Mercury MerCruiser - 4.5L 250 HP

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Captain's Report

MerCruiser 4.5L 250- HP


A 6-cylinder sterndrive power plant has been released by the folks in Fond du Lac, Wisconsin. This purpose-built 4.5 L marine engine is made by Mercury Marine which has full control over its production. This opportunity has resulted in a more powerful V6 engine that had replaced the venerable 5.0L small block in the never-ending quest to shed weight and increase performance.

This also opens the MerCruiser 4.5L 250-hp to power vessels over 21’ (6.40 m) in singles, and twins will break the 30’ (9.14 m) barrier which was unheard of in the past.

The mill produces 250 horsepower and tops out at 5200 rpm at wide open throttle. It breathes through a redesigned, high-torque induction system and is fed by sequential multi-port fuel injection (MPI).

Key Points

  • Mercury in-house design

  • More power, less weight, greater efficiencies

  • Smartcraft integration for single or twin applications

  • Numerous drive units and ratios are available

  • Back Story in Brief

    So why is MerCruiser developing its own V6 engine and spending big bucks in the process you ask? In the past GM supplied long blocks (complete engine minus accessories) to Mercury Marine. As major changes to the automotive world evolved and new government CAFÉ regulations for economy and emissions standards were imposed, GM chose to eliminate some engine platforms completely. When this happened Mercury realized it would have to produce its own platforms to feed the needs of the pleasure boating world. The result? Mercury developed a marine proven, purpose built sterndrive engine with features and benefits that provide a better boating experience. All in all, this departure from truck based engines for the marine industry will only strengthen the quality of the product by moving the technology forward and allowing our industry to grow independently.


    Recently, was able to test one of the first MerCruiser 4.5L engines on a sportboat. Our test weighed in at just under 5,000 lbs. (2,268 kgs), which is heavy for a 23-footer (7.01 m). Turning the twin bladed Bravo III, we got on plane in an impressive 4 seconds and reached 40 mph in just a hair over 16 seconds. Wide-open throttle delivered 44.8 mph burning 19.25 gallons an hour or 2.33 miles per gallon. Our most efficient cruise speed gave us 3.63 miles per gallon at 3,500 rpm with a respectable 29.4 mph speed.

    MerCruiser 4.5L 250- HP

    Performance graph of our Cruisers Sport Series 238 Bow Rider equipped with the MerCruiser 4.5L.

    Block Talk

    The block and heads were designed by Mercury. They are mated to a structural, cast aluminum oil pan that does a better job of dampening noise and vibration than a stamped steel pan can. There are also front and rear engine mounts. These larger footprint mounts will absorb more vibration from the engine and result in less of it being transmitted into the boat’s structure. A lighter-weight flywheel is being used; this increases engine throttle response and is claimed to decrease shift clunk when going in and out of gear.

    Induction. The big news with the MerCruiser 4.5L 250-hp engine is its induction system; this is where air and fuel come to juice the cylinders and optimize power output. The intake plenum is made from Mercury’s exclusive low pressure lost foam casting offering reduced weight while packing in more cool air to deliver more power.

    Greater Torque/Less Noise. A cross-ram, tuned scroll runner design was used to lengthen the intake runners which results in greater torque at low rpm; better out of the hole response and quicker planning times, according to Mercury. They also relocated the throttle body to the back of the engine, which significantly reduces the audible engine noise heard by those onboard. A vortex style ‘Anti-Whistle’ throttle plate is installed to minimize that annoying high frequency whistle often heard at high speed.

    Fuel Module and Water Sensor. The multi-port electronic fuel injection incorporates a fuel supply module, and a revised fuel filter system, incorporates a ‘water in fuel’ sensor. No longer will contaminated fuel catch a boater by surprise and kill their boating weekend.

    Engine Management

    The brain of the 4.5L 250-hp engine is Mercury’s reliable PCM 09 engine control unit (ECU) with SmartCraft CAN capabilities, onboard diagnostics (OBD-M) and dual wide band oxygen sensors. There is even an Emission Control Technology (ECT) upgrade for those needing reduced emission compliance. This means you will be able to run 87 octane pump gas or even E10.

    Engine Management

    Adaptive Speed Control

    The 4.5L V-6 also offers Adaptive Speed Control (ASC), which automatically maintains the set rpm point regardless of load or condition changes, such as tight turns, tow sports and lower speeds on plane. The result is increased throttle response and a “sportier” feel for the driver, who no longer has to make continual throttle-control adjustments.

    MerCruiser’s cylinder block and heads are coated with a special electro- deposition paint that is electrically fused to the metal then baked on for a tough, durable finish. Since the intake manifold has no cross flow water jacket cast in it, corrosion here has been eliminated. In order to reduce corrosion damage from saltwater, an improved active cathodic system called MerCathode comes standard with certain Bravo-equipped units and as a kit on Alpha models. The Gen 3 MerCathode offers double the corrosion protection barrier. This higher efficiency system provides significantly higher power with less power drain on batteries. The 4.5L 250-hp Bravo engines can also be ordered with Mercury’s SeaCore technology, which is a comprehensive corrosion protection system that improves upon MerCruiser’s standard built-in corrosion features – all making for the ultimate in corrosion protection.

    MerCruiser 4.5L 250- HP

    The MerCruiser 4.5L 250-hp will be available with standard raw water cooling or closed loop fresh water cooling.


    Anyone who has spent time working on a sterndrive-propelled boat knows that accessing some of the service points can be challenging. So we were really glad that the 4.5L's engineers were listening to people at the docks. For starters, all the basic fluid checks are color coded and mapped out onto the front of the engine along with an indicator light for the MerCathode corrosion system.

    MerCruiser 4.5L 250- HP

    Fluid check and other important engine info is located on top of the engine.

    Other notable improvements include a top-mount 70 Amp alternator that relocates the unit up high and extends its service life by keeping it from being splashed by bilge water. A maintenance-free automatic belt tensioner, and a more efficient stainless steel water pump make their way to the front of the engine.


    In keeping with tradition, Mercury makes all of their dependable and proven drives available for the V6. Everything from the Alpha through Bravo’s I & II, to the counter-rotating wheels of the Bravo Three. Salt-water applications can upgrade and have the SeaCore treated drives available for maximum longevity with it’s hard coat aluminum anodizing.

    Customers choosing DTS can get the added benefit of electronic shifting while those with twins will be thrilled with the Axius option with joystick steering and pod-like maneuvering.


    Boaters seeking to repower the existing V6 powered craft will find the upgrade a fairly straightforward endeavor as existing bolt patterns have been retained. Those looking to repower a small block V8 vessel will get an added bonus of some 4.5” of cockpit space gained due to the V6 being that much shorter. Granted, engine box modification would be needed, but for some it will be worthwhile. Not to mention that each motor is a whopping 130 lbs. lighter than the 5.0L mill.

    Another plus for repower is that the engine can be controlled either with older cable systems or can be rigged with Digital Throttle and Shift (DTS). By using the DTS and Smart Tow, I see a lot of runabouts getting new life and the high-tech advantage.

    4.5L V6 250 HP vs. 5.0L V8 260 HP Comparisons

    MerCruiser 4.5L 250- HP

    Preliminary Mercury Test Data


    This departure from GM-supplied blocks should result in better boating for all. Certainly this is a costly move for Mercury but their track record should prove the effort worthwhile. With the MerCruiser 4.5L 250-hp's performance and its "made-for-boating" features, consumers will now have a choice and better engine options than before.