Expansion Tank Size

[Mad Scientist 106]

So it looks like I might need a custom expansion tank, as mine needs relocating. The question is, does it need to be 2 litres like the oe tank? I do have a radiator with a higher capacity but it seems excessive.

 

I have to admit, I blew one at oulton because I had overfilled the system, but since then it has been fine. By my recollection, it was about a quarter full when cold, rising to about half full at temperature.

 

Any thoughts?

 

 

Thanks,

 

Pete

[Henry Yorke 269 3 Cars]

I'm guessing the larger the tank then the more expansion you have as the air will compress easier than the water. The pressure you run would be governed by the expansion tank cap. I'm not sure what the ultimate water pressure would be for running your engine, if there is even such a thing

[Cameron 16]

I'd imagine the standard tank is large because car owners won't want to check and top-up the coolant very often. I guess a thing to do would be to work out how much expansion volume there is in the standard tank (fill to Max, fill to brim, measure difference) and then make sure your new tank has enough volume to account for that plus a reasonable head of coolant.

[Mad Scientist 106]

That sounds sensible Cam, but there are no max and min marks in my tank. Any ideas on that?

 

My system takes approx 7 litres, so I've worked out with 50:50 coolant it will expand .5 litres at 100 degrees, roughly. I guess I want half a litre of coolant in the tank, plus half a litre for it to expand, plus a suitable air volume.

 

 

[welshpug 1,655]

The cap is rated at 1.4 bar, can you work out the volume of air required from that?

[Mad Scientist 106]

Maybe, cheers Mei.

 

What's the gti6 cap rated at, do you know?

 

[Henry Yorke 269 3 Cars]

I think they are all the same. I know I put a standard 306 one on mine as the original metal 205 ones is not available

[welshpug 1,655]

most pugs are 1.4 bar, that or 1 bar.

[Mad Scientist 106]

So after a bit of revision of Boyle's law and the ideal gas equation, I've worked it out.

 

Guess what, it needs to be 2.1litres with my coolant volume!

 

The oe tank won't fit, so I'm going to get ah fabrications to make me a nice shiny one to my spec.

 

 

[Cameron 16]

Wow, that's surprisingly large! What coolant volume are you running?

[Mad Scientist 106]

It's 7 litres. It could be smaller if I ran less coolant in the tank, but I had some problems with it pulling air through on tight corners. Didn't matter too much, but would like to avoid any potential hot spots whilst I'm bouncing off the limiter at 8500rpm.

Just had a look under the bonnet of my BMW and the tank is HUGE!

[allanallen 528 2 Cars]

Didn't taylorspug (think that's his name) run with no expansion bottle on his car?? Just a filler cap in the top rad hose from memory.

 

Another thought was could you perhaps fit a 106 rad with a built in expansion bottle?

Edited May 19, 2012 by allanallen

[Mad Scientist 106]

The way coolant expands you really need either an expansion tank, or a bleed off type tank. In theory I guess you could get away with it, but there would be lots of pressure on your hoses and head gasket.

I've been running an alloy BBM radiator for a while now, so the built in tank isn't an option.

 

There's plenty of room for the tank I've drawn up because my battery is in the car. Plus, it's more shiny stuff!!

[petert 585]

It's a header tank, not just an expansion tank. Mi16's also have a header tank (integral with the radiator), although most people don't realise it. I've fabricated new thinner, aluminium header tanks using these:

 

http://www.mscn.com....p?idproduct=408

 

I've been looking for a picture but can't find one. The owner of this car wanted to keep the larger battery. So there wasn't enough room for the standard header tank and the brace mounted PAS tank. So I made one approx. 60mm wide which fitted in on the original mounts.

Edited May 20, 2012 by petert