preserves wave energy. This section of the system is called the "header pipe" (the exhaust port length is considered part of the header pipe for measurement purposes). By keeping the header pipe diameter near constant, the energy in the wave is preserved because there is no expansion needed until later in the cycle. The flow leaving the cylinder during most of the blowdown process is sonic or supersonic, and therefore no wave could travel back into the cylinder against that flow. Transfer Once the exhaust pressure has fallen to near-atmospheric level, the piston uncovers the transfer ports. At this point energy from the expansion chamber can be used to aid the flow of fresh mixture into the cylinder. To do this, the expansion chamber is increased in diameter so that the out-going acoustic wave (created by the combustion process) creates a reflected vacuum (negative pressure) wave that returns to the cylinder. This part of the chamber is called the divergent (or diffuser) section and it diverges at 7 to 9 degrees. It may be made up of more than one diverging cone depending on requirements. The vacuum wave arrives in the cylinder during the transfer cycle and helps suck in fresh mixture from the crankcase into the cylinder, and/or prevent the suction of exhaust gases into the crankcase (due to crankcase vacuum). However, the wave may also suck fresh mixture out the exhaust port into the header of the expansion chamber. This effect is mitigated by the port-blocking wave. Port blocking When the transfer is complete, the piston is on the compression stroke but the exhaust port is still open, an unavoidable problem with the two stroke piston port design. To help prevent the piston pushing fresh mixture out the open exhaust port the strong acoustic wave (produced by the combustion) from the expansion chamber is timed to arrive during the beginning of the compression stroke. The port blocking wave is created by reducing the diameter of the chamber. This is called the convergent section (or baffle cone). The outgoing acoustic wave hits the narrowing convergent section and reflects back a strong series of acoustic pulses to the cylinder. They arrive in time to block the exhaust port, still open during the beginning of the compression stroke and push back into the cylinder any fresh mixture drawn out into the header of the expansion chamber. The convergent section is made