Running a Fan while Engine is Off

Discussion in 'General Tech' started by mcglsr2, Mar 18, 2015.

  1. mcglsr2

    mcglsr2 Well-Known Member SN95 Supporter

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    I've got a question, been thinking about this. There are folks (specifically drag racers, but some corner-carvers too) that install a fan override switch that works even if the car is off.

    Here's my question: if the car is off, what's the point of running the fan? The engine runs the water pump, which circulates the coolant/water. So, with the engine running, I can see the point of the fan running, especially if the car is parked in the paddock/lot waiting for the next run.

    But if I turn the car off, and I leave the fan on - the water is not circulating. So the fan is running through coolant/water that's just sitting in the radiator. The water left in the engine block will still be hot. I suppose that if I let the fan run long enough (hours upon hours?) it will eventually cool all the water in the system.

    I would think, however, that leaving the engine running for a bit *and* the fan on will actually do more then if the car were off (it will also help to cool the oil). UNLESS: if one has an electric water pump that runs after the engine is turned off in conjunction with a fan that also runs, then that totally makes more sense, and is probably better than leaving the engine running. However, 99% of the times I see this, peeps are running a crank-driven water pump.

    So - do I have it wrong? Is my theory dashed upon the rocks of reality? (FWIW, I do have a fan override switch - but it only works when the car is on, for the reason I discussed above).
     
  2. ttocs

    ttocs Legend

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    I was going to say that you can always do the electric water pump. I probably would not have done it if I didn't come cross such nice deal on a polished one a buddy was selling that I could hardly pass up.
     
  3. mcglsr2

    mcglsr2 Well-Known Member SN95 Supporter

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    Oh, I'm not interested in running an electric pump at this point. Just curious why people run their fans with the car off, and if it really helps :p
     
  4. ttocs

    ttocs Legend

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    the majority of the coolant is in the radiator so it may not cool the motor directly as much as the radiator and the coolant in it.
     
  5. mcglsr2

    mcglsr2 Well-Known Member SN95 Supporter

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    Hmmm, good point.
     
  6. evilcw311

    evilcw311 Most Evil Mod! Staff Member Staff

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    Most drag racers at da strip are running electric water pumps tho. That's why you'd see it then. If not then they are just wasting battery power


    This message courtesy of crapatalk!
     
  7. D3VST8R96GT

    D3VST8R96GT Well-Known Member

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    Cooler temps are cooler temps....
     
  8. mcglsr2

    mcglsr2 Well-Known Member SN95 Supporter

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    Ah, okay. Yah that makes perfect sense. Electric water pump, I totally get that.

    Agreed, but if you have a crank-driven water pump, you will pull those temps down faster I suspect by letting the car idle and circulate the water. Otherwise, you are letting the hot water sit in the block and heat it some more, while your fan works on cooling what's in the radiator. Sure, I suppose the next time you start the car, the water from the rad will be a little cooler. But then you also let the hot coolant heat soak into the block. So I dunno :dontknow:
     
  9. ttocs

    ttocs Legend

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    I don't think at idle it will cool itself better. While its not at the redline stage it is still building heat and would only get cold to a certain point, what 170-180 at best if its a fairly easy build? Now if your motor is already hot from a pass or two I would think it would take even longer to cool when idling again because the engine is building heat. Its true that it only truly cools the car when its circulating but again with the majority of the coolant in the rad, with the key off and fan on it would not only cool the rad but then don't forget the fans exhaust blows right on the block which would help a little. When you turn the key back on it would then start circulating coolant that is now cooler then it would have been if it was just at idle it would seem to me it would be better off.
     
  10. D3VST8R96GT

    D3VST8R96GT Well-Known Member

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    The whole idle to cool thing is questionable it will only get you as cool as the t stat will stay open.....
     
  11. mcglsr2

    mcglsr2 Well-Known Member SN95 Supporter

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    Good points all, thanks!
     
  12. D3VST8R96GT

    D3VST8R96GT Well-Known Member

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    The whole idle to cool idea may come from diesel and turbo applications where the oil needs to cool. Running the turbo hard makes it hot and oil staying in place (turning the car off) can burn or solidify. This is compounded in turbo diesels with heui systems that use high pressure oil pumps to fire the injector pulses and cool the turbo.
     
  13. hotmustang95

    hotmustang95 Active Member

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    I have my fan on a switch only because years ago my CCRM went bad and rather then spend near 300 bucks for another one, I had my buddy wire it up with a relay. On the highway I turn mine off. It's not needed when the car is in motion and going at least 50 mph. The amount of air passing through the radiator at those speeds is more then any fan get produce. Imo it saves the fans motor and prolongs it's life. At a track event one could use it to help cool the engine down some after a run but this is on rare occasions though. Most times at the end of a run you can cruise around the track for one more lap to allow everything to cool down before entering the pits. I think drag racers benefit more from having the fan on a switch compared to open track events. I haven't drag raced in a long time but when I did I do remember the average waiting time from run to run was a half hour. Having a fan on a switch would deff help cool the engine down in that small amount of time. Plus they use ice also. For open track events the waiting time from run to run is usually about 45 minutes or more. Plenty of time for an engine to cool down on it's own.
     
  14. cameron57

    cameron57 Active Member

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    What is this fabled t stat you speak of? Lol I never run them. My old car use to have mech pump and electric fans. After a pass you let it cool the water in the radiator for say 10-15mins then you start let it run for a bit and do it over again after second time its cooled way down.
     
  15. RichV

    RichV Well-Known Member

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    When the radiator cools to ambient, it still blows cooler air on the engine/exhaust getting the hot air out of the engine bay.

    I'm not sure it's too effective, specially if you put the hood up while parked.