Will smaller diameter (seawtater inlet/outlet) oil cooler significantly affect flow to cool engine

Discussion in 'Inboards' started by Northeaster, Jul 3, 2018.

  1. Northeaster
    Joined: Sep 2011
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    Location: Eastern Canada

    Northeaster Senior Member

    Hi Folks,
    I had a failure of the oil cooler for my Velvet drive 71C 1:1 gearbox, attached to a Cummins 4BT.
    I have a spare oil cooler which is a bit shorter but also has approx 5/8" inlet / outlet for the seawater top pass through. The old one had 1" inlet/ outlet and the water left there to go on to the heat exchanger to cool the engine and then out the wet exhaust.
    Will using a small diameter oil cooler, before the heat exchanger reduce flow enough to cause inefficient cooling of the engine, or will the pressure just increase a bit?

    Also, the old oil cooler was placed after the raw water pump (pressurized side) but I see some diagrams have the oil cooler before the raw water pump on the suction side.
    Which would be preferred in this case?
    thanks for any help!
  2. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Short answer - yes it will reduce flow. Will pressure increase? Yes it will. How will it all shake out? Can't tell.

    What you need to do is get the pressure/flow curves for the old tranny cooler, the new tranny cooler, the engine cooler, and the raw water pump. You can then see what the change will be, and consulting the engine and tranny specs, decide whether or not the result is acceptable. Being in Canada, you may be able to cheat a little bit since you probably don't have 90+ degree raw water temps very often. Or you may decide that for the price saved, you don't mind changing the pump's impeller with religious fanaticism.
  3. Barry
    Joined: Mar 2002
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    Barry Senior Member

    I guess you could consider saving a few bucks and take a chance and risk a transmission and the engine failure due to inadequate cooling water volume.

    The 5/8 inch diameter offers 40% of the cross sectional area compared to the 1 inch hose. This is a serious restriction unless the raw water pump is an absolute positive displacement pump
    that will pump the required volume of water through the system independent of the restriction hence increased pressure that the 5/8 inch diameter hose fittings will cause

    Spend the money and buy a properly sized cooler or if money is the big thing, you could take the cooler to a rad shop and have them solder on 1 inch barbs. ( but note that on the oil coolers that we have had to repair, the barb came with a saddle whose radius matched the radius of the cooler to increase the area of solder contact between the barb and the cooler)
    Last edited: Jul 4, 2018
  4. Northeaster
    Joined: Sep 2011
    Posts: 265
    Likes: 8, Points: 18, Legacy Rep: 45
    Location: Eastern Canada

    Northeaster Senior Member

    I appreciate the help and advice. I have the old cooler in getting repaired, and will have it back within a week or so. In the meantime, I decided not to restrict the flow to the engine, but rather put a tee in the main line so that the oil cooler has flow, which then tees back to the main line. But some water also flows around it, straight to the engine heat exchanger. I am not running the engine at full speed, but did keep an eye on engine temperature and checked the oil cooler and gearbox which was only warm to the touch, after running for 20-30 minutes.
    I will run it cautiously until I get the larger oil cooler back in.
  5. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    Forget the tees idea, you are on an entirely wrong path.
    Look up a Cummins 4b diagram and specs of cooling system.
    Cummins has put serious thought and many research dollars into the design you will see, follow it closely for best results.

  6. Northeaster
    Joined: Sep 2011
    Posts: 265
    Likes: 8, Points: 18, Legacy Rep: 45
    Location: Eastern Canada

    Northeaster Senior Member

    Boat is out now for the winter. Lots of time to put the proper size system now. thanks
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