The Fascination With Fins

Before the introduction of such mainstream bikes as Honda’s liquid-cooled Gold Wing (1975) and its V45 Interceptor (1983), the vast majority of motorcycle engines were air-cooled. Of course there were exceptions, such as early Scott two-strokes, the last version of the AJS V-4 racebike, Suzuki’s 750 “Water Buffalo,” and Guzzi’s fab 500 V8.

Why did those exceptions come about? Two-strokes heat their pistons twice as often as four-strokes, so their designers are vigorously drawn to liquid-cooling. In air-cooled transverse vee engines the hot air streaming off the front cylinder bank’s fins cools the rear bank marginally at best.

Just as a steam locomotive’s in-your-face piston rods and drivers had been proof of their elemental machine nature, so too were the deep and numerous cooling fins that covered most motorcycle engines. Those who grew up in World War II’s postwar period grew accustomed to seeing those same swirls of cooling fins on the cylinders and heads of radial aircraft engines.

Fins still have visual marketing power. Harley-Davidson has never forgotten it. When Polaris revived Indian, the new engines sported nonfunctional cylinder-head-cover finning inspired by the finned heads of Indian’s last 80-inch Chief from the early 1950s. Motorcyclists drag a fingernail across engine fin tips to hear a sound nothing else makes. It may not be music, but it definitely has emotional content.

Although false fins exist, there is satisfying sincerity in real ones. Fins must be deepest and most numerous where temperatures are the highest: on an engine’s heads and specifically around their hot exhaust ports. Look at a classic Triumph 650 of the 1960s and you will see the famous head and fin shape dictated by the need to push cooling air between the two outward-splayed exhaust ports. When they appear on cylinders, fins are deepest at the top, closest to the fire, and taper toward the bottom. These contours drive the reverence many of us have for cooling fins: The very shape and location of the fins reveals where the greatest need for them exists. Engineering honesty.

Erik Buell and others fought hard to keep air-cooling compatible with modern emissions limits, yet the battle was eventually lost. Today, several classic engine types continue with their original fins, although their cooling is invisibly augmented by internal liquid-cooling passages and external coolers. Harley’s 1200 Sportsters gained reliability from circulating their engine oil through passages that helped keep their exhaust-valve seats stable.

While it’s true that engine heat is ultimately dissipated to air regardless of whether the engine is air-cooled or liquid-cooled, the latter holds an insurmountable advantage: density. Water is 830 times more dense than air, and that lets it achieve intensive heat removal in places cooling airflow can’t reach. Such spots include the very hot exhaust-valve seats and, in four-valve engines, in the very hot metal between the paired exhaust-valve seats and ports. In the case of the R-3350 engines that powered the B-29 bomber of WWII, desperation over warping valve seats forced Wright Aero to adopt “blast tubes,” sheet-metal ducts that carried fresh air from the front of the engine to a specific group of cooling fins behind second-row cylinders.

Why not just let metal get hot? Bad idea! When a particular limited region of metal becomes extremely hot, it naturally expands. But because it is surrounded by cooler metal, this becomes a push-and-shove match that engineers call “thermal stress.” The expanding hot metal pushes so hard that surrounding cooler metal is forced to yield—this results in permanent plastic deformation, not the elastic deformation we see with a spring. Now, when the engine cools after shutdown, the hot metal that had been in an expanded state shrinks, leaving it in a state of tension. A few cycles of this in a cylinder head and tensile cracking or valve-seat warping occur. The first leads to combustion-gas leakage and valve-seat loosening, while warping prevents full cooling contact between the very hot valve and its cooler seat. Robbed of its primary cooling pathway, the valve “overtemperatures,” loses strength, and soon fails.

To avoid this, engine designers must prevent such local overheating, especially with air-cooled engines. The simplest approach is to limit performance. This gave us the great big air-cooled engines of the First Superbike Era (1973–1983), producing less than 100 stock horsepower from 1,000cc. That may have been a big number back then, but is unimpressive today.

An alternative—fan cooling—is denied us for aesthetic reasons. Porsche, among others, proved that fan-driven air-cooling works well. But we can’t see the fins, and it reminds motorcyclists of the weak putt-putt engines in lawn mowers and golf carts.

Adding extra metal to function as a heat sink is one way to keep air-cooled motorcycle engines ideologically pure. This is why air-cooled cylinder heads are so heavy—they have extra-thick metal over their combustion chambers. This helps to avoid overheating by just storing heat until the throttle closes, and it also acts as a “heat highway” to rapidly conduct that heat away to where cooling fins can be located.

Much depends upon how long we need our engine to operate at high power. Highly modified Pro Stock Motorcycle drag racing engines, which lack any form of cooling at all, are able to produce more than 300 hp—but only for just under seven seconds. All the heat produced during a pass is stored in the engine’s metal. Back in the pits, large fans cool the engine in preparation for the next round. Fan cooling strikes again, just with a different kind of fan.

Back in 1983 Rob Muzzy’s air-cooled Kawasaki Z1-based Superbike could complete 50-mile roadraces making 150 peak horsepower at 10,250 rpm. The modern liquid-cooled ZX-10RR, modified for World Superbike, is also built to its thermal and structural limits. It produces nearly 100 more horsepower from the same 1,000cc displacement, at 15,000 rpm.

At the end of the Sportbike era, a production 1,000cc liquid-cooled engine could make close to 180 peak horsepower at 13,500 rpm, all while protected by factory warranty.

If I feel a sudden desire for fins, I can walk up to the shop and stare at my 1965 Yamaha TD1-B. Sometimes I even run my fingernail over its fins. It sounds almost like music.