This effect is called the 'reverse supercharging effect', more commonly known by the English term scavenging. To make the most of the scavenging effect, it is therefore necessary to have exhaust manifolds of equal length and a perfect synchronisation of the opening and closing of the intake and exhaust valves according to engine timing. The best conditions are dictated by the geometry and performance of the machine, which influence the setting of the valve timing (overlap).

Overlap is that condition in which, for a certain amount of time, both the intake and exhaust valves are open. During the exhaust stroke, as the piston approaches top dead centre, the intake valve starts to open while the exhaust valve is not yet fully closed: this happens because it is physically impossible to bring the exhaust valve from its closed position to its fully open position instantaneously. This is the reason for the use of valve opening advances and closing delays, which allow the condition of
crossover to exploit the suction effect generated by the exhaust gases leaving the combustion chamber: the outgoing gases in fact draw in the fresh charge which, taking its place, performs a dead space scavenging operation.

On the basis of the above, one would say that exhaust pipes should have as small a cross-section as possible in order to speed up the fluid flowing through them, thus favouring emptying. In reality, physics always surprises us, even in this case: narrower pipes generate greater resistance to gas flow, better known as 'back pressure'. When the engine is running at high engine speeds, for which the exhaust gas velocity is very high, the back pressure generated becomes great enough to outweigh the benefits of scavenging, which is why exhaust pipes are only beneficial at low engine speeds. Conversely, exhaust pipes with larger sections reduce back pressure at high revs and increase maximum engine power, but at low revs the exhaust gas flow is too slow to generate scavenging.

For these reasons, variable backpressure exhaust manifolds were developed. They consist of a series of pipes arranged in parallel with different cross-sections which, through the use of a bypass valve, are crossed or not crossed by the exhaust gases in order to be able to make the most of the benefits of scavenging without running into the problems associated with back pressure.

 

 

 

(Source: Technical Car)