How long do temperatures have to stay at or slightly below the freezing point, in a daily temperature cycle, before water would freeze in a block?
Not long and not much below freezing. I lost a Fronty head when the temp drop to 30 degrees for a few hours a couple of years back. Granted the cast iron wasn't as thick as a Ford block but that was a $900.00 repair. Drain the radiator or add antifreeze. Why chance it!!!!!!!!
That's not too easy to answer without getting into thermal dynamics of materials. It depends on the temperature variation throughout the day and whether the car is subjected to other heat loss factors such as wind or humidity. Cast iron retains heat for a long time. If the high was 60 degrees, the water temperature may not dip below 40 all night at 30. It's different if the high was only 33 degrees.
Put a drop light with a 60-75 watt incandescent bulb (if you can find one) under the hood and put a blanket over the hood and radiator until you can get antifreeze into it.
Cast iron does not like hard water! (ice).
I do plan to remove water and replace with anti-freeze for winter storage as I always do. But I was looking at the 30 day weather outlook and saw a few days with highs in the mid-50's and lows around 30degF. It got me thinking what sort of bandwidth the water temperature remains in on those days when the car is inside of an uninsulated garage, shielded from the sun and wind.
There is no doubt some thermal lag due to the mass of the cast iron, but what the parameters are, I'm not sure.
My experience is somewhat different than Mike's. We often get to 29-30 deg for a few hours here in North Florida for a few hours at night (typically in Jan - Feb). For a while I had radiator leaks and used straight water until I got the leaks fixed. I checked in the mornings by draining a little fluid from the radiator petcock, nothing ever froze. Car was in uninsulated garage - but well sealed.
But that being said, it was silly to take a chance. If I run into similar problems again, I'll put a hundred watt bulb under the oil pan and not worry!
Ever hear of icebergs? Ice is at least 10% less dense than water, so it floats. Your rad would have to be froze solid to freeze up at the petcock.
That reduced density is also why ice cracks and breaks stuff in a confined area. It expands with great strength. You would be better to look in the top of the rad.
Hmmm - good point Ricks!
"the car is inside of an uninsulated garage, shielded from the sun and wind."
Here in Canada I've found that overnight in the winter it is typically about 10ºF warmer inside an uninsulated, unheated garage than outside. YMMV
I'm in Minnesota and my experience is similar to Ken Todd's.
I believe that the concrete slab of an uninsulated garage radiates some heat from the ground (or any heat absorbed during the day when the garage is heated by the sun). The garage structure itself holds the heat in.
The inside of my garage will stay above freezing well into November, even if the daytime or nighttime temperature is below freezing.
For the rest of the winter, the inside of the garage is typically warmer than the outside, day or night.
Also - I don't use antifreeze in my Model T. I drain the water in the fall, as was done 95 years ago. No big deal. If I ever rebuild the motor and buy a brand new radiator, I will switch to antifreeze.
Some of my brass era car buddies in Minnesota run water during the touring season. When they drain the cars in the fall, they fill them with antifreeze and drain them again. With many early cars, cooling system leaks are unavoidable and some opinions are that it is better to run them with water and leave the cooling system empty during the winter. Flushing with antifreeze eliminates any pockets of water in the system.
How much fluid remains in the block after opening the petcock to drain the system? I used to live in Wisconsin and I also drained the system via the petcock but I still found cracks in the block. If you live in cold country, make sure you have a good mixture of antifreeze in the block before you drain the system.