achris
More fish than mountain goat
- Joined
- May 19, 2004
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This document is for people new to repairing boat engines, and who may not understand the differences, or the consequences of fitting non-marine parts (in certain places)...
What makes a marine engine different to its car equivalent?
From the bare block, a marine engine is built up with slightly different parts.
The core plugs are brass or stainless steel, as opposed to steel. And the head gaskets are also non-steel, either composite or stainless. The backing plate and seal in the engine water circulating pump are also stainless steel. This is done because most marine stern drive and inboard engines will be cooled by seawater (salty/corrosive) and not a ‘steel friendly’ coolant. The thermostat in a marine engine is also a lower temperature to try to reduce the amount of disassociation (to acid) that occurs with seawater at elevated temperatures. Marine engine are also required to run under high load for extended periods of time. Unlike a car engine where high load is usually only experienced during acceleration, a boat engine is under high load at all times. Internally a marine engine is also different in that it may use different valve springs, hydraulic lifters and pistons. Even crankshaft bearings can be different. The camshaft profile of a marine engine is closer to a truck or RV engine than a ‘street performance’ engine. Excessive valve overlap, which can contribute to an engine’s higher power output is bad news in a wet exhaust marine engine. Excess overlap can pull water, which is there to cool the exhaust manifolds and gasses, back into the cylinders leading to extensive engine damage. If an aluminium intake manifold is used, then the water passages in it must be sleeved with a material that is less prone to corrosion. Crankcase ventilation must also be done back into the intake side of the engine, lest crankcase vapours accumulate in the boat bilge.
For electrical system components. Unlike an automobile, a boat engine sits in a mostly closed environment. This can allow fuel vapour to accumulate, which if ignited can be very bad. To that end, all electrical equipment must comply with a standard that reduces or eliminates the possibility of a initiating a fire or explosion. Items like starter motors and alternators inherently produce arcing on their commutators and slip rings, it’s just how they work. Screens are put in alternators that allow airflow but will not allow a flame front to pass through. Starter motors are ‘sealed’ to the point where a flame front cannot propagate. Obviously a starter motor can’t be completely sealed due to having to allow for thermal expansion, both of the material and of the air inside. Distributors also can’t be sealed, so the vent on the side of the distributor is a right angle, since a flame front can’t move through a right angle. All electrical equipment on a boat should comply with the SAE-J1171 standard. This also includes the trim pump motors, any circuit breakers, relays and contactors. Fuses are also covered, and are different to a standard automotive fuse. A standard blade fuse has an open base. That is, the fuse element can be in direct contact with the surrounding air. Any fuel vapour present would easily be ignited should the fuse overload and the element ‘blow’. Marine fuses are closed across the base, so there can be no direct contact with the surrounding air. Even things like spark plugs are made of a different material to help reduce corrosion. And spark plug leads must be radio noise suppressed, so as not to interfere with marine transceivers.
Fuel systems. When carburettors were common place they were also subject to ‘marinization’. A standard carburettor vents the fuel bowl to atmosphere, which allows fuel vapour to escape. In a car, that’s fine, the vapour just falls out the bottom of the engine bay and is dissipated, but in a boat that vapour accumulates in the bilge, potentially with serious consequences. A marine carburettor vents the fuel bowl internally, so no vapour can escape. A marine carburettor also has no ‘vacuum advance’ port, but does have another port near the top of the venturi. That port may be connected to a thin hose from the mechanical fuel pump. A standard fuel pump cannot be used on a marine engine because the internal design of the standard pump allows for fuel to be dumped to the crankcase in the event of a diaphragm rupture. In a marine pump there are either 2 diaphragms of different materials with a sight glass between them, or a thin line up to the carburettor port designed for this purpose. In the event of a diaphragm rupture, a large quantity of fuel will be dumped into the engine, stalling it. If an electric pump is installed, it must be wired with an oil pressure switch in circuit, such that should the engine either stall and the key is left on, or the key if left on inadvertently, then the pump does not run. The oil pressure switch will only allow power to the pump when the engine is running. The SAE standard for carburettors and throttle bodies is J-1223.
So yes, your Mercruiser 5.7 is very similar to that 350 SBC engine, but the differences are significant, and there for your safety.
Unfortunate a lot of these differences are internal and cannot be determined by a visual inspection without disassembly. Also, a lot of inexperienced people just pull something like a starter motor out, look at it and say ‘it’s a standard starter motor’, then unwittingly put a non-marine starter in, with dire consequences.
I'm sure I have missed a few things, and comments below will hopefully include those...
Obviously there are many parts that are the same as there automotive counterpart. Things like oil pans, timing gears, chains and covers, valves, head and block castings, oil pumps, vee and serpentine belts, harmonic balancers and water hoses to name a few.
