Few topics spark as much discussion among Harley-Davidson enthusiasts as engine design. Walk into any dealership, join any owners' group, or stop at a country pub where a few Harley trikes are parked outside, and before long someone will ask the inevitable question:

"Is the Twin-Cooled engine better than the Milwaukee-Eight?"

The answer isn't as straightforward as many would like. Both engines represent significant milestones in Harley-Davidson's engineering journey, yet they were developed to solve different problems at different points in time.

For trike riders—particularly those on Tri Glide Ultras and Freewheelers—the choice between these engines isn't just about horsepower. It affects heat management, maintenance, riding comfort, reliability, sound, and even resale value.

Let's take a closer look at what separates Harley's Twin-Cooled Twin Cam from the Milwaukee-Eight, and which engine may be the better choice depending on how and where you ride.

Why Harley Introduced Twin-Cooling

For decades Harley-Davidson relied exclusively on air-cooled V-twin engines.

Air cooling is beautifully simple.

No radiator.

No coolant.

No water pump.

The fins on the cylinders dissipate heat naturally as air passes over them.

That simplicity became part of Harley's identity.

Unfortunately, motorcycle regulations don't care much about tradition.

As emissions standards became stricter worldwide, Harley engineers faced a significant challenge.

To reduce emissions, engines generally run leaner.

Lean mixtures create more heat.

At the same time, customers wanted:

• more power
• smoother engines
• greater reliability
• better fuel economy

Those requirements often conflict with purely air-cooled designs.

Something had to change.

The Birth of Twin-Cooling

Rather than abandon air cooling altogether, Harley developed what became known as the Twin-Cooled High Output Twin Cam 103.

Introduced on selected Touring models in 2014 and later adopted for the Tri Glide Ultra, the system remained primarily an air-cooled engine.

Only the hottest parts of the engine were liquid cooled.

Specifically:

• cylinder heads
• exhaust valve areas

Everything else continued relying on traditional cooling fins.

It wasn't water-cooled in the way most Japanese motorcycles are.

Instead, it was a hybrid approach.

Think of it as giving the engine targeted cooling exactly where it needed it most.

How Twin-Cooling Works

Twin-Cooling uses:

• coolant passages through the cylinder heads
• electric water pump
• thermostat
• discreet radiators hidden inside the fairing lowers

Many riders don't even realise the motorcycle has radiators because Harley concealed them remarkably well.

Unlike large exposed radiators seen on sports bikes, the system preserved Harley's classic appearance.

The engine still looked air-cooled.

The iconic V-twin shape remained unchanged.

That mattered enormously to Harley buyers.

Benefits of Twin-Cooling

The biggest improvement was heat management.

Anyone who has ridden an older Harley on a scorching Australian summer day understands just how much heat these engines generate.

Now imagine sitting stationary at traffic lights on a Tri Glide.

Three wheels mean you can't lean the bike to encourage airflow.

Heat builds rapidly.

Twin-Cooling significantly reduced temperatures around:

• rider's legs
• passenger comfort
• cylinder heads
• exhaust valves

That translated into:

• improved comfort
• less heat soak
• reduced chance of engine knock
• more consistent performance

For touring riders crossing inland Australia, those advantages were substantial.

Twin-Cooling Wasn't About More Power

One common misconception is that Twin-Cooling dramatically increased horsepower.

It didn't.

Its primary purpose was thermal management.

By keeping combustion temperatures under control, Harley could:

• meet emissions requirements
• improve durability
• maintain performance under heavy loads

Any power increase was relatively modest.

The real gain was consistency.

Instead of losing performance as temperatures climbed, the engine could continue delivering predictable output.

Then Came the Milwaukee-Eight

In 2017 Harley-Davidson unveiled one of the biggest engine redesigns in decades.

The Milwaukee-Eight.

The name comes from:

• Milwaukee
• eight valves

Previous Harley big twins used:

• two valves per cylinder

Milwaukee-Eight uses:

• four valves per cylinder

With two cylinders, that's eight valves altogether.

