Closed Cooling, or Not

Discussion in 'Sterndrives' started by Blue Heron, Feb 23, 2010.

  1. Blue Heron
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    Blue Heron Junior Member

    I'm restoring an older 25' Seacraft cuddy cabin that has a Bravo 1 drive and a blown 350 CI engine. I want to repower with a Mercruiser MPI engine but haven't decided on a small block or big block yet.

    My question is whether closed cooling is something I should spend the extra money for. My boat experience up to now has been all outboards. I understand I/Os can have corrosion issues within the cooling system, mainly in the exaust manifolds, that can kill an engine if not corrected in time. What I don't know is to what extent closed cooling prevents those problems, or if closed cooling presents a separate set of problems.

    The boat will mostly be used for offshore fishing in salt water. A typical trip will involve a run 20-30 miles offshore followed by 4-6 hours of trolling at 4-6 knots and a run back in. The boat will be stored on a trailer with the exception of two week-long trips annually when the boat will be moored in salt water.

    It does not presently have a mercathode system, so I am going to retrofit it with one. If I go with the closed cooling, I would opt for the warm manifold system with only the riser elbows exposed to raw water. I will always flush the engine with fresh water as part of my post trip cleanup.

    Thanks for any input you can provide.
    Dave
     
  2. CDK
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    CDK retired engineer

    A closed system is the only way to have the engine reach a decent working temperature. It prolongs engine life, avoids frost damage, but more important: it improves fuel economy significantly.

    And you can skip the fresh water flushing...
     
  3. Ilan Voyager
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    Ilan Voyager Senior Member

    You're more than right CDK. True words of wisdom. Closed cooling (or FWC) is the lone way to have an efficient engine working at the good temperature. Other interesting fact: the life of the engine (or Time Before Overhauls) becomes longer. The little cost of the FWC is paid by the savings of gas and maintenance costs in a very short time.
    The Mercruisers with electronic injection and FWC have now rather good specific consumptions. The 5.0L and 5.7L have the best specific consumptions, followed closely by the 6.2L. The others are far behind. Other important fact; the Bravo II with the possibility of "big diameter" propellers is the best, followed by the Bravo III, whose dual propeller system is useful on a mono engine boat. The Bravo I is inefficient (too small propellers).
    At my knowledge the FWC (Fresh Water Cooling, a misleading name) do not create new problems, but it solves all the problems made by using a very corrosive fluid ( ie salt water) inside an engine.
     
  4. Blue Heron
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    Blue Heron Junior Member

    Thanks

    Thanks for the replies, Gentlemen. I was not aware that there was a difference in operating temperature between open versus closed cooling. I assumed the thermostat would be the same and that would make them run at the same temp. Is it because of the cooler incoming raw water, or because the manifolds are cooled with water that hasn't been heated in the engine block?
     
  5. marshmat
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    marshmat Senior Member

    A quick note on the drives- as IV points out, the Bravo I is a relatively inefficient drive at lower speeds; it's meant as a sport boat drive and seems to be best on relatively light boats that run north of 40 mph. Counter-rotating drives (Bravo III, most Volvos) or the Bravo II with its 20-inch prop tend to be more efficient on most boats. I don't think I've ever seen a single Bravo II, though- they're usually installed in pairs.

    Given the choice, I would invariably pick closed cooling, even for a fresh water boat. Flushing a raw water system- essential in zebra mussel territory- gets very tiresome very quickly.
     
  6. TollyWally
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    TollyWally Senior Member

    Fresh water cooling is the best but you still have corrosion problems with the risers. Mercruiser does not, in my opinion, have a well engineered exhaust system. Castings are too thin and prone to failure. My BBC was marinized by Volvo. The swedish engineering is good but they are too proud of their spare parts. My first choice would be a Crusader.
     
