Harley-Davidson’s New Milwaukee-Eight Big Twin Engine

The motor company builds its third all-new Big Twin in 80 years

It doesn’t happen often, but when it does, it’s big news. We’ve all learned the great names—Knucklehead, Pan, Shovel, Evolution, and Twin Cam—and now there’s a new one: “Milwaukee-Eight,” which tells us this new engine, like Harley’s legendary board-track racer of long ago, has eight valves.

Displacement is 107ci (1,750cc) or 114ci (1,870cc). The 2017 touring models get these engines first, but this is the Big Twin that will, no doubt, power Harley-Davidson’s future.

Harley-Davidson Milwaukee-Eight engine close-up

Harley-Davidson’s New Milwaukee-Eight Big Twin Engine

Harley-Davidson's new eight-valve Big Twin in 107-inch variant.Courtesy of Harley-Davidson

The standard 107 uses precision oil-cooled cylinder heads and will be found in Street Glides, Road Glides, the Electra Glide Ultra Classic, and Freewheeler trikes.

A Twin-Cooled version with liquid-cooled cylinder heads and radiators will power Ultra Limited models, the Road Glide Ultra, and Tri Glide models.

Harley-Davidson Milwaukee-Eight engine
The top view of an engine shows stiff tubular rockers, each of which operates two valves. Valve clearance is maintained by hydraulic roller tappets. There is one central spark plug and one side plug per cylinder.Courtesy of Harley-Davidson

CVO Limited and Street Glide models are equipped with the Twin-Cooled Milwaukee-Eight 114 featuring liquid-cooled cylinder heads and radiators.

“It’s a brand-new motor, tip to tail,” says Alex Bozmoski, Harley-Davidson chief engineer of new products. He has 34 years with the company and was involved in both Evo and Twin Cam development.

The new eight-valve engine seeks two broad goals. One is to make greater power and torque while being emissions-compliant, fuel-efficient, and highly reliable. The other is a trend visible across the vehicle industry—to achieve world-class “ride feeling” through chassis, suspension, and driveline refinement.

The design of Harley’s Big Twins has tracked the development of America’s highways. When most roads were dirt and average speeds low, the Knuck’s iron heads and cylinders handled the heat and “wore like iron.” As roads were paved and four-lane highways began to appear, riders could ride farther, faster. More power being used required increased cooling, so higher-heat-conductivity aluminum replaced iron, first in heads and then in cylinders. Design evolution of the last generation of Twin Cam was anchored by improved cooling.

The new eight-valve engine seeks two broad goals. One is to make greater power and torque while being emissions-compliant, fuel-efficient, and highly reliable. The other is a trend visible across the vehicle industry—to achieve world-class “ride feeling” through chassis, suspension, and driveline refinement. Customer research, covering 1,000 riders in seven cities, was distilled into “The Voice of the Customer,” telling The Motor Company that riders want more power for two-up riding and more back-road agility. They want the bike to fit more sizes of people. They want cooler operation.

Harley-Davidson Milwaukee-Eight engine
Cooler operation is the goal. Less combustion heat is picked up by a shallower four-valve combustion chamber, and more heat is dissipated by large-area head cooling fins. Note the dual spark plugs and short exhaust port.Courtesy of Harley-Davidson

Every manufacturer collects service information and tracks specific problems, such as overheating or any mechan­ical issues. Discussion of particulars can be found on internet Harley forums. When necessary, simple design changes are adopted, but no design is permanent, so eventually a comprehensive redesign becomes necessary to integrate many needed changes into a fresh package.

Harley-Davidson Big Twin owners love the look of an air-cooled engine’s fins but have accepted “strategic cooling” (intensive cooling of specific areas) as the price of keeping that look while improving function. In the recent Rushmore series of changes to the Twin Cam engine, this took the form of circulating liquid coolant in passages around each cylinder head’s hot exhaust valve seat and then to external radiators, as a means of keeping valves and valve seats well-sealed and warp-free as more power (and therefore increased heat) was sought.

