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Repowering with a Mercruiser Big Block V8 Marine Engine

Feb 15th, 2012 by admin

TOPIC: Repowering GM 454/502 V8 Marine Engines with the various models of the newer GM 496 C.I.D. Marine Engines

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Whether customers are shopping for a new Mercruiser Marine Longblock Engines  or Mercruiser Marine Bobtail Engines we are consistently faced with the questions of whether the 7.4L/454/502 Marine Engine can be replaced with the newer style 496 Cu-In (8.1L) big block V8 engine. For the sake of this article the terminology 8.1L and 496 are used interchangeably. Here's a little background and direction that will hopefully help you with your repower project.


In 2001 General Motors displaced the 7.4L Base (310 Hp, 330 Hp), 454 Mag (385 Hp, 392 Hp), and 502 (415 Hp) engines to make way for the newer 496 Cu-In (8.1L) engine platform. This product shift was mainly done in support of the pickup truck market, realizing that the marine industry "piggy back's" product from the GM's automotive truck product line. The 454/502 engines were being produced from GM's manufacturing plant located in Tonawanda NY. Once the 8.1L engines were introduced for long term production the tooling and assembly line processing for the 454/502's was converted over to support the 8.1L engines. To support the  existing 454/502 markets GM mandated that their supply network pick up the assembly of the older 454/502 engines. Therefore these new longblocks are still available (see links below) but the pricing increased since they are now being manufactured in much lower volumes than before.

The marine industry and GM did a relatively good job of keeping as much common componetry and sizing bewteen the engines as possible (realizing that making this a completely new layout would greatly effect their own truck markets as well as the overall boating industry).  With this in mind, the main things to consider when replacing a 454/502 engine with the 8.1L are:

  1. If you're replacing a base 7.4L (310 or 330 Hp) engine you're pretty much out of luck because the lowest Hp version of the 8.1L makes about the same Hp and more Torque than the 385 Hp 454 Mag.  Mercruiser tried to deal with this by introducing the 6.2L engine specifically to replace the base 7.4L engines.  If you try to replace your 310/330 hp 7.4L with the base 496 engine, the outdrive or transmission gear ratio may be off (depending on the boat) but the propeller sizing will definitely need to be changed due to the considerably higher torque of the 8.1L engine. We would recommend using a new 7.4L longblock or look at the 6.2L engine options (see links below).
  2. If you're replacing a 454 Mag we would recommend either the base 496 or H.O. 496. When you look at the Hp and torque curves for the two engines the 496 H.O. is really a new engine introduction because the base 496 matches up with the 502 better than it does with the 454 mag. Realizing this, the 496 H.O. is like a new, higher HP model as it outperforms the 502 in pretty much every respect.
  3. The 496 engine is about 1"- 1-1/2" wider, 1-1/2" taller.  The engine protrudes to the front about 1" further overall but this is due to the longer water pump shaft and not the actual engine block itself.
  4. The flywheel/bell housing mounting and location is exactly the same.
  5. The engine mounting footprint and mounts are exactly the same.
  6. The exhaust height is very close to the same however the width of the exhaust riser connection for the exhaust hose is a little wider but will match up fine in almost all cases.
  7. The weight of the engine is very close to the same when considering Fresh Water Cooling is standard on the 8.1L.
  8. The main engine electrical connections are the same however Mercruiser is now using the 14 pin Deutsch connecter so you will likely have to adapt to this if your current engine is older than 2005.
  9. The 8.1L engine does have some nice features such as the 555 engine controller, dry joint style exhaust, integrated closed cooling system, and easy drain system.




The easiest alternative when replacing your 454 or 502 Mercruiser Engine is to replace the longblock rather than trying to fit in a complete Bobtail Marine Engine. Available options for this include either a  Remanufactured Mercruiser Longblock or a New Longblock. For the customer looking for a complete drop-in replacement Bobtail Engine Option there are size considerations you have to keep in mind. If you cannot fit the 496 Marine Engine into your boat, another alternative to consider is the 450 Hp Partial Engine Package. If you are replacing the 310/330 Hp 7.4L V8 Mercruiser Sterndrive or Inboard Engine the 383 Mag Stroker MPI Bobtail is also a great option as well.

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