Tuned exhaust

In an internal combustion engine, the geometry of the exhaust system can be optimised ("tuned") to maximise the power output of the engine. Tuned exhausts are designed so that reflected pressure waves arrive at the exhaust port at a particular time in the combustion cycle.

Two-stroke engines

Expansion chambers

In two-stroke engines where the exhaust port is opened by being uncovered by the piston (rather than by a separate valve), a tuned exhaust system usually consists of an expansion chamber. The expansion chamber is designed to produce a negative pressure wave to assist in filling the cylinder with the next intake charge, and then to produce a positive pressure wave which reduces the amount of fresh intake charge that escapes through the exhaust port (port blocking).

Uniflow scavenging

An alternate design of two-stroke engines is where the exhaust port is opened/closed using a poppet valve and the intake port is piston-controlled (opened by being uncovered by the piston). The timing of the exhaust valve closure is designed to assist in filling the cylinder with the next intake charge (as per four-stroke engines).

An opposed-piston engine uses uniflow scavenging, however this design uses piston-controlled cylinder ports with one piston controlling the inlet port and the other the exhaust port. Similarly, split-single engines use uniflow scavenging, with the piston in one cylinder controlling the transfer port (where the intake mixture enters the cylinder) and the other piston controls the exhaust port.

Four-stroke engines

In a four-stroke engine, an exhaust manifold which is designed to maximise the power output of an engine is often called "extractors" or "headers". The pipe lengths and merging locations are designed to assist in filling the cylinder with the next intake charge using exhaust scavenging.