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  1. #1

    454 magnum vs 7.4 litre Mercruiser

    Searching the internet, I find that 454 Magnum block and heads are different than the 7.4 litre.
    I currently have a tired 1988 7.4 litre and my mechanic says the best way to freshen the engine is to buy a 7.4 litre long block and replace it with the present engine. Which is in a '88 Formula F223 LS.

    I ask him if I can replace the current engine with a 454 magnum block. He is not sure. The only difference he sees is the oil cooler is on a 454 magnum and not the 7.4 litre.

    Does anyone know what else is different between a 454 ans a 7.4 litre?

  2. #2
    This question doesn't have an exact answer, because it varies with year model. The TRUE 454 magnum was rated at either 360 or 365 horsepower at the flywheel while the 7.4 was rated at 330. There IS IN FACT more horsepower difference between these two engines than the horsepower rating suggests, BUT that's a different issue. In the late 80's, Merc packaged some 330's in certain boats with the "Magnum" designation. Some of the SeaRay Pachangas of the late 80's said magnum on the cover over the flame arrester, but were just the 330.

    The true Magnum had the rectangular port heads and alum intake very similar to the 427/425 & 396/375 intake that Chevy used on so many of their muscle cars in the late 60's. The marine version had the bronze inserts in the water jackets, and some had a dual pattern on top allowing EITHER a Quadrajet or Holley to be used. The real magnum also had a different camshaft as well as other components. I've been told that the magnums had 4 bolt mains and the 330's had 2 bolt, but have seen what I believe was an original 330 that had the 4 bolt mains also, so I'm unsure of that.

    You can in fact replace the 330 long block with a magnum with most all components interchanging, however you would need to buy the magnum intake also, assuming the long block includes the cyl heads. I would think you could find and original Mercruiser alum. high perf intake used for not much money because they were also used on other engines, such as the 502, and a lot of people used aftermarket intakes with em.

    The confusion is going to be in getting the real magnum stuff and NOT the 87/88 330's that were labeled magnum. I believe the 330 in my '89 Pachanga may have been labeled magnum also, but 3 engines later I can't remember for sure and don't know if I still have the cover.



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  3. #3
    Join Date
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    If your'e going with a longblock, the easiest way to go will be to look a the current engine. If it's an oval port, replace it with an oval port, if it'a a rectangular port, replace it with a rectangular port. All the block configurations for that year range is the same (as far as accessory bolt-ups). If you want more power and you have an oval port motor, you can get the rectangular port motor and buy a matching intake, re-jet the carb and your'e good to go.
    "One of the penalties for refusing to participate in politics is that you end up being governed by your inferiors". Plato .

  4. #4
    Quote Originally Posted by flabum1017 View Post
    If your'e going with a longblock, the easiest way to go will be to look a the current engine. If it's an oval port, replace it with an oval port, if it'a a rectangular port, replace it with a rectangular port..................
    I thought the question was "can I replace the current block with a magnum?".



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  5. #5
    Join Date
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    Quote Originally Posted by flabum1017 View Post
    If you want more power and you have an oval port motor, you can get the rectangular port motor and buy a matching intake, re-jet the carb and your'e good to go.

    The second part answers his question
    "One of the penalties for refusing to participate in politics is that you end up being governed by your inferiors". Plato .

  6. #6

    Angry Touching all bases.

    Quote Originally Posted by flabum1017 View Post
    If you want more power and you have an oval port motor, you can get the rectangular port motor and buy a matching intake, re-jet the carb and your'e good to go.
    So I don't need the Magnum oil cooler and Magnum camshaft?

  7. #7
    The long block would include the camshaft and the stock oil cooler is adequate. Reason I say that is that I upgraded my 330 to a stock 502/440, then to a 502 with twin turbos, and then to an HP 575 (540 cu inches) all with the oil cooler from the 330.