Harley-Davidson Milwaukee-Eight engine
The big new external-rotor 18-pole alternator makes plenty of juice at lower 850-rpm idle.Courtesy of Harley-Davidson

This is only one of two basic approaches to heat management—removing excess heat to keep parts at safe temperatures. An alternative approach is to change the design so it takes up less heat from combustion—an approach that may also increase power and efficiency by keeping that heat where it works for us—in the hot, high-pressure combustion gas that presses the pistons down to drive the crankshaft.

Therefore the new eight-valve engine abandons the large surface area of the traditional deep, modified hemi two-valve combustion chamber and puts in its place an almost flat chamber of minimum surface area with four valves. A true hemi chamber has exactly twice the surface area of a flat chamber, and to achieve the higher compression ratios needed to make strong torque, a piston in a hemi chamber may need a dome that increases its surface area as well. Thus, by switching from modified hemi two-valve to nearly flat four-valve, the surface area through which heat can enter heads and pistons has been substantially reduced.

Harley-Davidson Milwaukee-Eight engine
View of timing case on right side of engine, showing new four-lobe single cam.Courtesy of Harley-Davidson

Overall airflow capacity of the eight-valve is 50 percent greater than that of previous engines, and the throttle body now has a 55mm bore. Room for growth! A free-flowing intake airbox is engineered for low noise and to intrude less into the rider’s ergonomic space.

Another way to take up less heat from combustion is to speed it up, exposing heads and pistons to flame for shorter time. Faster combustion from two spark plugs per cylinder is one element used in the Milwaukee-Eight to achieve this, but a second is invisible: thousands of hours of flow and combustion simulation studies. Engineer Bozmoski said that ignition timing in this cooler-running engine is “more advanced, more of the time,” suggesting that in the past, systems intended to keep the engine safe from heat effects (such as detonation from advanced spark timing) were intervening frequently, resulting in some loss of performance. The irony of retarding the timing to keep the engine safe from detonation is often more heat.

Harley-Davidson Milwaukee-Eight engine
New 49mm fork with larger damping pistons and variable orifice damping now delivers good control at low speed yet does not become harsh at higher speed.Courtesy of Harley-Davidson

Engineers know that the crucial region of any four-valve head is the “exhaust bridge,” the narrow region that separates each head’s two exhaust valves. The obvious reason is that this bit of metal is heated by hot exhaust gas from both sides. In some air-cooled four-valve designs a brute force solution is tempting; to cast in a strong bronze “skull” with the valve seats and spark plug holes in it. But this degrades cooling because bronze transmits heat less well than aluminum. In the Milwaukee-Eight 107 (3.937 x 4.375-inch bore and stroke), this bridge is strategically cooled by pumping oil through it and then through a “chin radiator” ahead of the crankcase. In the 107 and 114 Twin-Cooled models (the 114 has 4.016 x 4.500-inch cylinder dimen­sions), water/antifreeze coolant is circulated through a cored heart-shaped passage that encircles the exhaust valves and then through coolers mounted forward to either side of the engine, as previously seen.

Because these new, larger-displacement engines take up less heat from combustion and employ strategic exhaust valve cooling, they can safely operate at high compression ratios (now as high as 10.5:1) that produce greater power and torque (the engineers' rule of thumb is that peak combustion pressure is roughly 100 times the compression ratio). As a result, the new bikes deliver a claimed 10 percent more torque, are two to three bike lengths faster from 0–60 mph, and one to two lengths quicker in top-gear 60–80-mph roll-ons. Harley states also that "improved fuel economy" comes with the changes—the reason being that more of the fuel's heat ends up pushing the pistons and less passes into metal parts and out the exhaust.

Harley-Davidson Milwaukee-Eight engine
Rear shocks have mechanical knob preload, tool-free adjustment.Courtesy of Harley-Davidson

Each cylinder has an acceleration-type knock sensor that, with ECM control, protects the engine from detonation. Within one or two detonating cycles the system pulls ignition timing back to prevent abnormal combustion. The system is an improvement over the previous ion-sensing knock detection.

