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.

Exhaust Manifold

Attached to the engine is an exhaust manifold with its exhaust inlets matching the exhaust outlet ports of the engine. This manifold is cooled by circulating cooling water through the water jacket surrounding the exhaust gas chamber.

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 outflow 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 manifold and the center line of the transom overboard exhaust fitting. When the manifold is appreciably above the overboard fitting, the exhaust connector can be an 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 or elbow is used. By design, the riser acts as a dam to keep both exhaust-cooling water and sea water 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 back pressure on the engine to a minimum. Back pressure is the pressure that builds up in the exhaust manifold and engine exhaust outlets. The lower this pressure, the more efficient the operation of the engine. Low back pressure is achieved by designing exhaust passages to minimize resistance to the flow of gases and to provide for a uniform gas flow away from all cylinders.

Replacement Exhaust Systems

When the need arises to replace a manifold and/or exhaust connector, one of three situations will exist:

• Replacement of both manifold and connector,
• Replacement of the manifold only, or
• Replacement of the exhaust connector only.

When replacing both manifold and connector, normally there are no problems in matching, and connection to existing exhaust lines is usually very easy. When replacing just the manifold or just the exhaust connector, problems can arise in matching the replacement part to the existing part. It is usually recommended to look at repair and replacement in terms of the complete systems and how the system is designed and balanced.

Original replacement parts reduce the cost, at times, because you would be assured of a somewhat perfect match to the original system. Some other companies produce duplicates to original parts and can be considered, if you know that the performance of the part is identical to the original, which is often hard to verify. The alternative at times is to replace the entire system which is desirable if a newer system is known to give better operation and performance. Be careful here to understand that all things new aren't always better.

If the reason for replacement of a single component is corrosion damage, you can figure that corrosion has and/or will force replacement of the rest of the components. It's better to replace as assemblies and not to piece-mill the engine back together.