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With a few tweaks to accommodate a life at sea, the GM 305 V-8 has been used by Mercury Marine's Mercruiser marque since 1974. The engine is configured both as the MCM 228 inboard-outboard and -- in engines with a serial number of 4931099 or higher -- an inboard configuration, the MIE 228. Caveats, options and numbers detailing the engine's performance characteristics are straightforward.
The direction in which the engine rotates is important in multiengine applications, since one engine will turn clockwise and one, counterclockwise. You can determine the engine rotation by observing the flywheel from the rear of the engine, looking forward toward the water pump. Note, too, that the direction in which the propellers turn is not always the same direction as the engine rotates. This is important when ordering a replacement engine, replacement short block or parts for the engine.
Engine serial numbers are located on the rear of the starboard cylinder head on the serial number plate and above the forward end of the starter. On the MCM 228, configured for a sterndrive, they are located at the starboard, rear top of the drive and at the forward end of the bell housing. On the MIE 228, with a propeller shaft and propeller, rather than a sterndrive unit, the serial numbers are found on the engine and front of transmission.
Transmissions available for the MIE 228 version of the Mercruiser 305 equipped with a propeller shaft and propeller, include the ZF/Hurth ZF 10 M, ZF 12 M, ZF 15 M and ZF 15 A straight drives. The ZF 15 MIV is a V-drive, packaged by Mercruiser as the Velvet Drive. A V-drive allows the engine to be mounted backward to save engine-room space. The propeller shaft exits the transmission in the direction opposite that in which the driveshaft entered the transmission.
Both the MCM 228 and the MIE 228 are available as seawater cooled engines and as engines equipped with a keel-cooler. The seawater cooled engines use a circulating pump to pull seawater through the engine, only to pump it back out of the engine and overboard. A keel-cooler is a closed heat exchange system mounted under the bottom of the boat, near the boat's keel, or spine.The hot water and antifreeze mix from the engine passes through the tubes of the keel-cooler to exchange the heat energy with the cooler seawater. Both engines are also available with a hybrid cooling system, where the water and antifreeze mix from the engine is pumped into a radiator with a twist that's located in the engine room. In this system, seawater is circulated through the radiator's tubes while the antifreeze mix surrounds the tubes.
Both the MCM 228 and MIE 228 produce 220 shaft horsepower, or 164 kW. The engine’s maximum rpm lies between 4,400 and 4,800. Its idle at neutral is 650 rpm. Its nominal operating oil pressure at 2,000 rpm is between 30 and 60 psi; at idle, the pressure drops to about 4 psi. The stock thermostat setting is 160 degrees F.
Its nominal timing is 10 degrees BTDC, although models with electronic fuel injection may be set at 8 degrees BTDC. Its firing order is 1-8-4-3-6-5-7-2.
It’s alternator produces 65 hot operating amps and 72 cold operating amps and the battery Mercruiser recommends is one with at least 375 cold cranking amps.
- Engines - GM V-8 305 CID (5.0L) / 350 CID (5.7L), Book 1 of 2, Sections 1 thru 4"; Mercury Marine; 1996
Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.