The Coming Crunch In Big Twin Engine Cooling

Why an air-cooled life is so unforgiving

Indian Roadmaster
The air-/oil-cooled Thunder Stroke 111 V-twin driving the Indian Chief, Springfield, Chieftain, and Dark Horse models is intended to satisfy those who view thick cylinder finning as requisite to the motorcycling experience.Courtesy of Harley-Davidson

I call it a "coming crunch" because makers of traditional big-inch air-cooled motorcycle engines are now having to stick on "Band-Aid" local liquid-cooling to keep their exhaust valves and seats well-sealed and healthy. Harley-Davidson began years ago, circulating engine oil around the exhaust-valve seats of the Sportster 1200 and now versions of its Big Twin do the same with antifreeze.

A few years ago, Harley's styling department vetoed oil coolers, so now it has had to accept the "space heaters" placed ahead of the rider's shins, radiators for the heat being extracted from the heads by circulating coolant. BMW on some of its ever-expanding flat twins has resorted to the same action, and Honda has called its version "spark-plug cooling," though in fact pulling heat out of the heads anywhere makes life easier for those hot-running exhaust valves.

Air-cooled life is especially tough if you adopt four valves per cylinder for its excellent ability to combine strong power (i.e., not wheezing out at 5,000 rpm) with broad torque. The reason is the vulnerability of the so-called “exhaust bridge,” the narrow isthmus of head material between the paired exhaust valves. If not actively cooled, its heat expansion can permanently distort the cylinder head around it, causing exhaust-valve leakage and erosion.

Meanwhile, Polaris/Indian appears to be taking The Big Step by giving its rumored upcoming “Raptor” engine full liquid-cooling. This will solve many problems in both performance and meeting emissions and noise standards. The big question is, can fanciers of American-made big twins accept it?

At my seventh birthday, my dad took me with him on a flight from Boston to New York. As we boarded, I stared in fascination at the cooling fins swirling over the cylinders of its big 2,800ci air-cooled piston engines. To the left of my keyboard, I keep an air-cooled bike-engine cylinder just so I can hear the ring as I drag a fingernail across its fins. It’s possible to fall in love with fins and never fall out.

SOHC, four-valve-per-cylinder V-twin
An all-new, liquid-cooled, SOHC, four-valve-per-cylinder V-twin will purportedly power the next-generation of Indian touring machines. Displacement is rumored to be nearly 1,800cc, with 120 hp targeted.US Patent Office

So far, Harley-Davidson and BMW have added power by making their cylinders bigger, but air-cooling makes this a difficult road. Why? The greater the cylinder displacement, the greater the volume of very hot exhaust gas that rushes under the lifting exhaust valve(s) and through the exhaust port. Half the heat that enters the cylinder head enters from the exhaust port’s walls. Designers have fought against this “hot zone” by making it smaller. Harley intentionally made its exhaust ports shorter in the Evolution engine and in later redesigns limited valve and port surface area, both of which are effective in keeping heat out of the head.

You could also ask this question: If giant aircraft engines of the past with 200ci cylinders reliably generated more than 1 hp per cubic inch, then where’s the problem? The answer has two parts. The first one is that those big aircraft engines quickly overheated at under 140 mph and were happiest at 200 mph. Those speeds generated ram pressure that could push cooling air through very closely spaced cooling fins, which were too closely spaced to work at highway speeds.

All these problems and the complicated Band-Aids now used against them are whisked away by full liquid-cooling.

The second part is that special and quite expensive techniques were needed to create cooling fins so closely spaced. Conventional casting of cylinder heads had to be abandoned in favor of machining them from solid forgings, a change that increased cooling fin area by two-thirds. And on the cylinder barrels, whose cooling was the key to keeping pistons alive, as many as 10 fins per inch were necessary.

Like those giant aircraft piston engines, Harley-Davidson’s Big Twins have a “minimum cooling speed.” In traffic, where movement and therefore air motion are slow, Harley’s ECU triggers “parade mode,” cutting out the rear cylinder before it can overheat, to operate as a single until conditions improve.

All these problems and the complicated Band-Aids now used against them are whisked away by full liquid-cooling. With it, not only can exhaust-valve seat distortion and leakage be eliminated, but compression ratio can be chosen for best power, rather than being limited by excessive cylinder-head temperature. With liquid-cooling, engine oil viscosity no longer drops in the heat of July and August but remains nearly constant because engine temperature is held constant by the automatic opening and closing of the coolant thermostat.

Air-cooled automotive engines have always been actively cooled by fans, as in the case of the old Volkswagen Beetle and the air-cooled Porsches. America’s last piston-engined strategic bomber, the B-36, used 10 percent of its gross horsepower to drive cooling fans.

If Indian is about to take this big step, it deserves praise for its courage in believing that riders, given the choice between cylinder style and cylinder function, will choose function because they’d rather be riding than looking.

What can we do with our love for fins? I keep them near me and can have a look at any time, like a picture of a childhood sweetheart.