Radiator Cap Pressure Help Please.

The rating of the cap depends on the temperature the engine should run at. As temperature increases so does the pressure. However, the curve is not linear. At 100°C the pressure of water will be around 1 bar (or 14.5 psi). Typically a modern petrol engine should run at between 85° and 95°C for optimum efficiency so there should be no need for a cap rated above 1 bar. I would suspect that a 7 psi (0.5 bar) cap will result in constant loss of coolant unless the engine is running too cool (in which case the cooling system is too efficient and preventing the engine from operating at its optimum temperature). If it is blowing coolant out of a cap rated at 1 bar it suggests that the engine may not have enough cooling capacity. If the engine is being over cooled it would be worth looking into changing the thermostat for one with a higher opening temperature.

In that case play safe and use a 1 bar (14.5 psi) cap. Better for the cap to release than to blow out the heater matrix. There really is no need to go any higher if the cooling system is working properly.

I would go with what the donor car came with and pay particular attention to your coolant hoses. With a higher pressure the coolant is less likely to boil over. If I am not mistaken are not the alloy BMW 6 pots known for being a headache to work with if they boil over?

Do you have a low coolant level warning system? If so, keep an eye on it. At 7 psi it's possible that all the coolant will get pushed out the overflow and you may not realise it until the damage is done.

Alot of modern header tanks have them built in though some (VAG that I known of) use some sort of conductivity device rather than a simple switch, so not quite sure how you "read" those?

You'd probably be ok to 13 psi, later Triumphs had those without any obvious system changes. When I fixed the Vitesse heater matrix I pressure tested it using mains water pressure which is just under 3 Bar here. It creaked a bit, but held!

Also quite a few Triumphs of that era use a double seal cap and unpressurised catch tank which allows water out when hot and then sucks it back in when cold. Vitesse and PI both have it - works ok.

Provided the rads big enough and it holds the right temperature it'll never be a problem anyway.

Cheers

Nick

And on thinking about it, the header tank did expel some water, but that was when I deliberately run the temp up to see if the rad fan would work.

But the water loss stopped of it's own accord, while the car was still hot enough to operate the fan and the level hasn't dropped any further, so I'm hoping that means the coolant system works OK, at least when the ambient temp is around 4c!

The girlfiend is hoping we can use the car in a warmer clime too!