduce idle-speed vibration even when the engine is fitted with rubber mounts, as on the touring models. This balancer represents a 75-percent balance factor, which leaves a little bit of rumble at idle and something for the rubber mounts to do. Presumably, Harley can bump this up to 100 percent compensation so this engine will work in the solid-mount Softail line. (When we asked H-D engineers about this, they resolutely refused to comment. We’ll take that as confirmation.)

Milwaukee-Eight oil pump
This is a view of the cam chest before the camshaft is installed. The overall architecture of the Milwaukee-Eight follows Twin Cam form but with improvements everywhere you look.©Motorcyclist

The balancer shaft represents an interesting twist for Harley. In the past, the way the engine shakes like a wet dog at idle would have simply been another of its character points. But market research, resulting in a complex set of directives under the term Voice of the Consumer, says that so-called conquest customers objected to this characteristic. Riders coming from other brands apparently felt the shaking was untoward, so Harley went about taming the idle quivers at no small cost in time and effort.

Such an effort is also reflected in the new engine’s ride-by-wire system, now calibrated as a “torque demand” system rather than one that tries to match hand throttle and throttle plate positions. Fitted with a 55mm throttle body (up from 50mm), the Milwaukee-Eight offers increased breathing potential along with improved rideability. New dual-spray fuel injectors (still one per cylinder) have their issue aimed directly at the intake valve stems. A new knock sensor bolted to the head replaces the previous ion-sensing system, which “read” the spark plugs electrically to determine if detonation was taking place. The new acoustic sensors are much more sensitive, giving the ECU better data, which results in quicker response times. Harley says the engine can detect and react to detonation events in just a couple of combustion cycles. With such quick responses, the engine can be tuned to run a much narrower detonation margin, which improves power and efficiency.

Milwaukee-Eight compensator
A ramped compensator mechanism resides outboard of the alternator and helps reduce shock loads to the primary drive.©Motorcyclist

Ergonomics were considered as well: The air cleaner assembly is narrower where it might contact the rider, while also freer flowing. To the same end (and to help with heat transfer to the rider and passenger), the rear header pipe is more tucked in than it is on the TC103. Together with a more efficient combustion chamber, catalysts pushed far back into the mufflers, and overall improved engine cooling, the M8 should be a lot easier on its pilot during those steamy summer months.

While the engine itself is the centerpiece of the Milwaukee-Eight effort, there are changes in the driveline as well. A new slip-and-assist clutch comes on board, along with a refined primary case that’s narrower than before. There are improvements to the charging system as well, with a new alternator producing much more current at low rpm. The old system made 17 amps at the nominal idle speed of 1,050 rpm, but the M8 arrangement makes 24.5 amps at the new, lower idle speed of 850 rpm, and more than 30 amps at 1,050 rpm. A 1.6kW starter replaces the 1.2kW piece to make the big engine spin easier, a job aided by centralized electric compression releases in each cylinder head.

Milwaukee-Eight stator
Along with updated electronics, the Milwaukee-Eight gets a massively up-rated charging system.©Motorcyclist

In hundreds of ways, Harley’s engineers sought to improve the Big Twin and prepare it for another decade or more of service. The big push in Europe for ever tighter emissions regulations (both for noise and pollutants) helped drive Harley forward, but a recognition that its core customers are changing, morphing as newer riders enter the fold without the historical perspective of what a Knucklehead felt like, or how a hot-running Panhead sounded as it clacked and clanged back to room temperature after a ride. For them, these are abstractions. What matters is performance and capability that fits with a perception of what a Harley engine should be: rumbly and charismatic, sure, but also in line with the competition.