radiator overflow bottle mounting

[maytag]

I have a hot-rodders' style stainless overflow bottle that's been lingering around on my shelf for decades. I'd like to use it on my car, but I have a question. In a closed-loop cooling system like ours, is it necessary for the "coolant loss" tube level to be higher than the highest part of the cooling system? IE: does it need to be mounted ABOVE the hose going to the head? Or can it be mounted at any height?
8th grade science class tells me it needs to be higher, of the coolant will gravitate out of the loss tube.

[mgbgtn]

It shouldn't make a difference as it is the vacuum caused by the cooling of the rad fluid that pulls it back into the rad. The volume of the fluid increases as it warms up forcing it into the resevoir and decreases as it cools.

[Daniel]

from my understanding the overflow was suppose to be at least the same level as the return on the rad .

[RRoller123]

deleted my post

[maytag]

ok... I'm rethinking my previous thinking....
look at this photo:


as I look at this, it seems pretty clear to me that whatever comes OUT of the radiator under expansion will NEVER go back in. this is a simple overflow / catch basin.
Hear me out: that cap is set to lift off the seat in the neck only under pressure. Some are (atmosphere plus) 12lbs, some are 15lbs, etc. The bottle is only at atmosphere. So as the pressure drops in the radiator, that cap closes back down onto the seat, and seals up well before it gets to atmospheric. So there is never a vacuum applied to the coolant that has made its' way to the overflow.

does that make sense? what am I missing?

[RRoller123]

The caps are supposed to have a one-way valve to allow fluid to get pulled back in.

[htchevyii]

Isn't that where the little flapper thing in the very center comes it? I think that vacuum pulls it open.

[maytag]

Rroller & HtChevy, y'all are both correct. After doing a little research, I've discovered what everybody else probably already knew: Not all radiator caps are created equal!

Teh one I have had on the car since I purchased it is a one-way only. What is required is a 2-way. I'll be finding me one of those.
So with that in mind, I'm still not sure it matters what height it is mounted at.

[RRoller123]

I don't think it matters much (the height) IF everything is working correctly. In the closed system, it is atmospheric pressure that pushes that fluid back into the radiator from the bottle when the rad vacuum is created, so the bottle can be above or below the rad top level, within reason. The column pressure of the fluid is negligible. But do a thought experiment here. Lets say the bottle was 50 feet higher than the rad top. The expansion presure might not get the fluid up to that height, and the column pressure might force fluid back. If the bottle is far lower, there could be siphoning problems if a small air leak developed (depends of course where the leak occurs), and all the rad fluid could overflow into the bottle and spill over onto the ground through the open cap. A leak in the heater hose or TPH might allow something like this to happen. So my take would be that the bottle is best set a little bit higher than the rad top to be safe? Granted it has been 37 years since my Fluid Mechanics course, so I eagerly await being corrected. <{:^)

[azruss]

you have the correct cap. the very inner rubber seal is the return valve seal. the middle one is your pressure relief seal. the outer just seals the entire system. Looking at the condition of your 3 rubber pieces, you are due for a new cap. As far as overflow tank location, there isnt enough space under the hood to vary the height by more than a few inches. 27" of water column is 1 psi, so you can see changing the height of the tank a few inches amounts to a change of a fraction of psi. As stated before, unless you are planning on puting the tank under the car or taped to your antenna, it shouldnt make any difference.

[RRoller123]

How about what this fellow did?