Once the “A” and “B” transfer pairs were more or less aimed at a common point on the piston crown, somewhat to the rear of center, refinements took place. While Yamaha tried to avoid short-circuiting to the exhaust out of the “A” pair of transfers by spacing them 8–10mm from the exhaust, Honda was severely squaring the corners of all its transfer area as a means of making maximum use of available cylinder wall. The improved durability of Nikasil over hard chrome plate allowed the thickness of the septa dividing port from port to be reduced to as little as 1mm.

Folks in the US, pretty much insulated from the coming of higher-performance options such as the A and B kits appearing in Europe, seldom got to see things like Yamaha's "FIM" kit for the TZ250, which included cylinders whose transfers were every bit as squared as Honda's. Beautiful objects.

V-twins became the rule in the 250cc Grand Prix class for two very good reasons. First was that this cylinder arrangement allowed use of wide, fuller-flowing transfer loops without having to resort to extra-wide cylinder spacing such as the 120mm of Kawasaki’s parallel-twin KR1. And second, a 90-degree V-twin architecture allows good primary balance (as in Ducati V-twins). The remaining problem was the rocking caused by the fact that the two cylinders, each with its own crankcase, had to be some distance apart with a seal and a bearing between. That was fixed with a balance shaft, a small price to pay for reducing vibration enough to allow modern aluminum chassis to be used (when Yamaha brought its first aluminum 500cc GP chassis to Europe in 1980, it cracked every day it was run). Those who were certain the balancer was “robbing big power” pulled it out and were rewarded with handlebars that felt like they were hot and 3 inches in diameter.

Yamaha, having carefully developed its “colliding-streams” transfer porting, now strove to further intensify it by making piston crowns perfectly flat. Away went the familiar slight convexity we’d all seen for years. This made sense to me because the more directly the streams collided, the greater might be the resulting loss of velocity. This, in turn, would further thicken and slow the column of fresh charge rising up the non-exhaust cylinder wall. That was the expected result.

But at the same time, Yamaha began to lose its way. As it was explained to me, Wayne Rainey’s adaptability meant that when Yamaha engineers sought his opinion (or that of the stopwatch) of new parts, there could not be a definitive answer because Rainey could adapt to and ride anything. This is not a unique problem. In 1982, Honda presented Freddie Spencer with a great number of chassis alternatives during preseason testing in Brazil, but he made essentially the same lap times on all of them.

In this period, I decided I should revive my Jante mapping and have a look at the latest Yamaha TZ250 cylinder. When I did so, what I found was a complete surprise. Instead of the expected D-section rising column of upflow on the non-exhaust cylinder wall, I got a ragged, off-center tongue flow that threatened to lick over to one cylinder wall.

I told myself, “This can’t be Yamaha’s fault. This terrible flow pattern has to be the result of a problem with my test method.” I tried to improve my rig, but each time I took a pattern (I have them all in a lab book; the pages would make nice wallpaper), it came out the same: an irregular center tongue that looked like an epiglottis.

I thought about trying to collide two circular jets of water so perfectly that the result would be a radially outflowing disc. But any slightest lack of symmetry would heavily bias the flow to one side. Shall we call this “sensitive dependency on initial conditions”?

Off I went one autumn to the SAE Motorsport meeting. Among others I met there was a very young Claudio Domenicali, who is now CEO of Ducati. I also watched with interest as a piratical-looking gentleman demonstrated an automated Jante mapping rig. His brochure informed me the rig could be mine for only $70,000.

My otherwise fruitless work with Jante mapping did teach me some basics. One was that when flow symmetry is lost, one easy way to get it back is to slightly raise the roof angles of the “A” and “B” transfer pairs. This, by giving more upward bias to the flows, encourages them to rise together rather than collide chaotically.

“No matter how careful you are doing manual porting with the little right-angle grinder, you can never be accurate enough to achieve flow symmetry. Only really good casting can do that.”

Another symmetry trick was to place a slight ridge of modeling clay on the piston crown aligned on the diameter from exhaust to non-exhaust sides of the cylinder. By establishing where both right and left streams must leave the piston crown and head upward, this instantly turned any other flow into a radical but stable center tongue. Otherwise, the flows would take turns deflecting each other off the piston crown creating a rapid oscillation. The oscillation was turbulent and noisy; the ridge on the piston stabilized and quieted the flow.

Then came accidental revelation. Bud Aksland, who worked for years with Kenny Roberts, told me that one of the outfits he’d worked for provided him with the $70,000 Jante mapping rig. When I told him about my unsymmetrical maps of the late TZ250 and my conclusion that my test method was at fault, he replied, “No, it’s real. And I learned another thing: No matter how careful you are doing manual porting with the little right-angle grinder, you can never be accurate enough to achieve flow symmetry. Only really good casting can do that.”

Other developments, not really scavenging-related, were changing things. In 1976, Yamaha had decided to raise its transfer ports almost a millimeter while leaving the exhaust where it was. Yet the transfer flow cannot begin until cylinder pressure has dropped below crankcase pressure (where the transfer flow is coming from). That drop in pressure depends upon the exhaust opening far enough in advance of the transfers to let this happen. In the C-model TZ, the reduction of blowdown (the number of crank degrees between exhaust and transfer opening) made the engine weak at peak revs.

