10-04-2012, 11:03 PM
Join Date: May 2007
My Ride: 330i
Please pay attention to the bolded statements below:
Originally Posted by lcoleman
Well, I do agree with your facts even if I disagree with your conclusions. But, help me understand: why would a 1 bar cap help anything at all, even if the system does stay <1 bar under normal conditions? By your own logic, since neither a 1 bar or 2 bar cap would vent, nothing has changed overall. I just think that a 1 bar cap will vent, or BMW probably would've put a 1 bar cap on in the first place.
To further compound your thoughts on the radiator in high-demand situations, high rpms means more rpms at the pump, and thus more flow. But, higher rpms over a sustained period almost always means more air flow through the radiator, and thus more cooling/delta T/whatever (and lower system pressure, lower thermal capacity of the coolant, etc). Not particularly relevant with regard to cap pressure, but that's what the thermostat is for--to hold temperature as close to constant as possible to prevent venting from overtemperature or inefficiency from undertemperature.
Originally Posted by SeanC
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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