Honda committed to the advantages of water cooling in their 1976 Gold Wing flat four. This triggered the expected outbursts of traditionalism from the “coots” – older journalists and commentators for whom liquid cooling was too complex, sure to be expensive. And heavy. Fins are beautiful, right?

Then why were nearly all auto engines liquid-cooled, save for a very few exceptions such as VW and the Czech Tatra?

Parenting manuals insist that “change is so difficult for children.” In fact, change is difficult for all ages – we dislike having our certainties overturned. Fins look right. Bare cylinder blocks are offensive to the eye

As in most instances of engineering change, it takes something of a crisis to make it happen. It hit two-stroke road race engines in the early 1960s. As cylinder-filling and exhaust pipe design matured, power per square inch of piston area increased. I put it in this form because two-strokes were already at disadvantage because of high piston temperature. The first cause? A 4T fires every other revolution, but a 2T fires twice as often.

DKW’s fast-selling post-war 2T putt-putt, the RT125, had roughly 3.5 square inches of piston area and made 4 hp. That’s 1.14 hp per square inch. At the end of the FIM 125 class in GP racing, that had risen to 16 hp per square inch of piston area.

The present 81 mm bore 4T MotoGP engines have 8 square inches of piston area and each cylinder makes roughly 70-75 hp. Its power-per-square-inch is 75/8 = 9.4, or just 60% as much.

An example of the adoption of water cooling for two-strokes was Bultaco, whose 125 single made low twenties horsepower. It’s bad enough that the piston crown is exposed to hot combustion gas, but making that worse was the scouring outrush of that hot gas across the piston crown and into the exhaust pipe – ideal conditions for heat transfer – a very large temperature difference between the hot gas and much cooler piston (melting-point of 1070 F) and a very high velocity turbulent flow that constantly keeps hot gas against the exhaust-side piston crown during outflow.

Suzuki engineers in the early 1960s described their GP engine pistons as “swelling like cakes” and we know that the exhaust side of the air-cooled cylinder was distorted by the near presence of the exhaust duct. If the ring can’t conform to this bulging, hot gas leakage might heat the piston’s exhaust skirt more than the cooler, lower part of the cylinder could cool it by intimate contact.

Switching to water cooling brought a new problem. During warm-up the piston temperatured-up much faster than could the heavier cylinder casting. The result was “cold-seizure” (unless warm-up was conducted gradually). This required another round of metallurgical R&D. In this period we saw oddities like air-cooled cylinders with water-cooled heads.

Bultaco, as always, were for simplicity, light weight and low cost. They just surrounded the finless vertical power cylinder with a sort of jug filled with water. There was no water pump (if it’s not there, can it fail?).
This reminded me of antique farm engines. Their cylinders were horizontal and their open-topped jackets mostly full of water cooled the engine by gradually boiling away. There was no radiator and no water-pump. Instead, a reliable child would be given the task of adding water every few minutes to keep the cylinder covered.

Bultaco’s system did have a rad but no pump. It relied upon thermal convection (fancy name was “thermo syphon”). Fat hoses connected cylinder and head water jackets to a radiator, giving a path for recirculating flow but only the density difference between hotter and cooler water acted to cause leisurely circulation.

Now began a process that would take a surprising number of years – figuring out how to cool higher and higher power densities as engine development marched on. In the snowmobile industry it became almost a crisis, for leisurely circulation allowed stagnant regions in the flow path to become steam pockets. As steam formed, taking up volume, water was expelled through the pressure relief radiator cap.

Yamaha water-cooled their 350 road race engine from 1973, with the TZ250 appearing a year later. In both cases, water jackets were generous – I can poke a finger far down into them or into the water passages of the head. These Yamahas all employed pumped cooling water circulation.

Despite this, the per-cylinder weight of the first water-cooled is surprisingly lighter than that of the air-cooled it replaced, with the two making similar power. The air-cooled TD3 cylinder of 1973 weighed 1896 grams to the 1435 gm of the 1974 TZ250 (which had two 125 cylinders in a single block, weighing 2870 gm).

As power continued to rise (on snow as well as on pavement and dirt) it became advisable to devote real study to water circulation. What, for example, is considered a best coolant velocity? I am told that the range is between 5 and 8 ft/sec. Transparent water jackets with particulates in the water allow laser velocimetry to measure flow velocities.

When Yamaha gave up building its TZs on the $105 production crankcase (TZ250G, 1980) it adopted separate-cylinder construction as on Kenny’s 500-4 of 1978-‘80. The 5F7 cylinder I have here weighs 1482 gm. The new, dedicated crankcase cost roughly ten times more than the production-based case. Low production = high production cost.

The water passages are smaller than in the previous design, but the real change came when they switched from the madly-rocking parallel-twins to a 90-deg V-Twin design(with a balancer to take out the rocking caused by cylinder offset).

The 1990 3TC cylinder weighs 1058 grams—only 74% as much as the previous design, with power rising 40%. It has much closer-fitting water jacketing, which comparably increased the water flow velocity.

The following year, they made the water jacketing even closer, which required reducing the head bolts from five 8 mm to six 6mm. Weight was about as before—1078 gm, only 57% as heavy as the last of their air-cooled cylinders in 1972.

Constant improvement in cooling was required by steady power development. By 1994 Honda had set 100 hp as the goal for its NSR250. Eventually cooling became so effective that engines could be jetted down until they slowed down.