Fins ain’t what they used to be

by Simon Hargreaves

Why air-cooled bikes are having a water-cooler moment

In the news of their 2014 Touring range upgrades, Harley-Davidson smuggled a phrase rarely associated with the Milwaukee ironmongers (VRods aside): liquid-cooling. Before purists protest, Harley limit the ‘Twin-Cooled’ engines to four top-end models: the Ultra Limited, Electra Glide, Ultra Classic and Tri Glide trike. Which isn’t even a bike. But then it’s not really liquid-cooling either.

It’s fair to say the 45° V-twin has never enjoyed a reputation for cutting-edge tech; Harley are reputed to employ more designers than engineers.

But it’s also unfair to say. Designing a Harley engine might not be as challenging as building a 250bhp MotoGP engine that does 26 laps on 21 litres of fuel. But making an air-cooled motor sound and feel true to the Harley archetype yet meet stringent noise and emissions regulations is a challenge nonetheless.

In 2010 Harley applied for a patent that previewed the new cooling system. Coolant is piped over the front cylinder head, ducted around the exhaust valve area (the hottest part, especially when one large, air-cooled exhaust valve runs several hundred degrees hotter than two smaller ones in a smaller, water-cooled cylinder), then back into two radiators. Harley call it ‘precision cooling’.

So do BMW. The 2013 R1200GS boxer twin channels coolant around its cylinder heads the same way, leaving finned barrels out in the wind. Air-cooling is an important selling point for both manufacturers.

As more power is demanded from an engine it gets noisier and hotter (both manufacturers claim 10% more torque and bhp). Without a sound-insulating water jacket every tap, knock, whir and bang is passed to the outside world. For normal people this is great because we like the symphony of internal combustion (within reason). But some think an engine should be seen and not heard. A water jacket – not, sadly, around the complainants, but the engine – is a good way of appeasing them.

Heat management is a bigger problem. Three quarters of the fuel energy used up in combustion is waste heat – half through the exhaust, half into surrounding metalwork. The ratios are similar in a 5bhp 50cc single or a 180bhp inline four, but the actual numbers are far higher: the more work you ask of your engine, the hotter it gets.

They also get hot if you run them lean (high air/fuel ratio) to meet emissions targets. And if an engine relies on airflow to control thermal stability it can get too hot. Which, ironically, may cause exhaust valve seat distortion and poor emissions control (among many other things). An old school remedy would be to reduce flame temperatures by adding fuel… and we’re back to emissions again. A more radical solution would be to detune the motor, splay the valve angle, increase the finning, improve oil-cooling efficiency and set the motor across the frame. Rather like a Honda CB1100F.

But by channeling coolant to a specific area, Harley and BMW can maintain temperature control, keep emissions down, retain the engine layout its customers demand… and hang on to that all-important, hand-on-heart, air-cooled vibe. Literally.