coolant overflow logic
 

I am workinon my track car and I just want to clarify my logic with the cooland overflow tank.

Lets omit the ast for now to simplify the logic and just pretend we have a radiator and an overflow tank.

So the way it works is pretty simple, when the coolant expands (heats up) it 'pushes' coolant into the overflow tank, when the coolant contracts (cools down) it 'pulls' coolant back in the sytem.

Now let me get to my question, it sems to me that in order for this to work the overflow tank is designed so that hose (or a tube from where the hose connects to) goes all teh way to the bottom of the tank so that it pulls coolant back in and not air.

My track car had a custom metal overflow tank that had a hose nipple on the top of it. However, there was nothing that went from the nipple down to the bottom of the tank. When I play it out in my head it seems that it would dump coolant in there fine but would not pull back coolant since the coolant is at teh bottom of the tank and there is nothing from the nipple to the bottom of the tank, so it just pulls air back in.

on a realted note, i thought the overflow needed to be vented too, and this tank is not. seems like you need it to be vented in case the overflow neeeds to 'overflow' and for to allow air to replace the coolant that gets sucked back in the system when it cools (contracts).

am I way off with my thoughs here?

u can't drink from a glass when the straw is above the pop.

Sounds like the metal overflow tank was installed upside down, honestly. You can vent it if you want, although it probably won't make that much of a difference. But yes, flip it over.

http://homepage.mac.com/foko1/.Pictu...C4C24C11D7.jpg

http://homepage.mac.com/foko1/.Pictu...CAC24C11D7.jpg

Pictures of foko's (now owned by damian) original setup.

As you can see, the lines are run in a very simple intuitive fashion (from a line routing perspective) - but that tank should be flipped over.

thanks for the reaplies guys, and for verifying my concern clayne...

my though was that is was connected to the top but had an internal 'straw' to the bottom of the tank....but i took it apart to check...nope.

I will now run the line to the bottom and use a 'vented' cap on the top.

>>You can vent it if you want, although it probably won't make that much of a difference. But yes, flip it over.

but I though you would need it vented so that air can replace the coolant that gets sucked back in the system, otherwise it wil not be able to pull coolant in? no?

Scientifically, yes.

Pragmatically, I'm sure the coolant arriving into the tank will compress the small amount of air inside anyways.

agreed....

do you know if the stock overflow tank is vented?

Yes it is.

The stock overflow is of such crap design that they couldn't make it sealed if they wanted to.

Just overfill it sometime and you'll see how vented it is. ;)

That plastic pop on cap with the dip stick can't be air tight, thus it's vented.

Raj

ohh believe me i know, I have had it dump all over before when i overfilled it ..... i guess if i would have remembered that i would have answered my own question about it being vented LOL (duh....hehehhe)

good point raj

does anyone still make an alumiunum overflow tank?

I believe Summit Racing has a few. They aren't direct drop-in replacements but shouldn't be that bad if it's something you really want.

On this subject something is not correct have you noticed the pipe that goes to the overflow tank also goes to the AST top nipple (when studying the AST i cant understand how it works,,, i mean when the water expands due to heat and the pressure cap spring with the rubber seal will be pushed back and allow water to flow into the overflow tank,,,,but when it cools how will it suck water back in ,,, the rubber seal will surely prevent this can someone please explain)

good question, I guess it gets sucked backed in as the spring is slowly de-pressurized

anyway, I lose about a half-cup to a cup of coolant every time I take the car out (coolant is not being sucked back into the system), and I believe a faulty AST cap is the culprit (bad seal/spring) so I just replaced it...I'll let you know if that fixed the problem

also I believe the cap on the filler neck has to be the same or higher pressure than the AST cap, or the same thing will happen

If you are still using the AST, the filler neck cap should be a total seal cap, not a spring-loaded pressure cap.

Take a closer look at the pressure cap (ast cap), There is a small metal disk on the pressure seal that you can lift with your finger nail. That is a oneway valve that will let coolant back in.

ha haaa right i see,,,,,

ok, that's news to me...I'll try it

There's a difference between "recovery tank" and catch tank. For it to be a recovery tank the hose has to go to the bottom to be able to draw coolant back into the system when it cools. If the hose termenates at the top of the tank then it will just accept the excess coolant when hot and won't drw back anything but air when it cools

#1 The hose should enter the overflow tank at the bottom. This is preferable to attaching the hose at the top and then installing some sort of pickup inside the tank. With the hose at the bottom the weight of the coolant itself will ensure coolant is easily pulled back into the system as it cools. It also ensures the coolant level doesn't fall below the level of the pickup so air cannot be sucked into the system.

#2 The overflow tank must be vented. Contrary to what clayne says you must vent the overflow. If the overflow isn't vented the air inside of it must be compressed just to allow the hot expanding coolant to enter the tank in the first place. You now have the pressurized tank fighting against the caps and the coolant. It doesn't work this way. The overflow tank is not a pressurized part of the coolant system. Vent it.