Just did a quick calculation. Apparently 1 bar cap is good enough. Take it for what it's worth:
Assuming 96 degree Celcius coolant temperature, which is pretty much the max, the pressure of an ideal gas (e.g. air) is 1.3516 atms. That means you'd need a cap capable of holding 1.3516 bars at the minimum. This would be called a 0.3516 bar cap. This would, however, increase the boiling point of the coolant by a mere 8.5 degrees (Celcius):
So %100 water as your coolant would boil at 108.5 degrees Celcius instead of 100 degrees.
%50-%50 mix would boil at 114.5 degrees Celcius instead of 106.
These numbers are ok theoretically, but they are too close for comfort in real life. Locally, temperatures could be higher than these (e.g. engine block), and your coolant will turn into gas whenever this occurs. Since gas occupies more space than liquid, you'd be replacing your hoses quite often. I am assuming the engine block can take a beating.
On the other hand, 1 bar cap increases the boiling point by about 24 degrees Celcius. So for 100% water, you'll get 124 degrees Celcius as your new boiling point. For 50-50 mix, it will be 130 degrees. Similarly, 2 bar cap raises it by 48 degrees, so your new boiling points for 100% water and 50-50 mix will be 148 and 154 degrees, respectively.
Since the operating temperature of your engine doesn't change, using a 1 bar cap will lower your chances of blowing the head gasket. You'll probably see your car smoking before the temp gauge hits the maximum. But there is more chance of introducing excess air (gas) into the system, which could require you to use heavy duty hoses in lieu of what comes from the factory.
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