Automotive Versus Marine Engines
What's the Difference
Between Marine and Car Engines?

The component parts of a marine engine are much different than that of a car engine. The reasons are as simple as the sea is salt: durability and safety.

GM's Vortec 5700 is a common choice for marinization

Anyone contemplating re-powering an inboard or stern drive has likely been tempted to replace a dead marine engine with a less expensive car engine. The difference in price is compelling, a fact that gives rise to the question: Is there really such a big difference between marine and automotive V-8s? The answer is a resounding, yes. In fact the differences between the two breeds are so striking, that one ought never place an automotive engine on stringers where a marine engine used to lay.

The biggest difference is that marine engine cylinder blocks are based on heavier-duty truck blocks, replete with four-bolt main bearing support of the crankshaft instead of just two. Keep in mind a car only uses about 15 of its one or 200 horsepower to sustain a speed of 55 mph (rpm = horsepower). Conversely, a boat is always under load. Its marine cycle can be compared to hitching a 10,000 pound boat and trailer on the bumper of a car and trying to climb the Rocky Mountains at 80 miles an hour.

Marine carburetors, like the Holley carb above, meet USCG specifications.

Besides the severe duty cycle there are other important differences. A marine engine's core plugs (also called freeze plugs) are corrosion resistant bronze. The camshaft is ground to different specifications, most often to maximize low end torque instead of high rpm horsepower. Valve overlap (the time when both intake and exhaust valve are open) is shortened in order to minimize the chance of water being sucked out of the exhaust and into the combustion chamber. Gaskets are premium quality for better sealing and corrosion resistance.

Most important of all the marine grade components are the starter, alternator and distributor. All three are fitted with special screens that quench internal sparks that might otherwise vent into the atmosphere and light-off gasoline fumes present in a boat's engine compartment. For the same reason a marine carburetor bowl vents its overflow down its own throat, instead of externally to the atmosphere. Marine carburetors meet USCG specifications for safety.

The bottom line, an unmodified automotive engine is totally inappropriate for a boat motor. Its torque curve won't meet the needs of a boat, its light-duty components won't long survive the rigors of marine usage, and you risk blowing yourself out of the water. You decide.