HARLEY-DAVIDSON LIQUID-COOLS THE BIG TWIN
Two years ago, the Internet circulated patent drawings of a liquid-assisted, cylinder-head cooling scheme from Harley-Davidson, as applied to its Big Twin. This system will now become part of select 2014 Harley models.
Purists rebel at the idea of water cooling. They love the air-cooled look of the generously finned Big Twins, which has evolved over the 76 years since the EL became The Motor Company’s first big production overhead-valve engine. They associate water cooling with Japanese-made “faux Harleys” and want nothing to do with that. In many ways, Harley’s appeal is that, in a confusing, changing world, it remains steadfastly itself.
Years ago, I spoke with Earl Werner, Harley’s chief engineer at the time. “The job of engineering is not always to seek ultimates,” he explained. “Engineering exists to give people what they want.” If Harley riders want the classic air-cooled look, engineering must find ways to give it to them.
Every Harley owner knows that engines heat up in stop-and-go traffic because there is so little air moving through the cooling fins. Once traffic clears and the rider can accelerate, the engine seems to make less-than-normal power. This is completely natural for an air-cooled engine. Intake air, entering the very hot cylinders, expands and loses density. Power is proportional to air density, so the result is reduced power until the heads and cylinders cool down in the open road’s rush of cool air.
COOLER HEADS PREVAIL: Strategically liquid-cooled 103-cubic-inch Big Twin makes five percent more horsepower and torque than the air-cooled-only High-Output engine and is fitted to bikes in which it can be hidden—the Ultra Limited, CVO Limited (with 110-inch engine) and Tri Glide Ultra. Electric pump circulates coolant to pod-hidden radiators.
As engines make more power, they also make more heat. Aside from the sparkplug electrodes, the hottest part of any engine is its exhaust valves and seats. In the big air-cooled radial engines of WWII B-29 bombers, hours of operation caused exhaust valve seats to distort and leak. Exhaust valves failed. Enormous R&D was necessary to control these problems.
Harley has long experience with heat. Improved cooling was required when 1930s’ dirt roads became blacktop, when 1950s’ two-lane roads became turnpikes and when riders expected their bikes to cross the nation at interstate highway speeds. Hot-running iron heads and cylinders were replaced by aluminum because it conducts heat so much faster. Fins grew larger. Oil coolers appeared. As displacement has increased, greater volumes of glowing exhaust gas rush past the exhaust valves, requiring even more cooling.
As a part of Harley’s Project Rushmore, the next step is “twin-cooling,” to place a ring-shaped water passage around each exhaust-valve seat and to pump a 50/50 mixture of water and anti-freeze through it, cooled by two fairing-mounted radiators (Harley prefers the term “heat exchangers”) and fans. Appearance of the engine is just as before. The result is sustained power and increased torque. Compression ratio—an important variable in engine torque—is increased from 9.7:1 to 10.1:1 because the engine can now handle the increased heat flow of the higher ratio. For the 2014 Twin-Cooled High Output Twin Cam 103 engine, torque is increased by 10.7 percent versus a standard 103.
This is engineering—giving people what they want. Modern performance from a classic design.
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