For decades, General Motors has been supplying engine blocks for MerCruiser sterndrives, an arrangement MerCruiser liked largely because of the economy of scale and ready availability. While marine engines are typically run with higher loads and at higher rpm, the task of pushing a boat and pushing a car is close enough that it was a pretty good arrangement. But starting in 1975, Congress enacted the Corporate Fuel Economy Average Act (CAFE), which demanded better future fuel economy for cars. Detroit responded by making more complex engines that started drifting away from what is good for marine customers, which resulted in many of the small-block V-8 engines, such as the 5.0L and 5.7L, mainly used by trailerable boats, being slated for extinction by GM. Same goes for the 4.3L V-6. So what did MerCruiser do? It built its own made-for-marine-only 4.5L engine, a V-6 that our tests discovered can out-accelerate an identical V-8-powered boat with more horsepower.
Front and Center
Touted by its engineers as “not having a single GM part,” the engine is a tremendous accomplishment, tempered by the possibility of higher parts prices until the aftermarket catches up. But building a true marine engine has a significant number of advantages, the most obvious relating to placement of service items. Virtually every item that needs routine servicing is front and center, which is a huge plus for do-it-yourselfers and pros. Before, the oil filter was designed to be replaced from the bottom with a car on a lift, but now it sits high on the right, within easy grasp. Need to add fluids? No worries. All the fills and dipsticks are right in front of you. In addition, the 70-amp alternator has been relocated from the bottom to the top, to keep it away from sloshing bilge water.
No ’Tude Like Quietude
Sterndrive engines have frequently been plagued by high decibel levels, especially while getting a boat on plane, and a lot of the noise has to do with the placement of the throttle body. On car engines, it’s placed facing forward, toward the front, which is great when the engine is in front of you. But on boats, the engine is behind the passengers and directs the sucking and whistling sound of rapidly flowing fuel into the cockpit. The 4.5L MerCruiser has the throttle body positioned in the back, facing aft and has dramatically reduced engine noise by an average of 3 decibels, which makes it nearly 50 percent quieter. In addition, there’s an anti-whistle plate to prevent that annoying teakettle sound.
Precious Metal Protection
The 4.5L V-6 uses a cast-iron block, which Mercury chooses to outsource even though it has its own state-of-the-art foundry, to prevent cross-contamination of its aluminum-block outboards. It costs more but is a prudent precaution, since the least bit of residual iron in an aluminum block can weaken it. Elsewhere, where components don’t come into contact with water, weight-saving aluminum and composite components are used. To fight galvanic corrosion, MerCruiser uses a system called MerCathode, which uses a controller to send a signal to the sacrificial anodes, creating a protective field around the sterndrive unit to protect it. A green light at the top, front of the engine lets you know the system is working. The 4.5L’s heads and cylinder block are protected with a special paint that is fused to the metal and baked on to form a barrier to corrosion. You can get the 4.5L with either a raw-water or a closed cooling system, and saltwater users can get its outdrives protected by the SeaCore system, which is a hard-coat aluminum anodized coating, much like a high-end skillet.
In Oshkosh, not far from Fond du Lac, Wis., where MerCruisers are built, we had a chance to test the new V-6 on Lake Winnebago. Our first test was a drag race between a 250 hp 4.5L-powered Sea Ray 220 Sundeck and an identical boat equipped with a 5.0L V-8 that had 10 more horsepower. To simulate real-world conditions, both boats had four passengers whose total weight was nearly identical. But since the V-6 weighs 130 pounds less, it already had a big advantage. Time to plane was a half-second faster for the 4.5L, at 4.6 seconds, and the time to 30 mph was 1.2 seconds faster, 12 seconds, for the little V-6 that could. Toward the end of the run, the extra horsepower finally exerted itself, and the 5.0L-equipped Sea Ray hit a top speed of 47.7 mph, which was 3 mph faster. During the sea trial, we got to test MerCruiser’s Adaptive Speed Control function, which helps maintain a constant engine speed during turns, so when towing skiers, the driver doesn’t have to compensate by adding throttle. Although the 4.5L engine has less peak torque than the 5.0L engine, it has a fatter power curve, so mid-range acceleration seems snappier. Another big plus is the 20 percent better fuel economy.
This is the first MerCruiser V-6 engine to be Digital Throttle and Shift (DTS) compatible, so you can use it for twin-engine applications and add the Joystick Docking System. You can get it with the dual-prop Bravo Three outdrive or a single-prop Alpha One.
MerCruiser 4.5L 250 HP
Type: Fuel-injected V-6
Full-throttle range: 4800-5200 rpm
Weight w/fluids and Alpha One outdrive: 940 lbs.
Gear ratio: 2.2:1
Price w/Alpha One outdrive: $12,540
Tested with Sea Ray 220 Sundeck
Time to plane: 4.6 sec.
0-30: 10.4 sec.
Decibels @ idle: 58 dB-A
Cruise: 29.9 mph/3500 rpm/85 dB-A
Peak: 44.7 mph/5100 rpm/ 94 dB-A
Mercury Marine; www.mercurymarine.com