Exhaust components, including the catalyst, have been relocated to move engine heat away from rider and passenger.

Like the original Big Twin—the EL of 1936—the new engines have a single four-lobe camshaft in place of the Twin Cam’s pair. Drive is by chain with automatic hydraulic tensioner. Fewer parts mean reduced noise and lower friction. What was said when the Evo engine first appeared still applies: “We’re killing the noise so we can keep the music.” The EPA sets a maximum sound level, measured in a specified way. Radi­ated sound is partly exhaust rumble (good) and partly mechanical. Mechanical noises often include energetic high frequencies (clicking, gear whine, etc.), so identifying and suppressing them allows that energy to go into a “richer exhaust note.” Years ago, when Big Twin cams were gear-driven, noise could be generated by excessive backlash (loud clicking from the constant rapid reversals of cam torque) or by tooth form irregularities. Both were expensive to control. Swiss gear-grinding machines were imported to achieve accurate tooth profiles, and time-consuming selective fitting was necessary to control backlash. An automatically tensioned chain eliminates backlash and spreads loads over many sprocket teeth, making operation quiet. We love the precision and beauty of gears, but we also love simplicity.

Harley-Davidson Milwaukee-Eight engine
Crank in right-hand crankcase reveals gear-driven crank-speed counterbalance shaft.Courtesy of Harley-Davidson

For the first time in a rubber-mounted Big Twin, a single counter-rotating internal balancer is used to cancel 75 percent of the engine’s primary shaking force. When an engine with complete vibration cancellation was tested, riders rejected it; some vibration is an essential element of Harley’s unique feel. Riders of long ago accepted lots of raw vibration, but most riders today do not—and Harley-David­son here appears to be satisfying the faithful customer with some of the shake they love while also smoothing the engine to the degree that will attract new customers with an increased level of refinement. Heavy vibration is tiring, so long-distance riders want engine smoothness—but still want the drama of handlebars shaking at idle—and the relationship with the moving parts.

Incorporating a balancer into the rubber-mounted Milwaukee-Eight engine in this way may allow it to be used in the Softail line, which uses solid-mounted engines. The solution with the Twin Cam was a second, dual-counter­balanced engine, the “B,” introduced in 2000. Harley-Davidson did not comment on this theory.

Harley-Davidson Milwaukee-Eight engine
View of timing case shows Gerotor oil pump below, inner cam bearing above.Courtesy of Harley-Davidson

The new crankcase is close to the weight of the old one, but sophisticated redesign has made it 23 percent stronger than before. Major castings are “shrink-wrapped,” revealing the shapes of the parts within to achieve a muscular functional look. The transmission case has been strengthened and a scissors anti-backlash gear in the has been adopted to eliminate a possible hot rattle.

Small details are important. In the past, if the oil was overfilled, excess oil could wet the air filter via the breather. This has now been rerouted through the transmission case.

Idle rpm has been cut from 1,000 to 850 rpm in the interest of “better sound and less heat.” A capable new alternator supplies 24 to 25 amps at that lower idle, with 50 percent more capacity available at highway rpm. “It will drive all the accessories,” Bozmoski said. A new 1.6 kW (2.14 hp) starter replaces the previous 1.2 kW (1.6 hp) unit and an automatic compression release speeds starting.

Because markets are now global, riders coming to the Harley-Davidson brand (this includes many abroad) expect more than tradition; they expect modern levels of refinement.

