A lot of two-stroke research took place in the diesel engine business. It quickly became obvious that the more fresh air you blew through a cylinder after it had fired, the purer the mix trapped in it would become after all valves/ports had closed. The ratio of the air delivered to the cylinder displacement was called the delivery ratio, and since power was required to blow this air through the cylinder, it was desirable to get the greatest effect from the least air delivered.

But with the rules adopted for motorcycle racing in 1947, two-stroke design was limited to the simple crankcase-scavenged type. This limited the volume of scavenge mixture to what the crankcase, operating as a piston blower, could pump.

One response to this limit was that engineers and tuners worked hard to reduce crankcase volume to the very minimum. Balance holes in crank wheels were filled with cork, “horseshoe plates” were installed to fill any clearance between the connecting rod’s big end and the inside of the crankcase, and deck plates were installed at the mouth of the crankcase, leaving only a narrow slot through which the shank of the rod worked. Upward-projecting fingers were sometimes attached to this plate, poking up to occupy volume inside the piston. All this, it was hoped, would make the crankcase into a more efficient pump.

The measure of their success was the “crankcase compression ratio,” which was the ratio of crankcase volume with the piston at bottom center to its volume with piston at top center. By fanatical machining, it was possible to push this as high as 1.75:1.

Because Schnürle’s flat-topped piston concept worked so well, engineer Richard Küchen at Zündapp was ordered to develop a variant of it that would break the patent. This was a third transfer port, located directly opposite the exhaust port but aimed steeply upward at 60 degrees to the horizontal.

With only the two transfer ports of marine diesel engineer Adolf Schnürle’s flat-top piston scavenge scheme, such a high crankcase compression ratio gave very high velocity to the fresh charge shooting into the cylinder from the transfers. Because it didn’t take long for the transfer streams to complete the scavenge loop and reach the exhaust port, the time during which maximum charge volume was present in the cylinder was very brief. The result was a narrow powerband. If the engine exceeded that speed, more fresh charge was lost out the exhaust. If it turned at a lower speed, transfer of charge from the crankcase would be less complete.

Because Schnürle’s flat-topped piston concept worked so well, engineer Richard Küchen at Zündapp was ordered to develop a variant of it that would break the patent. This was a third transfer port, located directly opposite the exhaust port but aimed steeply upward at 60 degrees to the horizontal. This simple three-transfer design would remain the last word in two-stroke scavenging pretty much until the end of the intensive factory Grand Prix racing of the 1960s.

At the end of 1951, a new idea arrived: the creation of DKW engineer Erich Wolf (who had previously been a private builder of race-modified DKW RT 125s). Knowing that mechanical valves could not move fast enough to stop fresh charge from being lost out the exhaust port, he decided to devise a nonmechanical valve based on pressure waves moving in the exhaust pipe. He knew that when the exhaust port opened, it released a pressure pulse of 50 to 100 psi into the exhaust pipe. When that pulse reached the expanding megaphone, its expansion propagated back up the pipe with its sign reversed as a low-pressure wave. That "suction wave" worked only too well, inviting more fresh charge to get lost out the exhaust. So why not place an "anti-megaphone" after the megaphone? It would reflect a positive pulse back toward the exhaust port. If timed to arrive just as the piston was closing the exhaust port, it would stop the loss of fresh charge and maybe even stuff some lost charge back into the cylinder. Thin sheet metal, metal snips, welding, and dyno time were all it took to evaluate Wolf's idea, and soon it was ready for track testing. It worked. German engineers called Wolf's converging cone a gegenkonus or a "counter-cone."

The positive exhaust wave reflecting back to the port from the gegenkonus acted as an exhaust valve, putting an end to the old compromise between exhaust port height and loss of fresh charge. The way to increased two-stroke power was now open.

All over postwar Germany, enthusiasts were building homemade 125cc roadracers out of DKW RT 125 singles. In East Germany, one of the foremost was Daniel Zimmermann. One of his ideas was to avoid the limitations of the RT’s symmetrical piston-port intake timing by switching to a rotary disc intake. By cutting away a sector of the disc (which mounted on one end of the crankshaft and rotated with it), he could achieve a nonsymmetrical intake timing of I.O. 125 degrees BTDC/I.C. 60 degrees ATDC. The idea was not new, having been used by engineer Albert Roder at Ziro motorcycles before 1925.

Although earlier writers have credited Walter Kaaden with innovations that were not his own, what he did accomplish was the successful integration of these several concepts into a whole with world-beating potential.

Zimmermann’s ZPH engines won local championships but when the IFA factory, located in Zschopau, hired an engineer to develop racing machines, party bosses directed Zimmermann to hand over all prototypes, drawings, and test results to him. And just go away. The engineer IFA hired was Walter Kaaden, who had previously worked at the wartime German missile research center at Peenemünde. Zimmermann turned to outboard motors.

At DKW, the potential of the gegenkonus exhaust pipe (which the late Gordon Jennings called an "expansion chamber" in his classic how-to book, Two-Stroke Tuner's Handbook) was far from fully realized. That company's revolutionary piston-port 350cc two-stroke triple, although making as much as 42 hp, failed to take a GP championship from Moto Guzzi's ultralight single-cylinder four-stroke making 38 hp. DKW's effort was a serious one by a very large racing organization, but it was trying to simultaneously race and develop the emerging two-stroke technology.

Although earlier writers have credited Kaaden with innovations that were not his own (third transfer port, rotary-disc intake valve, expansion-chamber exhaust), what he did accomplish was the successful integration of these several concepts into a whole with world-beating potential. And he did it using the skimpy resources of a defeated nation under authoritarian rule.

At first, Kaaden tried some “advanced” ideas such as an opposed-piston uniflow design. Two pistons compressed charge between them in a single cylinder, with one piston controlling exhaust ports and the other fresh-charge ports. Yet at the same time, a simple RT 125-based single had reached similar power! Occam’s razor suggests that the simplest solution is the best, so development was focused on the single. By 1955, using Wolf’s reverse-cone exhaust, he had achieved 15 hp at 9,000 rpm. Three years later, output had been pushed to 20 hp and parity with the best four-strokes.

How did Kaaden do it? In this kind of two-stroke, every system has a resonance—the filling of the crankcase through the rotary valve and its timings, the transfer of charge from crankcase to cylinder, and the pumping-then-blocking action of waves in the exhaust pipe. By bringing them all to peak function in the same band of rpm, their separate benefits added together to give impressive output.

Kaaden also discovered that with effective blocking action from the exhaust pipe, he could raise the exhaust port to open far earlier than anything previously contemplated by DKW. In the past, low exhaust ports had been the barrier to higher revs. But with the higher exhaust-port height made usable by the new pipe, the cylinder could expel its exhaust in less time at much higher revs. By 1959, Kaaden’s simple three-transfer 125 was making 22 hp at 10,000 rpm. This was more than the output of the best 125cc four-strokes of that time, the Mondial and MV Agusta singles.

Championship-winning success, however, depended upon reliability as well as upon power, and little IFA (which became Motorrad Zschopau or MZ in 1959) lacked the industrial clout necessary to trample the many remaining problems to death—cooling, piston and cylinder metallurgy, durable con-rod bearings, ignition. That task would shortly be taken on by the emerging two-stroke makers of Japan, namely Suzuki and Yamaha.