During the operation of any marine engine-gas or diesel-exhaust gases are produced and must be removed from the engine and conducted outside the boat. While there are two basic exhaust systems, wet and dry, the wet system is the one most often found in pleasure boats as well as many work boats. This system mixes the exhaust gases with engine-cooling water and then discharges them through the transom of the boat via an exhaust line.

Exhaust Manifold
Attached to the engine is an exhaust manifold with its exhaust inlets matching the exhaust outlet ports of the engine. Circulating cooling water through the water jacket surrounding the exhaust gas chamber cools this manifold.

Exhaust Connector
At the exhaust end of the manifold is an exhaust connector. This connector is an intermediary between the manifold and an exhaust hose or pipe. This connector also mixes the exhaust gases with water to further cool the gases, and it operates as a means for directing the gas and water outflows so that a connection can be made between the manifold and the exhaust tubing.

Particular attention must be paid to the relative height difference between the center line of the exhaust end of the center line of the transom overboard exhaust fitting, the exhaust connector can an be exhaust dump or drop of a convenient angle.

In those instances where the manifold is not appreciably above or is below the overboard fitting, an exhaust riser is used. By design, the riser acts as a dam to keep both exhaust-cooling water and seawater from back flooding into the engine and destroying it.

Back Pressure
From an engineering viewpoint, the most critical design requirement for exhaust systems is to limit the backpressure on the engine to a minimum. Backpressure is the pressure that builds up in the exhaust manifold engine exhaust outlets. The lower the pressure in the exhaust, the more efficient the operation of the engine. Low backpressure is achieved by designing exhaust passages to minimize resistance to the flow of gases and to prove for a uniform gas flow away from all cylinders.

Barr manifolds and exhaust connectors are designed to minimize backpressure. In many cases, after replacing an OEM manifold with Barr replacement parts, more r.p.m.