Cooling the “Authentic” Way

Technical Editor Kevin Cameron shares his wealth of motorcycle knowledge, experiences, insights, history, and much more

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Technical Editor Kevin Cameron shares his wealth of motorcycle knowledge, experiences, insights, history, and much more.Cycle World

At the moment, old is good. Old is authentic. Modern water-cooled stuff is plastic and marketing hype. Get with what's true and pure. Pass me that patina kit from KwikRust and my copy of For Whom the Bell Tolls.

Part of that scene is air-cooling. Real motorcycles, so folk are telling me, are air-cooled. I walked around my shop with Vernier caliper in hand, noticing that all the old air-cooled cylinders and heads have their fins spaced about 10mm apart. S’pose there’s a reason for that?

Yup, there is. As you tool along at 60 mph (88 ft./sec.), air does not flow at that speed through engine cooling fin spaces. It takes pressure to push air through such fairly rough, confined conduits. For one thing, the boundary layer of stagnant gas near a surface resists being accelerated and offers resistance to flow. So what happens is this: Air hitting the front of your air-cooled engine decelerates, its original kinetic energy now becoming pressure. That 88 ft./sec. air velocity, if fully converted into pressure (called "stagnation pressure" or just "Q"), becomes 9.6 pounds per square foot. Not very much, so fins have to be spaced far apart enough for that little pressure to push air through the spaces between them.

Full conversion of kinetic into pressure energy doesn’t take place in front of your engine because most of the air spills around the finned assembly rather than taking the resisting path between fins. By trial and error, engine makers found that the 10mm spacing (two and one-half fins per inch) worked okay at highway speeds.

Surface vehicles can’t use much of their power most of the time, but just in case they do put on a burst of speed, engine makers provided a "heat sink" (a heat storage system) in the form of a thick layer of metal right above the combustion chambers of the cylinder head. As you exuberantly accelerate up an empty on-ramp and zoom up to some unmentionable speed before caution gets the better of you, the presence of that extra metal keeps your cylinder head temperature from quickly reaching numbers at which your engine would detonate. As you coast back to the usual police-acceptable speed-limit-plus-9-mph, that stored extra heat is gradually conducted out to the cooling fins where it heats the air passing through the fin spaces. When Harley’s iconic aluminum XR-750 was being designed, racing manager Dick O’Brien wanted an inch thickness of metal above its combustion chambers but in the end settled for 3/4-inch. This is why air-cooled cylinder heads are heavy—because they are designed as finned heat sinks.

Engines in other situations are designed quite differently. Air-cooled Porsche and VW engines are rear-mounted where free-stream air cannot cool them, so they are given cooling blowers and sheet-metal baffles to make sure that all the blower air passes through fin space, rather than spilling past the bare engine. Because blower pressure is greater than the stagnation pressure of the free stream, the fins can be spaced more closely (blower pressure overcomes their higher flow resistance) and so engines can be made more compact.

No motorcyclist would be caught dead on a bike with a cooling blower. Horrors—cooling blowers are for golf carts and lawn mowers! Every human endeavor has its own aesthetic.

Cooling fins on high-power air-cooled aircraft piston engines were spaced as closely as 7.5 per inch—a lot closer than on any motorcycle engine. How was this possible? Look in the B-29 flight manual and you will see that a minimum airspeed of 140 mph (205 fps) is required for engine cooling during climb. Stagnation pressure from that speed is 50 pounds per square foot, five times more than is available to a motorcycle engine at 60 mph. That extra pressure is required to push air through the higher resistance of the aircraft engine’s more closely spaced cooling fins and air baffle system.

Back in the days of the classic American sit-up Superbike 1975–’82 (Kawasaki Z1, Suzuki GS, Honda 900) air-cooled engines were king, and builders took care to fully open any air passages that were partly blocked (as was often the case) by casting flash. Proper cooling depends on the free passage of air through all fin spaces.