It may sound like a small engineering change.

In reality, it transformed the engine.

Bigger Breathing

Four valves per cylinder allow the engine to breathe far more efficiently.

Benefits include:

• improved airflow
• faster combustion
• greater efficiency
• more complete fuel burn

That translates directly into:

• increased torque
• stronger acceleration
• smoother throttle response

Harley didn't simply redesign the cylinder heads.

Almost every major component changed.

A More Modern Engine

Compared with the Twin Cam, Milwaukee-Eight introduced:

• redesigned combustion chambers
• central spark plugs
• better oil circulation
• improved crankshaft balance
• lighter valve train
• higher compression
• revised camshaft

Collectively, these changes made the engine:

• smoother
• stronger
• quieter mechanically
• more efficient

Without losing its unmistakable Harley personality.

Torque Is King

Harley riders rarely obsess over peak horsepower.

Torque matters far more.

Especially for trike riders.

A Tri Glide typically weighs well over 550 kilograms once loaded with:

• rider
• passenger
• luggage
• accessories

Getting that mass moving requires low-end pulling power.

Milwaukee-Eight excels here.

Compared with the Twin-Cooled Twin Cam, riders generally notice:

• stronger roll-on acceleration
• easier overtaking
• less gear changing
• effortless highway cruising

The engine feels more relaxed despite producing greater performance.

Reduced Engine Heat

One interesting aspect of the Milwaukee-Eight is that many riders find it actually runs cooler than older Twin Cam engines—even before considering the later liquid-cooled versions.

Why?

Because the engine itself is simply more efficient.

Better combustion means:

• less wasted heat
• improved airflow
• reduced internal friction

Harley also improved:

• oil circulation
• combustion efficiency
• exhaust flow

The result is an engine that naturally generates less excess heat.

Oil-Cooled vs Twin-Cooled vs Liquid-Cooled Milwaukee-Eight

This is where things become confusing.

Not every Milwaukee-Eight is the same.

Depending on model and year, Harley has produced:

Air/Oil-Cooled Milwaukee-Eight

Uses:

• cooling fins
• engine oil

No coolant. No radiators.

Twin-Cooled Milwaukee-Eight (Early Touring)

Some early Touring models retained the Twin-Cooling concept.

Reflex Defensive Rider Systems (RDRS) Era Liquid-Cooled Variants

Harley later introduced even more advanced cooling systems for selected models where additional heat management was beneficial.

Understanding exactly which system your motorcycle has depends on its model year and specification.

Smoothness

One criticism occasionally aimed at older Twin Cam engines was vibration.

Harley riders often love mechanical character.

Too much vibration, however, becomes tiring after hundreds of kilometres.

Milwaukee-Eight significantly improved refinement.

Riders commonly report:

• smoother idle
• reduced buzzing
• cleaner acceleration
• less fatigue

Importantly, Harley retained enough engine pulse that it still feels unmistakably like a Harley.

They didn't sterilise the experience.

Maintenance Differences

Twin-Cooled engines introduce extra components:

• coolant
• hoses
• water pump
• thermostat
• radiators

These systems require periodic inspection and coolant replacement according to Harley-Davidson's maintenance schedule.

Although highly reliable, they add complexity compared with a purely air/oil-cooled engine.

Milwaukee-Eight air/oil-cooled variants avoid most of that complexity while still offering improved thermal efficiency.

That makes routine servicing slightly simpler.

Reliability

Both engines have earned solid reputations.

Twin-Cooled Twin Cam engines benefited from years of Twin Cam development.

By the time Twin-Cooling arrived, Harley had addressed many earlier Twin Cam issues.

Milwaukee-Eight has also proven highly reliable.

Like any modern engine, early production years saw some owner-reported concerns—including occasional oil sumping on certain models—but Harley introduced updates over time, and the platform has matured into a dependable engine when maintained correctly.

