Mercedes V8 & V12 Big Block Conversions

Discussion in 'DIY Marinizing' started by ATMHC, Jul 5, 2007.

  1. Northwester
    Joined: Sep 2007
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    Location: Vancouver Island

    Northwester Junior Member

    Don't get me wrong as I like German engineering.....

    I own a '91 BMW ///M5. It comes from the factory equipped with a 3.6l inline 6 cylinder DOHC engine that develops 310 HP @ 6900 R.P.M. You can run the engine all day @ 6000 R.P.M. if you want as it is basically a detuned racing engine designed for that kind of duty cycle. Great German engineering but I digress.....

    Back to your project. Your concept is neat and I appreciate what you're trying to accomplish. However unless the following details are considered and reengineered, your engine will probably not last very long in service.

    -at minimum, you will need stainless exhaust valves and hardened exhaust valve seats otherwise the stock seats and valves will probably burn, the valves and seats are probably going to be very difficult to source
    -a thicker head gasket made of stainless steel to lower the compression ratio and provide the required corrosion resistance
    -a custom fabricated water cooled stainless tubular exhaust system
    -if the cylinder head(s) or water circulating pump impeller are made of aluminum, raw water cooling is out of the question so add a raw water pickup pump which you'll need anyhow and a heat exchanger from big block chevy marine engine
    -you'll need a different thermostat which is probably tough to source
    -don't forget the vapour proof starter and alternator
    -aftermarket programmable fuel injection system that allows removal of distributor and uses coil packs to fire the plugs
    -replace all the frost plugs with ones made of brass
    -replace the oil cooler with one made for marine use

    If Mercedes engineered all of the cooling passages properly and all of the heat that is generated can be carried away, your engine has a chance of lasting provided that you do lower the compression ratio to a modest 8.0:1. Otherwise as I said before, the engine will likely suffer serious damage from detonation unless you plan to run high octane gasoline.

    A crate big block chevy marine is probably going to cost less than it will cost you to marinize the 6.9l Mercedes engine in the end. For the little guy, it's seldom cheaper to do-it-yourself marinize because you don't have the volume purchasing power the big corporations have.
     
  2. marshmat
    Joined: Apr 2005
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    Location: Ontario

    marshmat Senior Member

    I don't like the thought of such a special motor getting raw seawater rammed through the block. You shouldn't need many custom parts to convert a stock freshwater cooling heat exchanger, perhaps one meant for the popular 7.4 V8.
    What's wrong with the fuel injection system that you wouldn't want to use it? That's as much a part of the engine's character as anything else...
     
  3. ATMHC
    Joined: Jun 2007
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    Location: Eastern North Carolina

    ATMHC Junior Member

    I simply don't know enough about the mechanicals to answer your question.. If I put the boat in the water 10 times a year, for no more than a half day at a time, and then flushed it out thoroughly with fresh water.. how could it really be damaged by the salt water..? I assume there would be a strainer on the intake to prevent anything bigger than plankton from getting in the engine...correct ?

    The only reason I'd consider switching from F.I. to carbs would be to achieve more of a vintage race boat look...
     
  4. marshmat
    Joined: Apr 2005
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    Location: Ontario

    marshmat Senior Member

    It probably wouldn't be damaged by the saltwater in that kind of service. But if it were my boat (your priorities may differ) and I were the one spending the thousands of dollars to marinize a special engine, I would think the peace of mind and the knowledge that I didn't have to flush, would outweigh the extra few hundred bucks and few hours of routing coolant hose involved in adding a separate, generic (not engine mounted) heat exchanger, and piping corrosion-resistant coolant through my motor instead of nasty filthy seawater.
    I agree with Northwester that the spark-proof starter, alternator etc. are essential; however, I'm not convinced that so much internal reworking is needed, given the duty cycles you've described for the engine (hauling really heavy, really fast cars at supralegal speeds) and the boat (leisurely loafing around in the mid-planing speeds with the occasional quick flyby past the Donzis). Someone who knows this engine would be better suited to comment than I, though.
     
  5. ATMHC
    Joined: Jun 2007
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    Location: Eastern North Carolina

    ATMHC Junior Member

    If it is indeed that easy to create a safe & reliable fresh water cooling system, then I'd probably do so... I've just been under the perhaps mistaken impression that it isn't that simple to do...
     
  6. ATMHC
    Joined: Jun 2007
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    Location: Eastern North Carolina

    ATMHC Junior Member

  7. old750
    Joined: Sep 2007
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    Location: Cobourg, Ontario, Canada

    old750 Junior Member

    I know this is an old topic, but has anyone seen a modern mercedes 3.2L or 4.0L CDI (210-250hp) in a boat? I'm thinking it would be great on gas and fast enough for a 20' runabout. I've been searching all day for an engine source and so far only found the one give on this thread

    (http://www.hartsautoparts.com/mercedes/Engine.html)

    Anyone in Germany have anything?
     
  8. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    extra few hundred bucks and few hours of routing coolant hose involved in adding a separate, generic (not engine mounted) heat exchanger, and piping corrosion-resistant coolant through my motor instead of nasty filthy seawater.

    All you need is a thru hull, water circ pump heat exchanger and water outlet.

    The heat exchanger Mfg will direct you to correct sized unit.

    FF
     
  9. TollyWally
    Joined: Mar 2005
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    Location: Fox Island

    TollyWally Senior Member

    I realize this is a bit flippant but if one wants a mercedes v 12 diesel in a boat why not use a M.A.N. ? A wee bit spendy but... just asking.
     

  10. Jimbo1490
    Joined: Jun 2005
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    Location: Orlando, FL

    Jimbo1490 Senior Member

    I know 400 Lb-ft seems like a lot of torque to you, but remember that a stock Chevy 502 makes well over 500 Lb-ft. and is probably within 50 lbs dry weight if the M-B 6.9 V8. With the plethora of aftermarket high performance and marine parts available for the big block Chevy engine, it's hard to justify committing resources to something different that is highly unlikely to be better than the current standard.

    Jimbo
     
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