FNM 300 in a 27’ uniflite... why does this scare me?

Discussion in 'Inboards' started by crankshaft, Jul 1, 2019.

  1. crankshaft
    Joined: Apr 2011
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    Location: Big Lake, Alaska

    crankshaft Junior Member

    I once had big dreams of putting a 6bt in my 27’ hardtop express, but after hours of staring, it just was not a good fit. For some reason, mine is one of the few that got a single 318, and, needless to say, while underpowered, the darn thing wont die. But, here I am, finally tired of going so slow to the point of “no fun”. The ol’ boat is about to get a major overhaul, paint, new interior, and getting rid of those ugly sliding windows, glassing in the holes, and installing more classy, round corner glass, that opens, not slides. Anyway. Plans are, to replace the single 318 with a chrysler 413. Hey, I am a chrysler guy, what can I say. And then...... a good friend of mine, replaced the 454 in his 28’ Bayliner, with a FNM hpep300.... needless to say, it burns half the fuel doing the same work. But I have concerns. My 28 is a wide, hard, heavy semi displacement tub, and that Bayliner is a water skipper. So, I am afraid of two things. (1)Overworking a little, hard to work on, tiny ITALIAN diesel in a big water plow(2) a chrysler big block will last 20+ years with ease in my application, is carbureted, has serviceable ignition, etc. will that scary little Italian thing give the service life, of an old, rock solid design like an RB chrysler?

    My boat is 318 power. My pickup is a 74 dodge. My semi has a 3406e. And I ride Honda 3 wheelers..... as tempting as it is, to buy a used twelve thousand dollar diesel, and roll the dice, I am worried that this new “tech” would leave me hating it somewhere offshore a few years from now, with no box of parts to fix it... anyone have good solid experience with these things?
     
  2. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    It's not just the engine, it is the transmission and maybe the prop also as well as the engine beds. The transmission is what links the engine's torque delivered @ rpm curve to the prop's torque absorbed @ rpm curve. Installing an injected 4-cyl turbo diesel which delivers 295hp in place of the V-8 318 small block marine which delivers ~225hp should improve performance as long as the transmission is changed to match the diesel's flatter torque curve. The big gain in fuel economy is coming from the injection vice the carburetor so while the 413 would be a performance improvement (also note that the engine mounts on the big-block are different from the small block), fuel economy won't improve much. On the other hand, you could see if someone marinizes the injected 5.2l/5.9l LA engines which also would give up to 250 hp or the 5.7l hemi "Eagle"/"Hellcat"/"Hellcrate" crate engine (360-707 hp) all of which should be a bolt in as well as having similar torque curve shape as the 318.
     
  3. crankshaft
    Joined: Apr 2011
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    Location: Big Lake, Alaska

    crankshaft Junior Member

    Theoretically the little fiat has the same operating power band as the 318, or any gas v8, just more torque at a lower rpm. He is running his Bayliner at the exact same rpm as the 454. (3200) and getting the same speeds. I should be able to use the same gear ratio, and pitch the prop to compensate. My fear isn’t if it will work, or gearing it accordingly, as much as it being reliable in the long term. Plus, my uniflite really doesn’t have a “plane” speed. Literally, the more power you give it, the faster it goes, at no point between displacement and top speed of 20kts, are you able to trim it, or break free and back off some. The single 318 is able to cruise 15-16kts, at 3600 rpm, at the point right before the secondaries open on the little AFB. That is asking a lot from a motor I built out of the best used junk, from 3 different truck engines. My theory, is the 413 would do the same amount of work, at a lower (2800rpm). Fuel economy really isn’t as important, as, are these little fiat diesels living, in fairly heavy semi displacement boats?
     
  4. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    While it may be true that at 75%-80% throttle the diesel and the 454 perform the same, diesel engines can be damaged if they are run too lightly loaded. This is why you may need to change gearing (or prop), to load up the engine at lower rpm where the torque curve is higher. While a carbureted engine will use vacuum to pull only the necessary fuel to match the load, an injected engines need to have the rack or EFI tuned to prevent cylinder temp or wet stack issues. FWIW, most diesels have a operational rating (service rating B or C) closer to max output (i.e. rpm) than automotive derivative gasoline engines, so I don't think longevity is a real issue as long as the engine is properly loaded to prevent piston burn.
     
  5. crankshaft
    Joined: Apr 2011
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    Location: Big Lake, Alaska

    crankshaft Junior Member

    I am thinking, the 27’ uniflite would load up a diesel just fine. It is a prime example of a water brake dyno. I have yet to find anyone who has put a little fiat (it is a marinized car engine) in a semi displacement hull.
     
    kapnD likes this.
  6. 7228sedan
    Joined: Nov 2009
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    Location: New Jersey USA

    7228sedan Senior Member

    I have a 1972 28 Ft Luhrs Sedan bridge with a single 454 & Velvet V drive 1.5:1. Very similar to your Uniflite, semi displacement, wide beam, was originally powered by a 318/360 etc. I re-powered last year with a reman 454 after wanting a 6BT as well. The height of the inline diesel made it prohibitive without a raised hatch. Even with my 72C transmission, it was questionable if the torque of the diesel was too much for the transmission. Not sure why, but marine gears are rated at a gas limit and a diesel limit. There are a few threads with discussions on that subject in the forum. There is a shop that upgrades the 72C with additional clutches to handle the diesel torque. With the 454, I cruise at 2850-3100 RPM 13-15 knots dependent on wind, current, weather etc. She burns 10 GPH at cruise.
     
  7. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    The transmission ratings are based on the assumption that the diesel generates power at a lower RPM. Therefore there is more torque input.
     
  8. crankshaft
    Joined: Apr 2011
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    Location: Big Lake, Alaska

    crankshaft Junior Member

    The transmission issue isn’t a big deal with this little fiat. The rpm range is a lot higher than a Cummins, so torque, is lower. I’m worried about longevity. Big Chrysler’s are known for longevity. I cant find anyone who has ran the same little fiat for ten years plus.
     

  9. crankshaft
    Joined: Apr 2011
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    Location: Big Lake, Alaska

    crankshaft Junior Member

    What I am wanting out of my uniflite, you have. 1.3nmpg is not bad at all. I will also be building my 413 to take advantage of the rod ratio and big lifters, having a custom cam ground (narrow LCL, etc) for low end grunt, to try and keep my cruise under 3200.
     
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