So just what is the difference between a car engine and a stern drive engine?
You might think that your Volvo Penta powered boat has a Volvo engine in it and your Mercruiser powered boat has a Mercury engine. While the engines have been modified extensively for marine use, in almost every case, they are engines built by General Motors Corporation. Yes, the car company.
Over the past 50 years or so, many companies have built stern drive engines including Mercury Marine, manufacturer of Mercury Outboards and Outboard Marine Corporation, whose stern drive products evolved into some of the current Volvo Penta line. OMC modified outboard motors to run mounted on their sides and power stern drives back in the 60’s. Merc built a 3.7 liter engine using their own aluminum engine block and a few Ford components. All of these have fallen by the wayside today and we see GMC engines pretty much across the board in current production.
So what is different between the engine in a Chevy Tahoe and the one in a boat? There are several things. The engine block “freeze” plugs, for example. These are not really plugs to allow for freezing but are plugs that cork up holes left in the outside of the block during the manufacturing process. These plugs are usually steel on car engines because the block is filled with anti freeze which inhibits any corrosion. In stern drive engines, these plugs are changed to brass because most boats run raw water through the block. Raw water is pulled into the engine from the lake, ocean or river and after a pass though the cooling system is blended with the hot exhaust at the back of the manifolds and discharged. Steel plugs would rust out quickly so brass is used.
Because there is no downhill coasting in a boat, the engine is under load constantly while underway and because there are no gears other than forward and reverse, a boat engine’s torque characteristics must be different from automotive engine. This change is accomplished a number of ways but the most significant variance from automotive is in the camshaft. Ignition and fuel injection tuning is also calibrated to adapt the engine’s performance to the marine environment.
All components of the engine mounted fuel system have to endure a burn test where the engine is placed on what could best be described as an industrial barbeque and is set afire. No component of the fuel system can leak for a specified amount of time while it is on fire.
The electrical system has to be modified to prevent any sparks that would normally be generated by electric motor brushes, alternator brushes, relay contacts, and the like. This prevents them from igniting any fumes that may accumulate in the engine bay.
The exhaust system is fully water jacketed to isolate the red hot exhaust from the engine box area. Water is blended into the exhaust just before it enters the stern drive’s exhaust passages to cool it to manageable levels.
A specially designed bell housing adapts the rear of the engine to its mounting place in the boat.
There are many similarities and many components are exactly the same but there is also much dissimilarity. Many components that appear outwardly to be the same are in fact significantly modified for marine use and should never be interchanged.