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What is the purpose of marine manifolds and how do they work

What is the purpose of marine manifolds and how do they work?


A marine petrol engine is basically a car engine that has had certain components added to it to make it suitable for work on a boat. Mostly this includes the use of a marine gearbox, a shaft connected to a propeller and a modified cooling system replacing the wind blown through a car's engine bay.


A manifold takes the exhaust gasses from the engine head and leads them to the exhaust pipe. In a car they are kept cool by air forced around them when the car is in forward motion. Most engines in a a boat are enclosed. A car manifold is unsuitable for most boats as it can not be sufficiently cooled. The solution is a water jacketed manifold where cooling water is pumped around the outside of the exhaust chambers. This water is provided by an engine water pump and is expelled from the boat through the exhaust.


A riser is used where there is a risk of sea water being pushed up the exhaust pipe and into the engine's head- which would be damaged. The riser also increases the height of the exhaust above the waterline.   A riser can sometimes add more torque (pulling or pushing power) to an engine by slightly increasing the back pressure to the engine head.



 Why do manifolds fail?


Marine manifolds, especially those in continuous salt water environments, can fail for any of the following reasons:


bulletIf any seams are welded, the weld may fracture due to vibration or corrosion.
bulletIf there is insufficient body mass that causes the manifold to cool faster than the engine block (see photos under Horror Story)
bulletCast iron manifolds over time will rust from the inside of the water jacket to the outside. This causes water to leak into the heads and cylinders.
bulletThe caustic effect of exhaust gasses mixing with raw water cooling will corrode the risers and/or exhaust elbow.
bulletThe exhaust gasses from the head will eventually degrade the inside of the manifolds exhaust chamber towards the water jacket.



Cast iron, Aluminium or stainless steel?


In Australia, it appears that cast iron after-market manifolds are imported chiefly from the USA. With our low A$ exchange rate this makes them quite expensive. Original manufacturer manifolds are extremely expensive. Cast iron manifolds and risers are also very heavy and can add over 50 kilos in weight. Like all manifolds, those made from cast iron will corrode over time if left permanently in a salt water environment. The cast iron manifolds we market are very well made by a reputable manufacturer. These manifolds will give a good service life and in most cases carry a manufacturer's warranty.


Stainless steel manifolds are a new product in the Australian marketplace. They can be made for just about any engine as they are generally custom built, whereas iron and Aluminium manifolds are made from moulds. They can be expensive, but if properly made will provide a good service life.



There are generally two types of Aluminium manifolds available- either die-cast or sand-cast.


Sand-casting reduces gas build up (and hence bubbles) as the casting cools. This reduces the porosity and chemical impurity in the casting which leading to a longer service life, especially in salt water conditions. The cost of making sand-cast aluminium manifolds is far more expensive, but results in a more robust product.


Aluminium manifolds are starting to be installed on some marine engines, but generally require a pencil  anode to stop corrosion.


Die-cast manifolds not made of marine grade aluminium generally do not last as long as sand-cast manifolds as the pouring and cooling process can create air pockets. Further the aluminium used is not suited to the marine environment.



What about cost and service life?


As you are undoubtedly aware, marine manifolds have never been a cheap boat accessory. Every so often you are faced with the problem that they need to be replaced- either because they are getting old or they have begun to leak. A manifold which allows salt water to enter the cylinder head and hence the cylinder may result in hydraulic lock (engine is seized), corroded pistons, rings, cylinder walls or valves. Irreversible damage can be caused In just a few days by salt water leaking from the manifold or riser.


The cost of owning manifolds can be calculated.


Stainless steel manifolds can have an inbuilt riser incorporated into their design.

A set of very good American after market manifolds and risers for a Mercruiser V8 engine might cost A$4,000.


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