Each time the exhaust opens high pressure waves are generated.

The expansion chamber is a double pulse action device made of mainly two parts: a part with diverging walls called diffuser and a part with reverse cone.

The diverging part or diffuser inverts part of the positive pulse from the exhaust in a negative wave that helps evacuate the cylinder but part of the positive pulse continues on to be reflected by the reverse cone and arriving before the exhaust port closes it forces escaped mixure back to the cylinder increasing power.

The system needs a certain ammount of back pressure to work and this can be achieved by any form of restriction, but the ammount of back pressure is critical for it increases e combustion temperature. And the system has to have the correct volume for the engine to breath and for the process to be generated at the needed values.

(C) 2006 Bimotion

The need for the return waves timing to target the engin desired power range implies specific angles and lenght making the design of the exhaust more articulated.the first diffusor (L2) acts in the lower rpm range and the last diffusor (L4) in the upper range.

The rear baffle (L6,L7) angle decides the top end rpm range and power 'hit'. Steeper angles will increase the pulse strength and decrease the pulse length, i.e. shorten the rpm range at which the usable power is produced.

The internal length between the diffusors will also decide the power production characteristics, so if the first diffusor is relative long then it will gain power in the lower rpm range etc. A pipe that is pressurized with an early opened port (long duration) will be able to retain the pressure through the steep angles and deliver a strong suction pulse back at BDC. With strong pulses engaged we don't need too large/high transfer ports. The pipe will then help and pull out the gas from the crankcase, even manage to open the reed valve and pull more air through the engine. When the transfer ports are closed, a second returning wave in the pipe pushes back the fresh gas that was spilled out into the header.