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  8. #8
    Join Date
    Jan 2007
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    In meinem geheimen unterirdischen Versteck, USSA
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    If your a Winni boater don't bother spending your money, buy a kayak.
    But, if your realy into it if you have a 4 bolt motor save some money and have
    your motor tweaked and rebuilt by somebody who knows their sh*t.
    .......................................................................LETS STORM THE CASTLE BOYS!

  9. #9
    Any mecanic that worth anything can rebuild that bbc and make it put out as much as u want or funds allow for much cheaper than a long block.

  10. #10
    Quote Originally Posted by rils View Post
    Any mecanic that worth anything can rebuild that bbc and make it put out as much as u want or funds allow for much cheaper than a long block.
    Maybe, maybe not. Over the years I've seen WAY TOO MANY "bubba" motors that either failed or were a boat owner's nightmare because the local drag motor or roundy round motor guru put all his voodoo on an engine and the boat ended up with something that was completely unsuited for marine use. The average hotrod engine builder (certainly including "any mechanic that is worth anything") doesn't have a clue what it takes to build a marine friendly engine........... ie: one that can operate steady relatively UPPER RPM levels for extended periods and deliver good mid range and upper RPM power while having a broad enough powerband to operate WITHOUT a transmission. If these guys had to build engines that were put in cars with ONLY 4th gear, that were run on a track at a steady 5K RPM (give or take) for extended periods, BUT STILL had to deliver reasonable acceleration, THEN they'd understand what it takes to build a good marine engine. The average automotive engine builder over cams and over carburates for marine use.



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  11. #11
    Join Date
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    the things you have to watch are gen 4,5 and 6, gen 4 was the big b locks most of us know, the 5 came out in 91(i believe) 1 pc rear main seal, no fuel pump provisions, the gen 6 was similar to the 5 except it had a hyd roller cam, the cylinder heads interchange(but not well) w/ a conversion head gasket, the oil pans are different, and no mechanical fuel pump on the 2 later engines, rect port heads differentiate the magnum(except the 330 models) from the base 330/310 hp models, and anyway, in a boat you'd be better off to find a set of early oval port heads(more torque/hp up to 5 k rpm)
    action mobile marine, 772 528 0754, dealers for aces fuel products,wolf efi, pro marine,latham and gaffrig/livorsi. we build gearcases and modify them(3 litre) to ratchet, and powerheads as well. 21 skater/3 litre wolf efi the engine build http://www.screamandfly.com/showthre...-3-litre-build

  12. #12
    Join Date
    Apr 2006
    Location
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    Mrcrsr is right. The Gen 4,5,6 thing is a pain in the butt. Gotta watch that. On top of not having a fuel pump provision the front cover and waterpump are completely different. Also some accessory brackets such as alternators and water pumps may not transfer as a result.

  13. #13
    This website answers a lot of my questions.

    http://www.perfprotech.com/store/art...placement.aspx

    Anyone had experience with this vendor? Any comments? Looks like the 392HP performance package will be my best bet?

  14. #14
    Quote Originally Posted by Raceman View Post
    Maybe, maybe not. Over the years I've seen WAY TOO MANY "bubba" motors that either failed or were a boat owner's nightmare because the local drag motor or roundy round motor guru put all his voodoo on an engine and the boat ended up with something that was completely unsuited for marine use. The average hotrod engine builder (certainly including "any mechanic that is worth anything") doesn't have a clue what it takes to build a marine friendly engine........... ie: one that can operate steady relatively UPPER RPM levels for extended periods and deliver good mid range and upper RPM power while having a broad enough powerband to operate WITHOUT a transmission. If these guys had to build engines that were put in cars with ONLY 4th gear, that were run on a track at a steady 5K RPM (give or take) for extended periods, BUT STILL had to deliver reasonable acceleration, THEN they'd understand what it takes to build a good marine engine. The average automotive engine builder over cams and over carburates for marine use.
    I agree with this thats why I said worth anything because if a guy builds engines and cant put the use of the engine into context weather its going into a boat, street car, race car, truck or whatever the case my be all having diffrent applications and needs hes not worth much in my book.

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