I had seen this from another perspective in my homemade Kawasaki H2R in the spring of 1972. When I turned its by-then-well-used cylinders upside down, I could see that carbon was accumulating in the tops of the transfers. That could only mean, as the transfers first opened, exhaust pressure remained higher than crankcase pressure. That being so, exhaust was flowing into the transfers as they opened, depositing the carbon I was seeing. The fix was obvious: Raise the exhaust ports a bit. I raised them a millimeter and was rewarded with quicker lap times.

To get faster blowdown without having to raise the exhaust ports, Yamaha added little “booster” exhaust ports, opening at the same time as the main oval port, located one on each side, right above the “A” transfers.

Kel Carruthers did the same at Daytona in 1976, raising the 250’s exhaust ports enough to give Roberts the tool he needed to beat the upstart KR250s (Ron Pierce qualified mine on pole). And he would do it again with the cylinders of Yamaha’s new disc-valve square-4 OW54 in 1981 when it too had suffered from too-high transfers.

Don’t ask me where my dyno results are. This was old-time middle-class racing by guys with day jobs, driving to races in a van on 29-cent gasoline. We got as far as talking to an outfit with a dyno, just as I had 10 minutes of conversation with people at MIT’s low-speed wind tunnel. Testing of those kinds remained a dream, never a reality.

To get faster blowdown without having to raise the exhaust ports, Yamaha added little “booster” exhaust ports, opening at the same time as the main oval port, located one on each side, right above the “A” transfers. Tune-up info from Yamaha advised squaring those booster ports just as had been done with the transfers on FIM-kit TZ250 cylinders.

Richard Schlachter
Richard Schlachter finished third at Laguna Seca in 1980 on this Yamaha TZ750. I cut the cylinders with the “B” pair of transfers swung around to point slightly toward the exhaust, with their roofs flattened as much as the casting would allow. Stuart Toomey made the exhaust pipes. The longer he made the head pipes, the more the engine revved up! Bob MacLean was our beneficent sponsor.Cycle World archives

In the 1990s, Yamaha was well aware of its symmetry problem because the next thing it did was restore symmetry by slightly raising the transfer roof angles on later-model TZs. And on the last of its 500cc reed-valve engines, the big-oval-plus-boosters exhaust arrangement was abandoned and the T-shaped port with center divider adopted.

The big trend near the end of the two-stroke era was to open the exhaust as late as possible to pull piston-crown temperature down, increasing the engine’s margin of safety against detonation (anything that heats the fuel-air charge makes detonation more likely). The later the exhaust port opens, the cooler the exhaust gas has become, and the less heat it transmits to the piston as it rushes across its crown toward the exhaust. The ultimate goal of this work was to create enough detonation margin to safely use a higher compression ratio, which boosts torque everywhere.

That extra compression caused changes in exhaust pipes: Their center sections became fatter and fatter. Diameters began in 1960 at as little as 75mm (3 inches), growing to 97mm in the 1969 Yamaha TD2 (3.8 inches). And in the final period of refinement they swelled to 130mm (5.1) or more. I look at this as a change in expansion ratio, which indicates how hard the designer was trying to extract energy from the exhaust gas. If we assume the cylinder’s exhaust outlet remained constant at 39mm, then the expansion ratios for the aforementioned examples are 3.7 (1960), 6.2 (1969), and 11.1 (recent). Those are big increases.

A major reason Yamaha had to try harder and harder to extract more energy from exhaust gas was that rising compression ratios steadily reduced the energy going into the pipe. As a concrete example, people working with TZ250s in the 1970s knew that if they raised the low stock compression ratio by milling 0.025 inch off the cylinder head, the engine would peak about 500 revs lower than before. The reason? The more energy you take from the combustion gas to drive the piston (by using higher compression), the less energy is left to go out the exhaust port. Because the speed of sound in a gas increases with temperature, cooler exhaust gas and the lower speed of sound in it made the pipe act longer, so the engine peaked at lower revs. To get those revs back, you had to shorten the pipes.

When this odyssey began, engines were air-cooled and unable to operate steadily on a maximum-power fuel-air mixture. They essentially had a zero-detonation margin because they ran so hot. To survive, they had to be run roughly two sizes rich (this reduces flame temperature). In those days, everyone associated leanness with seizure. At the end, water-cooling with giant radiators (bigger than what came stock on Yamaha TZ750) had finally created such effective cooling that an engine could be leaned out until it just slowed down.

What was achieved overall? The high point was Aprilia’s last 125cc single, giving roughly 55 hp at 13,000 rpm. To make that power at such rpm, the engine was filling its cylinder every revolution as well as a highly developed four-stroke racing engine of the same displacement can achieve only every two revolutions. In both cases, that degree of cylinder-filling maximally exploited intake- and exhaust-wave action. As Rob “Mr. Superbike” Muzzy once said, “The harder you tune on a four-stroke, the more it begins to act like a two-stroke.” It’s the same air, the same fuel, and the same laws of physics at work in both cases.