Chris........
What makes a marine engine different to its car equivalent?
From the bare block, a marine engine is built up with slightly different parts.
The core plugs are brass or stainless steel, as opposed to steel. And the head gaskets are also non-steel, either composite or stainless. The backing plate and seal in the engine water circulating pump are also stainless steel. This is done because most marine stern drive and inboard engines will be cooled by seawater (salty/corrosive) and not a ‘steel friendly’ coolant. The thermostat in a marine engine is also a lower temperature to try to reduce the amount of disassociation (to acid) that occurs with seawater at elevated temperatures. Marine engine are also required to run under high load for extended periods of time. Unlike a car engine where high load is usually only experienced during acceleration, a boat engine is under high load at all times. Internally a marine engine is also different in that it may use different valve springs, hydraulic lifters and pistons. Even crankshaft bearings can be different. The camshaft profile of a marine engine is closer to a truck or RV engine than a ‘street performance’ engine. Excessive valve overlap, which can contribute to an engine’s higher power output is bad news in a wet exhaust marine engine. Excess overlap can pull water, which is there to cool the exhaust manifolds and gasses, back into the cylinders leading to extensive engine damage. If an aluminium intake manifold is used, then the water passages in it must be sleeved with a material that is less prone to corrosion. Crankcase ventilation must also be done back into the intake side of the engine, lest crankcase vapours accumulate in the boat bilge.
For electrical system components. Unlike an automobile, a boat engine sits in a mostly closed environment. This can allow fuel vapour to accumulate, which if ignited can be very bad. To that end, all electrical equipment must comply with a standard that reduces or eliminates the possibility of a initiating a fire or explosion. Items like starter motors and alternators inherently produce arcing on their commutators and slip rings, it’s just how they work. Screens are put in alternators that allow airflow but will not allow a flame front to pass through. Starter motors are ‘sealed’ to the point where a flame front cannot propagate. Obviously a starter motor can’t be completely sealed due to having to allow for thermal expansion, both of the material and of the air inside. Distributors also can’t be sealed, so the vent on the side of the distributor is a right angle, since a flame front can’t move through a right angle. All electrical equipment on a boat should comply with the SAE-J1171 standard. This also includes the trim pump motors, any circuit breakers, relays and contactors. Fuses are also covered, and are different to a standard automotive fuse. A standard blade fuse has an open base. That is, the fuse element can be in direct contact with the surrounding air. Any fuel vapour present would easily be ignited should the fuse overload and the element ‘blow’. Marine fuses are closed across the base, so there can be no direct contact with the surrounding air. Even things like spark plugs are made of a different material to help reduce corrosion. And spark plug leads must be radio noise suppressed, so as not to interfere with marine transceivers.
Fuel systems. When carburettors were common place they were also subject to ‘marinization’. A standard carburettor vents the fuel bowl to atmosphere, which allows fuel vapour to escape. In a car, that’s fine, the vapour just falls out the bottom of the engine bay and is dissipated, but in a boat that vapour accumulates in the bilge, potentially with serious consequences. A marine carburettor vents the fuel bowl internally, so no vapour can escape. A marine carburettor also has no ‘vacuum advance’ port, but does have another port near the top of the venturi. That port may be connected to a thin hose from the mechanical fuel pump. A standard fuel pump cannot be used on a marine engine because the internal design of the standard pump allows for fuel to be dumped to the crankcase in the event of a diaphragm rupture. In a marine pump there are either 2 diaphragms of different materials with a sight glass between them, or a thin line up to the carburettor port designed for this purpose. In the event of a diaphragm rupture, a large quantity of fuel will be dumped into the engine, stalling it. If an electric pump is installed, it must be wired with an oil pressure switch in circuit, such that should the engine either stall and the key is left on, or the key if left on inadvertently, then the pump does not run. The oil pressure switch will only allow power to the pump when the engine is running. The SAE standard for carburettors and throttle bodies is J-1223.
So yes, your Mercruiser 5.7 is very similar to that 350 SBC engine, but the differences are significant, and there for your safety.
Unfortunate a lot of these differences are internal and cannot be determined by a visual inspection without disassembly. Also, a lot of inexperienced people just pull something like a starter motor out, look at it and say ‘it’s a standard starter motor’, then unwittingly put a non-marine starter in, with dire consequences.
I'm sure I have missed a few things, and comments below will hopefully include those...
Obviously there are many parts that are the same as there automotive counterpart. Things like oil pans, timing gears, chains and covers, valves, head and block castings, oil pumps, vee and serpentine belts, harmonic balancers and water hoses to name a few.
Chris........
