1 Two Stroke Performance Tuning Chapter 3 porting and Cylinder Scavenging TODAY, when we take a look down the cylinder of a two- Stroke engine, we find its walls literally filled with ports to handle the induction, transfer and exhaust phases of gas flow through the engine. Those of us who have grown up in the Japanese two- Stroke era take it for granted that every cylinder has a huge exhaust port flanked by anything from four to six transfer ports' However, it hasn't always been this way. As far back as 1904 Alfred Scott patented his original two- Stroke vertical twin. Then in 1906 the French Garard motor appeared with a rotary disc inlet valve. Scott also developed a rotary valve engine in 1912, winning the Senior TT in that year and the following year.
2 However in spite of some very innovative designs being incorporated in two- Stroke engines they continued to be embarrassingly unreliable and this single factor stifled development right up until the time of World War II. In the mid-1930s, the DKW company set out to make two-strokes respectable. They were in the business of manufacturing economical two- Stroke motorcycles and stood to profit from changing the two- Stroke 's image. They engaged the services of an engineer named Zoller to build a 250 racer, which ultimately won the Isle of Man TT in 1938. This led to the development of a 125 single employing a porting arrangement originally invented for two- Stroke diesels by German engineer It was this concept which ultimately brought success to the two- Stroke , both as an economical power source for transport and as a powerful, light-weight power source for competition.
3 Schneurle's loop-scavenging method, patented in 1925, employed a single exhaust port flanked by two small scavenge or transfer ports, whose air streams were aimed to converge on the cylinder wall opposite the exhaust (FIGURE ). Being aimed away from the exhaust, the transfer streams had a natural resistance to short-circuiting straight out the exhaust. Earlier designs had used deflector-dome pistons to keep the fuel/air charge away from the exhaust port. This increased the piston's heat gathering area and meant that only low power outputs could be aimed for without continually risking piston seizure. After the war DKW moved to Ingolstadt in West Germany, while their old plant at Zschopau in East Germany was rebuilt as Motorradwerke Zschopau, or MZ.
4 In 1952 Walter Kaaden joined MZ to take over development. His early work concentrated on exhaust development and alternate scavenge methods. After much experimentation he proved that the Schneurle loop-scavenge system yielded the best power and reliability. Then in 1957 he added a third transfer port, opposite the exhaust. Its air stream joined with the two main transfer ports, directing flow up toward the head (FIGURE ). 1 Port and Cylinder Scavenging Fig. : The Schneurle loop scavenge system. 2 Two Stroke Performance Tuning Fig. : Komet K78 TT porting . Contemporary two- Stroke technology was introduced initially to Suzuki, and later to Yamaha in Japan when Ernst Degner defected from East Germany to join Suzuki.
5 By combining designs which Degner brought from MZ with Japanese technology in the field of metallurgy two- Stroke power outputs and reliability took a leap forward. During the '60s Suzuki and Yamaha both won world championships using exotic porting and rotary valve induction systems originally developed by DKW and MZ. The Yamaha engineers, however, went one step further. They added a pair of auxiliary transfer ports alongside the main transfers, which also directed mixture flow toward the rear of the cylinder and up (FIGURE ). The Japanese engineers then realized, as did Walter Kaaden back in 1957, that there was a section of cylinder wall at the rear which could also be filled with another one or two ports. Transfer flow improved and, as the velocity of the fuel/air charge entering the cylinder was reduced, mixture loss out of the exhaust was decreased (FIGURE ).
6 3 Port and Cylinder Scavenging Fig. : Yamaha TZ25O D/E/F porting . Fig. : Suzuki PE175 C porting . 4 Two Stroke Performance Tuning Back in Europe two- Stroke engineers were battling excessive ring and cylinder wear, due to the exhaust port width being too great. A narrow port reduced power but improved reliability. A taller port restored lost power but made the power band unacceptably narrow. To get around the problem Rotax engineer Dr. Hans Lippitsch added a pair of small auxiliary exhaust ports alongside the large oval exhaust port and above the main transfers. The two auxiliary ports connect with the main exhaust port before the exhaust flange (FIGURE ). Fig. : Rotax 124 LC porting . Yamaha engineers tackled the problem with their power valve system, which is basically a mechanism to vary the exhaust port height without narrowing the power band (FIGURE ).
7 As you can see, there is a drum-like valve up against the cylinder wall. At high rpm the port is raised, increasing hp while permitting a relatively narrow port width for good ring life. At lower speeds the port is lowered, which improves midrange power and widens the power band. The YZR500 works racer's power valve is controlled electronically by a battery-powered motor, but the TZ500 production machine utilizes a much simpler system. Cables run from the tachometer to a centrifugal governor that raises and lowers the port in harmony with engine rpm. Exhaust duration at higher speeds ( , above 10,500 rpm) is 202 , which is about average for a road racer. Low rpm duration is about 180 , or similar to that of a 400 motocross engine.
8 When it comes to modifying a cylinder, the most logical place to start is the exhaust port. A little grinding (or filing) at the sides and top of the port will yield large power increases if approached correctly. Exhaust ports come in all shapes and sizes; each type has its advantages and disadvantages. The port in FIGURE is really rectangular but it is usually referred to as a square port. This is the type that you will find in many low Performance engines. The size of it has to be small so that the rings won't catch on the top of the port and break. There are two ways this port can be modified: either it can be widened at the top or it can be ovalized. We have to be careful that the exhaust port 5 Port and Cylinder Scavenging doesn't get too close to the transfers, otherwise there will be excessive loss of fuel/air mixture out of the exhaust.
9 I like to see 8mm separation between these ports, but at times it is possible to go down to as little as 5mm without ill effect. Fig. : Yamaha powervalve. Fig. : Square exhaust port modifications. If port spacing is a problem, you will have no alternative other than to widen the exhaust port at the top. This type of port will give the engine good power from the upper mid-range to maximum hp. When you grind this type of port, the centre of the port should be 4 to 5 higher than the ends. The reason for this is that when the engine is on the compression Stroke the ring bulges out into the port to its greatest extent just as the port is being closed. However, by raising the centre of the port, the ring has less chance of hanging up on the edge of the port and breaking because the ends of the port actually begin pushing the ring back into the piston groove before the port closes.
10 The elliptical or oval port is the one which I prefer if the port spacing is suitable. It is the type which you will find in most competition two-strokes. The shape of the port is fairly gentle on rings providing it isn't made excessively wide. What is an excessive width? Well, I'm not sure; but I have found that a port of the bore diameter is a good compromise for most road race and motocross engines using ductile iron rings (the maximum safe port size is about with brittle cast iron rings). Some tuners take the port size up to but ring, piston and port damage is unacceptable. I have been able to take some ports out to of the bore size, but this is the exception rather than the rule. 6 Two Stroke Performance Tuning The square bridged port is fairly common in large displacement motocross and enduro engines (FIGURE ).