Harley-Davidson Milwaukee-Eight engine
This cutaway of one cylinder and head shows intensive cooling of critical areas—the exhaust valve seat and exhaust bridge—by circulating liquid, shown in blue. Note the lovely fin surface finish, lightweight piston with low-tension bore-conforming rings.Courtesy of Harley-Davidson

Many analytical hours were spent in completely retuning the chain primary drive system, refining its oper­ation, as well as making it narrower. The main problem for a large-displacement twin is to seamlessly convert the engine’s series of big torque pulses into smooth propulsion of the motorcycle—without creating unpleasant resonances or jerking. The intuitive solution—massive flywheels—isn’t enough, for the heavier the flywheels, the slower the accel­eration. Also, when two large masses—the crankshaft and the motorcycle itself—are flexibly connected (no driveline can be infinitely rigid), all kinds of oscillations can result.

There is a self-torque-boosting clutch with Brembo hydraulic actuation, resulting in a lighter lever pull. The engine ECM has been changed from a mapped (command) system to torque-based (rider throttle position is interpreted as a call for a specific torque level, not a specific throttle angle). Quiet roller-tipped hydraulic tappets continue to be used, and Harley-Davidson claims the forked rocker arms—two per head—will require no adjustment for the life of the engine. With eight smaller, lighter valves in place of the original four larger ones, total valve moving mass remains largely unchanged, but seat pressures may now be lower, leading to essentially no wear or recession. Bozmoski said operation “sounds quite a bit more refined.” The connecting-rod system remains fork-and-blade with roller big-ends as always, but the rods are larger and stronger to handle the increased torque.

Harley-Davidson Milwaukee-Eight engine
Harley-Davidson Milwaukee-Eight powertrain cutaway.Courtesy of Harley-Davidson

Engineers carrying out driveline analysis saw only numbers for months but when presented with the real final result were delighted to find obvious improvement that any rider can feel.

Front and rear suspension is all new, with the goal of improving control and eliminating harshness. The new 49mm fork contains “dual bending valve fork technology.” This means it employs cartridge-style variable-orifice damping valves, which can deliver excellent control at low speed (preventing any wallowing feeling) without harshness over sharper bumps—something not achievable with the old system of fixed damping orifices. The words “bending valve” refer to how the variable orifices are implemented; in this case, the damping fluid’s flow path through the damper pistons is covered at rest by a thin washer that is clamped at its (usually) inner edge. As suspension motion pushes damping fluid faster, the free edge of the washer is deflected by fluid pressure, forced into a shallow cone shape as fluid is forced past it, allowing the flow path to become bigger. Correctly done, this results in damping force that is proportional to damper piston speed, giving a well-controlled and comfortable ride.

Bigger damper pistons, which move more oil per inch of suspension movement, further improve low-speed damping control. Fork travel is 4.6 inches on standard models and 3.9 inches on low models.

Harley-Davidson Milwaukee-Eight engine
Harley-Davidson Milwaukee-Eight 107 engine.Courtesy of Harley-Davidson

The goal of the new rear suspension is to increase the range of preload adjustment while eliminating both the leakdown of air shocks and any need for tools. Twenty-three turns of a single knob give an inch of preload adjust­ment. As with the new fork dampers, larger damper pistons give the suspension increased ability to control movement.

We know that in addition to the thousands of hours of analysis and predictive computer modeling that went into these new engines, a similar investment of time was made in real-world rig and dyno testing plus exhaustive road operation to validate all hardware changes.

All this effort underlines the importance of the work for Harley-Davidson. The Big Twin, this new Milwaukee-Eight, is the very core of Harley’s business, the “engine” that ultimately drives sales all the way down to the last key fob. It’s the heart, soul, and sound that connects to the brand’s millions of fans around the world. All Big Twins have been designed to satisfy The Faithful, but perhaps more than any Big Twin before, the Milwaukee-Eight is designed and refined to capture a broader audience. If ever an engine inspired religious fervor, this is it. Did we just hear a hallelujah?

Harley-Davidson Milwaukee-Eight engine
Harley-Davidson Milwaukee-Eight 107 main air cleaner.Courtesy of Harley-Davidson
Harley-Davidson Milwaukee-Eight engine
Harley-Davidson Milwaukee-Eight 107 fins detail.Courtesy of Harley-Davidson