Mercury Remanufacturing - 383 MAG Stroker Sterndrive

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Captain's Report

Mercury Remanufacturing 383 MAG Stroker full engine
The MerCruiser 383 MAG Stroker is a remanufactured version of the MerCruiser 377 MAG V8 that has been re-engineered, re-patterned and re-built to produce the horsepower and torque of the discontinued 454.

Ben Franklin once said that the only thing predictable in life is death and taxes. That person was not a boat owner, as they left out the fact that new boats aren’t getting any cheaper, and the value of a new boat predictably goes down as soon as one buys it. So repowering the owner’s favorite vessel is a good hedge against both the sticker shock and depreciation of a new boat. Further, veteran boaters from time-to-time may come across a boat in great shape except for the engine(s). Here, there may be an economic opportunity if the numbers work out.

Mercury Remanufacturing can help owners in this situation, even if their old mill is no longer available. Sure, in some cases one can buy a long block and bolt on their old accessories like an alternator, heat exchanger and risers, but nothing beats a turnkey set-up that comes with the latest in computer technology and corrosion-resistant materials.

The Set-Up

For this test, we worked with a Mercury Marine dealer who was tasked with the repowering of a 1999 Donzi 275 Z, originally equipped with a MerCruiser 454 MPI (310-hp) and a Bravo 3 sterndrive. The single engine, pocket cruiser measures 29'3'' (8.92 m) overall and had a beam of 8’6” (2.59 m). She offers entertaining and over-night capabilities with a V-berth forward, a small aft cabin, a little galley, and a private head compartment. With the fuel and water tanks topped-off, the boat weighs approximately 8,200 lbs. (3,719 kg).

Mercury Remanufacturing 383 MAG Stroker profile shot
The boat’s owner knew the old Donzi had good bones, it just needed a repower and a few system upgrades.

Road Blocks

Replacing an engine and drive sounds simple enough, until the first hurdle is that the big block 454 that we have learned to love and trust all these years is no longer available from Mercury. Buying a remanufactured long block from a third-party engine house was not going to be an option for the wise owner, as MerCruiser dependability and factory warranties ensure a pleasurable boating season. Who said caveat emptor?

Enter Small Blocks

Like so many power plants before her, the 454 MPI has exited stage left to be replaced by a lighter, cleaner, and substantially more advanced engine platform -- the 383 MAG Stroker. This is a remanufactured version of the MerCruiser 6.2L V8 that has been around for over 15 years and has staked its reputation in the marine market for its strong torque curve and unprecedented durability.

The engine develops 350 ponies and 390 ft-lb. of torque at 3800 rpms and weighs 932 lbs. (423 kg). It breathes through a long runner intake manifold with multi-port fuel injection, and is driven by the latest 555 ECM (electronic control module) with SmartCraft.

Mercury Remanufacturing 383 MAG Stroker running shot
Despite being heavily loaded on test day, the single-engine cruiser got on plane easily and accelerated quickly to her top speed of 44.7 mph.

Anatomy of a Reman Engine

1. MerCruiser Marine Block

CNC machined remanufactured MerCruiser marine block - Line bored cam and mains - Max 0.040" CNC overbore - CNC honed - CNC deck machining

Mercury Remanufacturing 383 MAG Stroker marine block

2. New, Marine duty gaskets

Mercury Remanufacturing 383 MAG Stroker gaskets

3. Hypereutectic pistons. Stronger than new.

Mercury Remanufacturing 383 MAG Stroker pistons

4. All new marine valves and valve springs.

Mercury Remanufacturing 383 MAG Stroker valves

5. Marine cylinder heads.

Mercury Remanufacturing 383 MAG Stroker cylinder heads

6. New marine water pump

Mercury Remanufacturing 383 MAG Stroker water pump

7. New or polished marine camshaft. Never reground. Exclusive 350 and 377 camshaft profiles available.

Mercury Remanufacturing 383 MAG Stroker cam shaft

8. Timing cover.

Mercury Remanufacturing 383 MAG Stroker timing cover

9. New high-flow oil pump.

Mercury Remanufacturing 383 MAG Stroker oil pump

10. Marine oil pan and windage tray.

11. 100% Inspection before Shipment.

• Hot tested

• Pressure checked

• Compression checked

• UV light and dye leak check.


Since the original power package in the Donzi was tired, performing a baseline test on the boat wasn't feasible and would have been futile. So we searched our archives for a pocket cruiser with nearly identical dimensions, weight and equipment. We found the Doral Monticello to be nearly a twin.


