Four Valves Per Cylinder, Part 3

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

As Japan’s postwar motorcycle industry emerged through the 1950s, important domestic races showcased the power and reliability of the new bikes. Of greatest importance among them was the Mount Asama Volcano Race, run on a cinder circuit.

New Reasons to Revisit Old Ideas

At the time, Yamaha and Suzuki were both building two-strokes, and consequently they had no trouble with the rough track. When their rear wheels bounced into the air and engine revs spiked there were no valves to float, or to bend or “drop” when whacked by a piston. On the other hand, Honda’s four-strokes definitely needed protection from these problems.

European roadracing engines of the time all displayed “industry best practice”: two valves per cylinder. The higher weight of such large valves led to valve float at as little as 300 rpm above peak power.

Were Honda ever to win at Asama, they needed a lot more protection than 300 rpm.

Why not just make the valve springs stiffer? Because Honda’s first purpose-designed race engines were intended to reach high rpm, their valve springs were already much stiffer than the equivalent springs in production four-strokes. And stiffer springs could lead to slow stretching of the exhaust-valve stems, revealed by slow loss of valve clearance. Something more subtle than a brute-force solution was needed.

Honda hired its engineers from respected schools. The principal engineers behind Honda’s early racing engines were Kimio Shinmura and Tadashi Kume. They understood that as parts are made smaller, their weight decreases much faster than does their length or diameter. The bones of a squirrel are but 5 percent of its total body weight, while those of humans make up over 15 percent.

Related: Four-Valve Beginnings, Part 1

The obvious solution, therefore, was to revive an idea then believed obsolete—the use of four smaller valves per cylinder. One of the reasons that four-valve designs were considered obsolete was because their intake flow coefficient was a bit low. If that was the case, why not just increase the cylinder bore and shorten the stroke enough to fit four slightly bigger valves? Honda chose bores 7 percent bigger than the stroke length for its earliest GP bike engines, increasing to 30 percent bigger by the end of 1967 when it withdrew its team. In those designs, the gains in bore area, and therefore the potential gain in valve area, were 14.5 percent in the first case and a whopping 69 percent in the second.

This work, and the conclusions to be derived from it, were not yet complete when Honda decided to send a team of four-stroke 125 twins to the 1959 Isle of Man TT races. Some bikes were fitted with two-valve engines, and only later did their four-valve replacements arrive. They were a bit quaint in appearance, having vertical cylinders and leading-link forks, but they ran reliably, winning a team prize for finishing sixth, seventh, eighth, and tenth.

The Honda Way

Honda’s “wrong way” of using four valves per cylinder was quickly validated. In 1961 it chased long-dominant MV out of the 125 and 250 European FIM GP classes, then established its own dominance a year later with championships in 125, 250, and 350 classes. Honda’s highest-revving engines peaked at 21,500 rpm.

Mr. Honda himself, on the occasion of his company’s first Grand Prix championship, noted in a speech that Japan had now proven they were “not a nation of copyists.”

As observed by Laurence Pomeroy, author of the two-volume The Grand Prix Car (1949), “The first instance of new principle is invariably defeated by the developed example of established practice.” It was not that Honda invented the use of four valves per cylinder, but rather that it saw a new use (its ability to operate reliably at very high rpm) and put it to work successfully, ignoring established opinion to the contrary.

Squish, Turbulence, and Combustion Speed

A chemically correct mixture of perfectly still air and fuel has a combustion speed in the order of 1 foot per second—too slow to make the spark-ignited internal combustion engine workable. Fortunately, turbulent charge motion naturally accelerates combustion, shredding and rapidly distributing the flame kernel originating at the spark plug’s gap. The source of this turbulence is the persistence of the fresh charge’s high-speed flow, entering the cylinder on the intake stroke. Achieving fast combustion depends upon not letting this high-speed flow decay rapidly from contact with obstructions such as a tall piston dome or other features of the piston crown.

As the piston nears TDC and the ignition spark occurs, rapid charge motion transforms into small-scale turbulence that very effectively increases the surface area of combustion.

By 1960, two-valve engines had been highly developed, and benefited from fairly rapid combustion accelerated by deliberate axial swirl of the incoming fresh charge. This swirl came about by offsetting the intake port, making its flow enter the cylinder on a tangent. Extra combustion speed was gained by the Polish engineer Leo Kuzmicki at Norton (though he had been a flight engineer and had lectured at university, up to 1950 he had been sweeping floors at Norton) in the form of piston-to-head “squish.” As an area of piston crown closely approaches a corresponding area of cylinder head at TDC on the compression stroke, the charge between them is forcibly ejected at high speed, giving the whole charge a last-moment stir. The faster combustion occurs, the less time there is for heat loss from the hot combustion gas to the piston crown and cylinder head, and the greater the fraction of power from that gas that reaches the crankshaft.