Routine servicing, quality oil, and following the manufacturer's maintenance schedule remain far more important than the badge on the rocker cover.

Fuel Economy

Milwaukee-Eight generally delivers slightly better fuel efficiency.

Reasons include:

• improved combustion
• four-valve heads
• reduced friction
• more efficient breathing

Real-world differences vary depending on:

• rider
• speed
• accessories
• load
• wind conditions

Across long touring distances, however, many owners report modest improvements in range from the same fuel tank.

Riding Two-Up

This is an area where Milwaukee-Eight shines.

When carrying:

• passenger
• luggage
• camping gear

the additional torque becomes immediately noticeable.

Hill climbs require less effort.

Highway overtaking becomes more confident.

The engine rarely feels strained.

Twin-Cooled Twin Cam engines certainly handle touring duties well, but the Milwaukee-Eight delivers its performance with noticeably less effort.

Sound

Ask ten Harley riders which engine sounds better and you'll receive at least twelve opinions.

Twin Cam engines have a distinctive traditional Harley rhythm that many enthusiasts adore.

Milwaukee-Eight sounds slightly different.

Some riders describe it as:

• smoother
• deeper
• cleaner

Others prefer the rawer feel of the Twin Cam.

Much depends on the exhaust system fitted.

Neither is objectively better.

This comes down almost entirely to personal taste.

For Trike Riders

Trikes place unique demands on an engine.

Unlike motorcycles, they cannot lean to aid airflow through slow corners.

They often:

• spend more time idling
• carry heavier loads
• tow trailers
• operate in hot climates

Heat management therefore becomes even more important.

Twin-Cooling represented a significant step forward for Harley trikes by addressing one of the biggest complaints from touring riders—excessive heat around the rider and passenger.

The Milwaukee-Eight builds on that progress with a more efficient engine design that delivers stronger performance while generally running cooler and smoother than earlier generations. For riders who spend long days on Australian highways, tackle mountain climbs with a passenger and luggage, or regularly encounter stop-start traffic in summer, those refinements can make a noticeable difference.

Should You Upgrade?

If you currently own a Twin-Cooled Twin Cam Tri Glide, should you rush out and buy a Milwaukee-Eight?

Not necessarily.

The Twin-Cooled engine remains:

• reliable
• comfortable
• capable
• enjoyable

Thousands continue to tour Australia every year without issue.

However, if you're shopping for a newer Harley trike, the Milwaukee-Eight is difficult to ignore.

Its advantages include:

• noticeably stronger torque
• smoother operation
• improved fuel efficiency
• more modern engineering
• better thermal efficiency
• refined ride quality

For riders covering significant distances each year, those benefits accumulate over time and contribute to a more relaxed touring experience.

The Verdict

The Twin-Cooled Twin Cam was an important bridge between Harley-Davidson's traditional engineering philosophy and the demands of modern emissions, performance, and rider comfort. Rather than abandoning its heritage, Harley found a clever way to preserve the look and character of its iconic V-twin while solving one of its biggest weaknesses- heat.

The Milwaukee-Eight, however, is more than an evolution. It represents a comprehensive redesign of Harley's big-twin platform. With improved breathing, increased torque, greater refinement, and enhanced efficiency, it delivers a riding experience that feels more effortless without sacrificing the character that has defined the brand for generations.

For owners of Twin-Cooled models, there's little reason to feel left behind. They remain excellent long-distance touring machines with proven reliability and strong real-world performance. But for riders comparing used and new Harley-Davidson trikes today, the Milwaukee-Eight stands out as the more advanced engine.

It doesn't replace the Harley experience-it enhances it.

Ultimately, whether you're cruising Australia's coastal highways, climbing through the Victorian High Country, or crossing the vast open stretches of the Nullarbor, both engines deliver the unmistakable pulse, soundtrack, and touring capability that Harley-Davidson riders have come to love. The difference is that the Milwaukee-Eight does it with a little more refinement, a little more efficiency, and a lot more engineering beneath those familiar cooling fins.