For our comparison here, we’ll use the Doral as the theoretical baseline. Her cruise speed was clocked at 3500 rpms at 27.8 mph and she burned 13.0 gph. Compared to the 383 MAG Stroker repower, we found a cruise speed of 27.3 mph at 3500 rpm while consuming 11.7 gph, nearly a 10% improvement in fuel economy.

The WOT Gap

At wide-open throttle, the 383 MAG Stroker delivered 44.7 mph at 5200 rpms while the 454 MPI peaked at 43.3 mph at 4750 rpms. Clearly the small block engine has no problem keeping up with the heavy big-block, while burning less fuel.

Mercury Remanufacturing 383 MAG Stroker
The chart above shows calculated performance numbers for the repowered Donzi 275 Z equipped with the 383 MAG Stroker engine.
Mercury Remanufacturing 383 MAG Stroker speed chart
The speed curve above shows the engine’s powerful torque curve which adds prop-twisting power in the mid range to get the boat on plane in a shot.
Mercury Remanufacturing 383 MAG Stroker ecm performance
The advanced performance of the 555 ECM (with SmartCraft) comes from this small computer the size of a slice of toast.

Block Talk

Mercury Remanufacturing begins with a thoroughly inspected, used GM marine iron block core that gets bored out another .030” to 4.030”. The cylinder deck is then machined for flatness and the crank journals are line honed and prepped for new bearings. After a multi-stage cleaning process, the assembly line installs a new 4340-forged steel crankshaft into the lower end, followed by a set of graphite coated hypereutectic pistons with a set of Moly-coated piston rings for improved durability.

Iron Man Reman

A new, high profile camshaft is installed along with a high-flow oil pump and new bearings. The used cylinder heads begin their second life by getting decked and machined for all new hardware. New stainless steel valves, along with springs and keepers are fitted with rebuilt roller lifters. 1.6:1 full roller rocker arms are installed for improved volumetric efficiency and less friction to deliver better performance.

Mercury Remanufacturing 383 MAG Stroker removable deck hatches
The Donzi 275 Z’s large removable deck hatches made engine replacement easier and faster.
Mercury Remanufacturing 383 MAG Stroker finished repower
Here’s the end result for the repower. Note the clean bilge – a sign of a pro installer.
Mercury Remanufacturing 383 MAG Stroker new motor mounts
The Mercury Remanufactured engine comes with new motor mounts that match the original footprint for easy installation.


A huge advantage to a turnkey system is the reduction, at least in the short term, in required service of the all-new electrical, plumbing and fuel systems. The 383 MAG Stroker uses the latest in dry-elbow exhaust to help prevent water being sucked back into the cylinder’s and ruining the engine. The folks at Mercury have also incorporated a new second-generation fuel pump/vapor separator/cooler that delivers good performance and prevents vapor lock on hot days.


The MSRP for the Mercury Remanufacturing 383 MAG Stroker is $11,118.75.


The engine comes with a one year parts and labor warranty which can be extended for an additional two years. Other than oil changes and fuel filters, this package should be headache-free for at least three years.

Mercury Remanufacturing 383 MAG Stroker engine flush
An easy to reach, engine flush quick-disconnect is fixed to the starboard side exhaust manifold.


From a look at the numbers we have on hand, the 383 MAG Stroker is a better performer than the old 454, we see roughly a 10% increase in performance and about the same in economy. Over time, fuel savings will add up, and the newer engine can also take 87 octane gas and 10% ethanol blends.

One caveat before embarking on a repower journey is to closely examine the cost/benefit of putting a new engine in an old boat. It can easily make sense if the old boat is in good shape, and, very importantly, if the buyer intends to keep the boat for a few years.

The market value of the boat only matters if one plans to sell it in the near term, in which case they shouldn’t be doing a repower. Also keep in mind that the labor to install the engine will be a big part of the overall cost, and there will be other components -- new wiring, bilge pumps, blowers, batteries, battery chargers, seacocks and so on that will make sense to replace while one is at it. The owner also may well find that some of the hull structure needs to be replaced, such as the engine beds or even the transom, which 20 years ago were invariably made of lumber-grade plywood that rots quickly when exposed to moisture.

That said, when done properly, repowering one’s fundamentally sound boat may well be worth the effort and cost. Pride and joy could be a rewarding experience, in addition to an excellent-performing vessel. In the long run, a repowered boat will certainly be worth more than it would be otherwise.