can't remember if i asked this before, and if i did, i can't remember getting the answer...

if my pressure cap on a header tank is not the highest point of the system, what will be the effects?

all i can think of is you might get a tiny air bubble in the top of the radiator and engine when the car's not running, but when its all up to pressure and stuff, that air should still be forced into the header tank, completely filling the engine and radiator with coolant. this would be helped by the water pump sucking some fluid out of the bottom of the header also.

i've got a air bleed on my thermostat housing and on the radiator also. i would intend to T them together to go to the top of the header

i can't get my head round why it would be a bad idea to not have the header mounted at the top

sorry if its a stupid question, but i just can't understand it lol

i tried drawing a diagram, but for some reason the photo archive ain't playing...

[Edited on 26/2/10 by blakep82]

if its not the highest point i would have thought you would have issues with gas expanding in the wrong places???

as far as i can tell, it would just expand into the header tank. i suppose the only other issue might be all settle and fill the header tank with water, leaving spaces in the engine and radiator, but again, once started should sort itself out i'd have thought

quote:

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Originally posted by blakep82
as far as i can tell, it would just expand into the header tank. i suppose the only other issue might be all settle and fill the header tank with water, leaving spaces in the engine and radiator, but again, once started should sort itself out i'd have thought

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i see what you mean, but if you think about your heater being above the header tank, the header is not the highest point, and so its a simialr set up to what i'm thinking isn't it? sort of? lol

i may have to get a custom, very tall header made then the only bracket (or place to fit it) i've got is about half the way up the radiator.

suppose i had a header tank, with a very long neck on it so the cap sits at the same level as the radiator. that could do it i guess, but i really want one very cheap

You will get airlocks in the higher points on the system, which will result in poor coolant circulation, coolant loss, and overheating.

I have a smallish pipe (8mm i/d) from the highest point (thermostat housing) to the BOTTOM of my expansion / header tank.

As I see it, air collects at the highest point and when heated expands along the pipe and bubbles into the half filled tank.

On cooling only water can be sucked back into the engine.

The only issue is filling the system from scratch, but if I squeeze the top rad hose this forces air into the tank, and releasing it
sucks water back. Repeat ten or so times and all the air is gone.

This works for me.

Cheers
Paul G

The header tank dosen't need to be at the highest point but it should be directly connected to the highest point --- ie air gets forced into the header tank by expansion but if the pipe is arranged so only coolant can drawn back on cooling all will be well.

In short the vent pipe from the highest point should enter the coolant tank at bottom of the tank.

[Edited on 26/2/10 by britishtrident]

so it CAN be done as long as the system looks a bit like this

Description

good times. i'll try and get some photos of the problem

the above pic won't work because gravity will simply fill the header tank and the airspace will move to the top of the system, i..e rad and engine............

its much better/easier if you get the header to the top of the system......how tricky can it be?

I've done it in an autograss car where the top of the radiator is only an inch below the roof of the car! Ok there's a hole in the roof to access the pressure cap, but hey!

its very tricky, because i've got no brackets on the chassis, and nowhere on the chassis to put a bracket higher than the radiator, because the radiator is right up to the bonnet as it is, and i'm not putting a hole in the bonnet for it. the only place i've got left is half way up the radiator. i could put a long neck on top of the tank to put it just above the radiator, but with the neck being about 30cm higher than the tank and bracket, i can see it breaking from fatigue and vibration

the air moving to the top, as it drains to the header tank, where will this air come from?

i don't know why no one can agree on an answer to this lol

Crap pic, but this is what I have.

Paul G
Rescued attachment water pipe.jpg

On the car
Rescued attachment IMGP1484-s.jpg

Methinks the pipe from the rad overflow looks a bit close to the timing belt Paul?

quote:

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Originally posted by rusty nuts
Methinks the pipe from the rad overflow looks a bit close to the timing belt Paul?

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quote:

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Originally posted by blakep82
i don't know why no one can agree on an answer to this lol

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The whole point of the header tank being higher than the rest of the cooling system is to collect air. An expansion tank does NOT do this.

Old cars had expansion tanks but the modern and much better way is to have a proper header tank which is pressurised (unlike an expansion tank) above the engine and with a couple of returns to the lower coolant pipes.

If you allow air into your coolant system then you are asking for trouble. Air pockets can form around the cylinders causing uneven cooling amd increased localised bore wear or failure. Cavitation in the water pump and poor head cooling also spring to mind. People often run swirl pots to help get rid of air in the system as it is such an important point with any engine let alone one that gets a caning!

While this is not the only way and an expansion vessel will do, IMHO it is by far better to run the later system with a proper header tank

[Edited on 28/2/10 by Strontium Dog]