[MagicRat]

Originally Posted by curtis73

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In actuality, most marine engines these days are carbon-copies of what gets put in the street version. They add the brass freeze plugs, a bronzed lining in any aluminum water passage, a marine water pump, and mechanically speaking - that's pretty much it. It used to be that they ran larger bearing tolerances requiring 20w50 oil, but metallurgy has come so far in the last 30 years that they can use stock street tolerances and just use 10w30.
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The differences between marine and auto engines (assuming the same architecture) is an interesting subject.

Traditionally, marine engines were considered to have more durable bottom ends, because marine engines must be able to run at full power, high (4500 +) rpms for extended periods of time, use that cars never see, unless in race conditions. So many components are more robust. However, I have wondered how necessary is this.... can one use a decent auto engine in a boat?

From this site:

Marine Engines 101: I was participating in a forum on-line recently and someone asked, “why can’t I simply drop an auto engine into my boat, and use that?” In response I put together the following which I called Marine Engines 101.

1. Marine engines are generally based on commonly used automotive blocks. Up until the late 80's it could have been a Chrysler, Ford, or GM block. Newer engines are almost all based on GM blocks. A good example is the GM 8.1 Liter engine that is in my motorhome, in a lot of pickup trucks, and a lot of other vehicles. But that is basically where the resemblance (other than outward appearance) ends.

2. Marine engines usually run at much higher continuous duty loads than automotive engines. Auto engines run up to speed, then the transmission shifts and the RPM drops to save fuel and because all that power is not needed to keep the car moving at the same speed on level ground. A car cruising on a level highway at 60 mph is running at about 2500 rpm (rpm depends on the gear ratio of the differential), much lower than it's max torque, and using about 20% of it's rated hp.
(MagicRat edit: I think the hp required is even lower. Most cars get down the road at 60 mph on 10-20 hp, which is even less than the 20% stated)
Cruising speed on most motor boats is around 3/4 throttle. A marine engine at 3/4 throttle is running anywhere from 3500-4000 rpm or more (depends on the engine, propeller size and pitch, and the reduction gear in the transmission) and near it's max hp. It is generally running at or near max torque. So the loads placed on marine engines are much higher and they must be built to handle heavier loads.

3. The cam shaft in a marine engine is designed for heavier loads. Whereas in a car the cam is selected for general use over a wide range of rpm, the cam in marine engine is selected for max rpm and torque.

4. Because of the heavier continuous torque loads, rods, pistons, etc are stronger.

5. Because they run under heavier load they need more cooling water. So the water passages are larger in a marine engine and a more powerful water pump is needed.

6. Valves are generally heavier duty like in trucks.

7. The head is usually heavier with larger water passages.

8. A higher capacity water pump is required.

9. A higher capacity oil pump is required.

10 Carburetor: The carburetor on a car and on a boat may look the same but internally they are not. If the float sticks in the bowl and fuel flows out the vent on a car it goes out of the carburetor. On a marine carburetor it goes down the throat. Marine carburetors are not allowed to spill any fuel into the boat. Plus that, marine carburetors have different jets and setup because of the different duty cycle. They need to be tuned to run most effeciently at three quarters throttle, usually about 4500 rpm (the rpm varies depending on the engine).