Marine Manifolds: How do they Work?

[sea horse]

I've studied the posts about manifolds with interest, but I don't have a good understanding of how they work, how they fail and what's going on inside. One thing that puzzles me is where the water goes - especially when they crack!<br />Thanks,<br />Seahorse

 

[Spidybot]

Re: Marine Manifolds: How do they Work?

There are two main principles:<br /><br />The motor exhausts, just like any car, into the manifold. Well away from the exhaust ports, behind a bend to make sure water does not get near the ports, the 'used' water is pumped into the manifold. The exhaust/water mix is then lead outside the boat - either via the outdrive (if any) or via some kind of waterlock and/or silencer.<br /><br />A crack in the manifold will, in almost all cases, result in water inside the motor compartment.<br /><br />If the system is with heat exchanger, it's just like a car ('dry' exhaust). In this case, the hot coolant is lead to a heat exchanger, where seawater is used to cool down the coolant before it's circulated back to the engine. Seawater never gets near the engine - only passes through the exchanger (which is made from alloy or alike that withstands the saltwater). <br /><br />Big difference is, that when seawater (saltwater) is allowed in the engine coolant channels it corrodes everything. Especially when getting hot (which is why, by the way, those engines often run at what seems to be a very low temp.). As exhaust is very hot from the combustion and gets mixed with the hot water in the manifold, those (often cast iron) manifolds tends to corrode before the engine. Next part to corrode is the head.<br /><br />When using a heat exchanger, the only thing inside the engine cooling channels is coolant. This prevents corrosion, lubes water pump and contains antifreeze.<br /><br />Systems designed for seawater cooling definately works best when used in freshwater as is not as agressive to the engine parts as saltwater.<br /><br />Add to this, that boats that are trailered/lifted out of the water, drains the seawater from the system (engine or exchanger) thus greatly reducing the risc of corrosion.<br /><br />Check www.vetus.com - they have a lot of good parts and pics. on the subject.

 

[Boatist]

Re: Marine Manifolds: How do they Work?

Sea Horse<br />I going to assume you are talking about I/O as outboard are much different. The standard setup for most manifolds is just like a car. The exhaust comes out of each exhaust port then each port is ran to a exhaust pipe and exhaust ran out the back. Main difference in a boat is each exhaust port is sourounded by water to keep it cool inside the boat. The ports all run together just like a car but have water around them to keep them cool. After they all run together most boats next run exhaust into a riser which rises above the manifold then makes a u turn and runs down and out the exhaust port on the outdrive. Again this riser is sourounded by water and then when starts going down dumps the water into the exhaust port to go out the outdrive thru the center of the prop. The main function of this riser is to keep a swell from forceing water up the exhaust port into the engine.<br /><br />Ok all this works very well in Fresh water and last a long time. In salt water if you always flush with fresh water will last 4 to 6 years. What usually happends is salt and heat cause the manifold to corrode and one of the ports will allow water to leak into the exhaust port. once it is in the exhaust port it runs back to the exhaust valve and that starts to rust. Also if the valve is open water runs into the piston and that rust or worse yet into the cylinder then the exhaust valve closes and intake opens suck in a little fuel, intake closes and start compression stroke but since you can not compress water it bends the rod breaks the piston and you are done.<br /><br />Most heat exchangers run coolent in the engine but still use salt water to cool exhaust manifold. The sea water will run thru the heat exchanger to cool engine coolent then run thru the exhaust manifold water passage to cool exhaust mainifod. This is called a half system.<br />As far as draining water out of the manifold ever dealer I have seen says not to drain it except in winter to prevent freezing. They say flush with fresh water but do not drain water as air will actually cause the rusting to be worse.<br /><br />That is the way it works and ofcourse want to replace manifold and riser before water jacket rust thru into exhaust port and wrecks the engine.