Keeping a cool head about the Harley-Davidson Twin-Cooled™ system

Claremont (December 18, 2013) - In August of this year, as a direct result of Project Rushmore, Harley-Davidson announced the arrival of a new breed of Touring motorcycles. Project Rushmore was the working title of a three-year study and design programme to improve the iconic range of Harley-Davidson touring machines. The results of which have received rave reviews from the press and also owners of these bikes who snapped them up as soon as they appeared in dealers across the globe.

Even though the ‘Rushmore’ bikes feature major changes to give the best ‘touring experience’ you can get, the silhouette of the machines hasn’t changed significantly. In other words, the revisions and application of up-to-date technology has been, for want of a better word, subtle. Cleverly subtle. So while there are in excess of 100 changes, some obvious, some not obvious, every machine encompassed by Project Rushmore is obviously new but still instantly recognisable because their iconic styling has been retained.

Stand close and you will see the latest technology and changes. The New colour-screen Boom! ™ Box infotainment system, new Batwing fairing with splitstream venting, a larger Tour-Pak® and panniers with convenient One-Touch latches, sleeker front and rear fenders, lighter cast aluminium wheels, revised intuitive hand control switches, new dash display layout… and a whole lot more. And a heap of these changes you can’t immediately see.

One such change, or rather technical addition, is Harley-Davidson’s Twin-Cooled™ system, where traditional air/oil cooling – even on the latest High Output Twin Cam 103™ engine – meets with liquid-cooling.

So why build in liquid-cooling on a Harley-Davidson? Heat is the main reason. Heat is the biggest thief of engine power. The thermal dynamics of an engine change considerably when it gets hot especially in stop-start traffic and can, at worst, cause engine damage. This is the reason why the ECM (Electronic Control Module – the bike’s electronic brain) retards the ignition to reduce maximum cylinder pressure from the resulting fuel detonation and temperature.

The hottest part of the engine is the cylinder head where the fuel (the mix of air and petrol) burns and most of the resulting hot waste gases disappear out of the exhaust valve. What heat isn’t pushed past the exhaust valve dissipates through the heads, cylinders and engine oil and is effectively retained by the engine, especially on bikes that have weather shielding fairings eg touring bikes. Another problem is air is a gas and not a very good conductor of heat, whereas fluid in liquid-cooled engines is more dense and better at thermal conductivity.

In order to retain the traditional air-cooled look of the bike (finned cylinders), Harley-Davidson decided not to make the Twin Cam engine totally liquid-cooled, where the cylinders feature an enclosed water-cooling jacket or cooling tubes, but rather cool the constant hottest part of the engine – the cylinder head. Harley then went further by only directing coolant around the area of the exhaust valve on both cylinders. This technical addition is better known as precision cooling and negates the use of miles of coolant hoses and a huge radiator to carry a large amount of cooling liquid. Why make something so complicated when less complicated will do?

The liquid cooling system is more commonly referred to as water-cooling, which isn’t technically correct. It is actually a 50/50 mix of glycol and water. This has to be the case to prevent the water content freezing in cold climates. There are also corrosion inhibitors in the glycol mix too.

In order to further retain the traditional air/oil-cooled look of the engine and the overall silhouette, Harley-Davidson engineers worked long and hard to disguise the liquid-cooling system. As you’ve already read, the plumbing required to carry the all important coolant is minimal (size and quantity) and can’t be seen until up close to your nose. An electronic pump used to speed up the flow of coolant is mounted below the front frame rails and carefully hidden by a small protective cover. The two radiators, yes two, are secreted within the leg shields that used to be hold small flap-covered cubby holes: don’t worry there is more and plentiful storage area all around on the new touring bikes to make up for this.

What you might not realise is both radiators also have small electronic fans behind them to draw air over the radiators and rapidly cool the coolant. These fans only come into play when the bike is really, really hot. And no, they are not as loud as car radiator fans; all you hear is a slight almost imperceptible hum from the fans when parked up.

The advantage of liquid-cooling is actually two-fold. Not only does it allow the engine to perform as it should in all conditions, but also allows engine designers to safely extract more power from the engine by raising engine compression and altering camshaft timing along with a few other tricks Harley and its engineers have amassed over 110 years.

The other beauty of the new Twin-Cooled system is it does not detract in any shape or form in the way a Harley-Davidson sounds and looks. This is a modern world indeed and Harley-Davidson rides every road within it.