The new MerCruiser 4.5L is the latest response from Mercury Marine to a change in the supply of engines it feels are suitable for inboard marinization. Designed and built in-house, the MerCruiser 4.5L is a 4.5-liter V6 that will have an initial rating of 250 hp. Mercury is pitching the 4.5L as an alternative to the 260-hp 5.0-liter V8. In the future expect a de-tuned version of the 4.5L to be offered by Merc as a replacement for the 220-hp 4.3L V6.

Mercruiser 4.5L

The new MerCruiser 4.5L, from Mercury Marine, will NOT be found in any automotive incarnation.


Mercury is confronting the reality that its inboard engine supplier, GM Powertrain, may soon be discontinuing some of the affordable base engines it has offered for years. The auto industry has a laser focus on meeting new federal fuel mileage standards that peak with a CAFE (Corporate Average Fuel Economy) standard of 54.5 mpg in 2025. Note than most half-ton pickup trucks are now equipped with high-revving V6 engines with VVT (variable valve timing), complex active fuel management controls and six-speed automatic transmissions. Mercury took a similar path when GM dropped its 8.1-liter engine. Rather than adopt the new GM LS3 6.2-liter V8, Mercury began assembling its own 8.2L based on an aftermarket block because it felt those “big block” cubic inches were essential for marine duty.

“The automobile industry is focused on squeezing that last one or two percent of fuel economy from its powertrains,” said David Foulkes, Mercury VP of Product Development, Engineering and Racing, who spent 17 years with Ford Motor Company before joining Mercury in 2007. “Much of the technology that will get the auto companies to 54.5 mpg does not add value to a marine engine, which typically operates at much higher engine speed and load than an auto engine. New auto engine designs are adding weight and friction that don’t help us.”

Unlike the GM Powertrain engines sourced by Mercury and Volvo-Penta that dominate the sterndrive and gas inboard market, this new MerCruiser engine was designed from the oil pan up for marine use. It’s a small-block style engine, with a cast iron block, pushrod valve actuation, and two valves per cylinder. The cylinder heads and intake manifold are aluminum and are cast by Mercury in its own foundry. The iron block is cast by a supplier. Bore and stroke are 4.0 x 3.6 inches (101 x 92mm). The engine will be offered with raw-water or fresh-water cooling, is compatible with MerCruiser Alpha and Bravo One/Two/Three outdrives, and can be rigged with cable or SmartCraft DTS controls. With DTS comes the option of Joystick Piloting for twin-engine installations.

In its lightest configuration – raw-water cooled with an Alpha drive – the 4.5 weighs 940 pounds, which is 134 pounds less than the 5.0L V8, according to Mercury. That’s a big weight difference in a mid-size runabout, and a really significant advantage for the 4.5L in a twin installation. The 4.5L is also about four inches shorter in length than the 5.0L, so boat builders who commit to the 4.5L package can also gain four inches of cockpit space if they’re willing to modify the interior of the boat.

The 4.5L has a WOT range of 4800 to 5200 rpm. Foulkes says that while the 4.5L makes less peak torque than the 5.0 V8, its torque curve is broader and flatter than that of the V8, good for mid-range power in the marine cruising rpm zone. An intricate scroll-type intake manifold is cast with the lost foam process (an area of Mercury expertise) and contributes to that broad torque curve. Mercury designed the intake manifold to place the throttle body facing aft so intake sound, which can’t be muffled much due to US Coast Guard flame arrestor regulations, radiates away from the cockpit. Mercury says other noise-reducing features include new engine mounts, and the rigid structure of the cast-aluminum oil pan. A special lightweight flywheel reduces the “clunk” sound when shifting into gear. The engine also features a new water pump that Mercury says is smaller, stronger and more efficient than previous designs, and is self-draining.

Another marine-specific feature of the 4.5L is the arrangement of key consumer “touch points” on a bar in the upper front of the engine – the dip stick, oil filter, oil fill and power steering fill – along with a detailed service label with a QR code for viewing maintenance how-to videos on a smart phone. The valve train is maintenance-free, and this engine does not require a traditional 20-hour service. The 4.5L is equipped with the same air-pressure drain system found on the MerCruiser 350 MAG, which allows the owner to clear most of the water from the engine to extend service at the end of the season when freezing temperatures could occur.

To provide a handy comparison of the 4.5L and the 5.0L engines, Mercury was kind enough (or clever enough) to provide a pair of Sea Ray 220 Sundeck boats with the two engines, in each case mated to a Bravo Three drive with a 2.2:1 gear ratio.

SeaRay 220 Sundeck
LOA: 22’ 6”
Beam: 8’ 4”
Fuel Level: 27 gallons

MerCruiser 4.5L (22.5 Pitch Props)

0 - 30 mph: 9.3 seconds. Top Speed: 44.8 mph. MPG @ 3500 rpm: 3.9

MerCruiser 5.0L (24.0 Pitch Props)

0 - 30 mph:10.4 seconds. Top Speed: 49.1 mph.  MPG @ 3500 rpm: 3.5

The numbers reflect what we saw in impromptu drag races on the water – the 220 Sundeck powered by the 4.5L was a little quicker out of the hole, but the 5.0L would eventually catch up with superior top speed, although the speed difference didn’t always seem as dramatic as the test data indicates. The 4.5L delivered better fuel economy at about 30 mph despite running smaller props. The 4.5L was significantly quieter at cruising speeds. At 4000 rpm we measured 88 dB-A for the 4.5L compared to 91 dB-A for the 5.0L. On the dB-A scale that’s almost twice as loud.

On the water I got to sample another new 4.5L feature, Adaptive Speed Control, which automatically holds engine rpm as load changes, so you don’t need to modulate the throttle when putting the boat into a turn, for example, to hold a steady speed – handy when towing tubers, I guess.

Lighter is always better in my book, and with this engine Mercury seems to have optimized the tricky balance between weight and power. A bonus – I was told the new MerCruiser 4.5L will cost about the same at the 5.0L V8 when supplied to boat builders.

For more information, visit: Mercury Marine.