  7. Blue Heron
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    Blue Heron Junior Member

    Actually,one of the repower options I'm considering is a Crusader Power Pack SD. It's a 330 HP 5.7 MPI engine with the fuel system, ignition system and ECM, but none of the engine accessories. They've developed it for the I/O repower market.

    Tell me why Crusader is better than Mercruiser.
     
  8. Ilan Voyager
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    Ilan Voyager Senior Member

    The reason of the very essential difference in temp between the FWC and the raw water cooling is that the salt of the sea water deposits inside the engine when the temp is above 55° Celsius. By security and for not having salt deposits on the hot points, the cooling water is kept at about 40-45° Celsius. It's pretty cold as the optimal running temp of a gas engine is around 85-90°, I won't detail the consequences for an engine of running cold, but it's nothing good, and does not help the consumption. An engine must run as hot as possible.

    Raw soft water has often salts, in hard water the sulfates (secondary hardness) are as as problematic than salt, the ebullition point is about 100°.

    The cooling liquid of a FWC is a mix similar to those of the cars; distilled water, plus monoethylene glycol plus anti corrosion additives. The boiling point is about 110-127°, the freezing point ranges from -17 to -34°. Thus the engine is protected from corrosion, blowing head gaskets by local ebullition and freezing...

    The Crusader is an excellent brand and worth the money with lot of nice details and features. The differences with Mercury are all those important details; placement of the filters, quality of the accessories (exhausts specially), adjustments and even the paint. I do remember but I'm no sure, a "detail" of the bearings of the crankshaft. Can be used with a Bravo transmission? I know it only for gear boxes.

    The Bravo II is rarely used in mono engines because of the lateral torque induced by the big propeller, the Brave III with the 2 counter rotating propellers has not this problem and is the best choice for a mono engine.
     
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  9. TollyWally
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    TollyWally Senior Member

    The blocks and internals are generally similar. The marinization parts are where larger differences occur. I can not speak knowledgebly about the crank bearing details that Ilan mentions but it would not surprise me if Crusader went the extra mile there too. In my opinion they use better engineering and better design sense, a higher commitment to using better components.

    The Achilles heel is the exhaust manifolds and risers. Despite FWC, you're still running raw water through the risers. Mercruiser uses thinner castings that are more prone to catastrophic failure and need replacing more often.

    When you say the Crusader repower motor lacks accessories I assume you are speaking of alternators, coolers, pumps etc. Everything costs money and you will undoubtedly be doing some bracket fabrication if you adopt the old stuff. FWIW if I was repowering a boat with a new motor it wouldn't be a mercruiser. Best of luck what ever you do.
     
  10. CDK
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    CDK retired engineer

    Ilan Voyager gave you an elegant and basically correct explanation for the lower operating temperature. My way of thinking is a bit more caustic:

    Mercuiser gives one year limited warranty on their products, unless forced by law to extend that period. They would get into serious trouble if the engine were allowed to reach normal operating temperature, so they install a thermostat that fully opens at 70 C. or 158 F.

    As TollyWally remarked, the risers will still be prone to corrosion with a closed system, but they can be easily replaced when the need arises.
     
  11. Blue Heron
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    Blue Heron Junior Member

    The water recirculation pump, fuel pump, and exhaust come with it, but none of the other bolt on accessories. I spoke with the Crusader distributor and he says all my brackets will bolt up.

    You can see it here:

    http://www.pleasurecraft.com/PEG_Press_Kit/sterndrive.pdf
     
  12. gonzo
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    gonzo Senior Member

    A modern 5.7 produces basically the same power as a 454 but is much lighter.
     
  13. TollyWally
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    TollyWally Senior Member

    Typical weight difference between a BBC and a SBC is roughly 110# using steel heads. Different torque curves for the weight of a small girl, sometimes size matters. :)
     
  14. gonzo
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    gonzo Senior Member

    If you are using a big block, might as well buy a 8.1.
     

  15. TollyWally
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    TollyWally Senior Member

    They certainly have pushed the cubic inches available in those motors.
     
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