Indeed, such was the respect for two-valve axial swirl and squish that Ducati’s revered engineer Dr. Fabio Taglioni rejected the four-valve revival on the solid basis that its combustion was slow and therefore inefficient.

Four valves per cylinder were reintroduced to auto racing by Honda’s entry to Formula One in 1964. Two years later established F1 engine builder Coventry Climax adopted the layout in its 1.5-liter V-8. Note that at this time the only applications for four-valve designs outside of racing were in the large cylinders of diesel engines.

In all of these “second coming” four-valve engines, Honda’s classic 1960s GP bikes and the following transfer of four-valve designs to GP cars, slow combustion was a constant feature, revealed by a need for ignition timing as early as 45–60 degrees BTDC. Bear in mind that well-developed two-valve designs of the 1950s and ‘60s had achieved faster and more efficient combustion, generally giving best torque in the vicinity of 36 degrees BTDC. In 1965, when Coventry Climax tested four-valve heads on its 1.5-liter F1 V-8, performance was inferior to that of its original two-valve heads. Only when ignition timing was advanced to 45 BTDC did greater power appear—enough to justify the experiment.

Duckworth and the Third Coming

Englishman Keith Duckworth (1933–2005) began his work with engine tuning as so many have: trying to improve production-engine performance. Having a fertile mind and being a disciplined thinker he soon progressed to designing high-performance cylinder heads for existing engines. One such head gave him fits for two years as he strove to make it burn faster and more efficiently through the use of squish. This lack of success made him return to first causes and to consider other ways to accelerate combustion. His own writings make it clear that he was not trying to revolutionize the design of internal combustion engines, but rather just to get his current project to work as well as he expected it to.

What he realized was that, of the original energy of the incoming stream of fresh charge, some had to be converted into pressure in the cylinder (cylinder filling) and some had to be set aside—stored—for use in the all-important task of achieving rapid combustion. This was a compromise: Best cylinder filling would transform all of the kinetic energy of incoming mixture into pressure, but fast and efficient combustion requires using some of that energy to keep mixture in motion all the way to TDC and the beginning of combustion.

Related: Four Valves Per Cylinder, Part 2

Little by little Duckworth realized there was another way besides axial swirl to store charge motion in the cylinder. What was at first called “barrel motion” resulted when mixture, entering the cylinder through a pair of side-by-side intake valves, flowed across the head, turned down the cylinder at the far side, flowed down to the piston crown, was defected back across it and up the near-side cylinder wall to complete the loop. It worked. When Ford decided “this is the guy we want to design our Formula One engine,” the resulting Cosworth-Ford DFV, entering competition in 1967, needed only 27 degrees of ignition timing to deliver best torque. Fan-freakin’-tastic. Nothing else came close.

The important variables in determining how much energy would be stored as “barrel motion” were the downdraft angle of the intake ports and their diameters.

This upset the pundits’ applecart and stomped on the apples. With the wisdom of Solomon they had always rated engines on total valve area per liter of engine displacement. On that basis the clear winner was Matra’s V-12, with valve area to burn. Trouble was, the little Cosworth V-8 made Matra’s V-12 look, well, feeble.

Cosworth’s “Barrel Motion”

Cosworth managed to keep its “barrel motion,” later renamed “tumble,” confidential for maybe two years, but after that, word got around.

What was wrong with the Matra V-12? Why did Honda’s GP bike engines of 1959–1967 have the same problem? And the Climax V-8? Its designers had all unthinkingly continued to give its engines wide valve-included angles (“industry best practice”) and the deep combustion chambers that resulted. To achieve a decent compression ratio, those engines needed tall piston domes that caused rapid decay of in-cylinder charge motion, leading to slower combustion. The extra surface area of a deep chamber and tall piston dome let a lot of heat leak away from hot combustion gas, reducing its pressure.

Duckworth’s design placed the valves at a much smaller included angle—32 degrees between intake and exhaust valve stems, rather than the traditional 58–78 degrees. The resulting relatively flat cylinder head and piston crown robbed less heat from combustion.

Soon everybody was doing it Duckworth’s way, and I don’t mean just specialized producers of esoteric racing engines. In a world suddenly obsessed with fuel consumption (the first “oiru shokku” came in 1973–74), a fast efficient combustion system was a gift from heaven.

Four valves actuated by dual overhead camshafts sat in nearly flat combustion chambers has become a nearly universal design. (Kawasaki/)

Today, the use of four valves in nearly flat combustion chambers has become almost universal. The ability of four smaller valves to reliably track the shorter duration, relatively high-lift cams that give wide, flat torque (rather than “bill-spike power”) has lately proved the way to combine acceptably high performance with low levels of unburned hydrocarbon emissions.

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