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Discussion Starter · #1 ·
From Gary (German Auto Solutions). It's a thing of beauty. The top is engraved steel and the relief valve is "Reutter" - the OEM supplier for the OEM expansion cap. The beauty is that "if" the temp is getting too high, it will release the pressure much sooner than the OEM cap, thus having a chance for the plastic parts to last longer. I would venture to affirm that if the cooling parts will rupture due to pressure, at a lower pressure the damage could be smaller, i.e. the crack could be just that: a smaller crack, which could allow you save the engine. At higher pressure, a crack will develop very fast in a big rupture, where one could loose cooolant very quickly.

Thanks Gary. Beautiful product. :thumbup:



 

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Discussion Starter · #7 ·
The consensus over at Bimmerforums is that lower pressure cap is bad and will result in engine failure due to overheating. Are you not concerned about that?
No.
Because the 1.2 bar cap holds the coolant pressure up to 126°C or 260°F. If you ever reach that temperature, chances are your engine has or starts to overheat. And if you reach that temperature, something else is wrong, not the expansion tank cap.
For example, I know 100% that if I see the KTMP going past 98°C, one culprit could be a bad fan clutch. And with a bad fan clutch I noticed KTMP as high as 102°C. Which is still below 126°C. I adressed the issue, the KTMP stays where it should be now. That's for i6. For v8 the normal operating temp is higher.
So, yes, I think it's pretty safe. Thanks for pointing it out though. One can never be too careful. :AF330i:
 

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This is a nice thread to cross reference with the following (found by typing /cap F3 in the best links):
- All about modifying the cooling system pressure cap (1)


EDIT:
noticed KTMP as high as 102°C. Which is still below 126°C.
I know Doru is aware but for others to reference, normal KTMP fluctuations are discussed, in detail, in this thread (found by typing /thermostat F3 in the bestlinks):
- What is the temperature of the coolant & when the thermostat opens under normal conditions (1)
 

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Alex, I did not say I was part of the consensus ;) and yes, I still run a 1.4 bar cap. going on 3 yrs now. I have not noticed issues with losing coolant.
Oh, that's good, at least we're on the same page. I don't see how a lower pressure cap is more vulnerable to over heat, to my knowledge it would help the situation. Thanks for the clarification buddy.
 

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I also ordered one of the 1.2 bar caps from German Auto and can't wait to get it in my car. But, remembering some of my college chemistry, I don't think it will result in increased overheating. It would probably increase the likelihood of overboiling, i.e. coolant out of the radiator cap due to the lower pressure. But, the coolant would still have to reach a certain temperature before that could happen. In this case, the 1.2 bar cap is still above 1.0 bar or sea level pressure and would work (like the 2.0 bar cap) to prevent overboiling.

Also, the point of antifreeze/coolant is to both lower the freezing temperature and increase the boiling temperature. It just seems like the new 1.2 bar cap will work. Still, there must be a reason BMW is using a 2.0 bar cap.

I will update my experience once I have replaced my current cap with the one from German Auto.
 

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Discussion Starter · #12 · (Edited)
I think BMW did a mistake with the 2 bar expansion cap, and fixed it on the newer BMW models to 1 bar expansion caps to prabably avoid the catastrophic cooling system failures (ruptured radiator necks, split expansion tanks to name a couple). Pretty much for the same engine (N52 vs M54 - not much different).

Edit: The above statement is wrong. The source I quoted was off.
 

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Doru, product review incompleted without link and price ;)
 

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Discussion Starter · #15 ·
If you choose the lower pressure cap, make sure the expansion tank is topped to the LOWER LIMITS.
Quote from the "Fanatics", real-life test:

Originally Posted by kmulder
Hello everyone,

We've done extensive testing on this subject, and wanted to share some of the results. For testing, we used a 1997 E39 at idle in a garage. The main radiator fan was removed, and the auxiliary fan blocked off in order to force the system into higher temperatures and pressures. We first tried with only the main fan removed, but the auxiliary fan was successful in preventing the temperature from rising above 103 degrees C. It would turn on at 103 C and lower the system temperature to 95 C at which point the fan shut off again. We monitored coolant temperature both through the on-board diagnostics and through external software. Each test began with everything at ambient temperature. The expansion tank was tapped and a low-pressure gauge installed to monitor the pressure in the expansion tank.

Results:

We saw a maximum temperature of 126 degrees C (~260 F) before ending the test. We were unable to increase the temperature above this due to the efficiency of the system even without airflow. Keep in mind that a 50/50 mix of coolant will boil at approximately 135 C (275 F), so we were very close. Pressure at this temperature (126 C) was 20.5 PSI. The temp gauge needle was in the red zone well before this and the coolant temp warning light was on. Pressure never increased above 15 PSI before a temperature of 120 degrees C. We ran the test several times and achieved virtually identical results each time. We also tested the system with purposefully low coolant to see how that would affect the results. At a given coolant temperature, the pressure is lower in a system with low coolant.

During normal operating conditions, the system never operates anywhere near 2 bar. Assuming normal operating temps of 105 C (a bit on the high end), the system won't see pressures above 10 PSI.
 

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Discussion Starter · #17 ·
I'm struggling with understanding all this.

Is it two things we should do?

One, keep the coolant lower (within limits) to keep the pressure lower?
Two, use a lower pressure cap so that it will vent before the radiator blows?
I think the 2 bar cap was designed to be "foolproof", as in some people overfill the cooling system (the expansion tank gauge at cold sits high). In this case, there is little room for expansion. If the coolant temp is too high, you would develop higher pressure (this leads to the much documented expansion tank cracks).
The high pressure ET cap is a good thing if you track the car, where you have high rpm's in lower gears steady. My car is a DD, and very seldom sees 4-5k rpm's, so the pressure-cooker ET is useless to me. On contraire, it's a ticking bomb.

Quote from the "Fanatics":

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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people here dont understand the cap doesnt cause pressure, the heat does. heat is produced and maintained by thermostat opening and closing to route water to the radiator. rad cap is simply a safety device to allow pressure to escape if necessary. venting in a car ejects precious coolant and introduces air into the system; this is NOT the solution to lower pressure. lowering temperature is the ONLY way to reduce pressure of a ethylene glycol/water mixture. this total failure of people's understanding may be especially bad for the 540is(as they run hotter) and hit that 1.2 bars sooner then the i6 guys.
 

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Has anyone actually had a pressure gauge hooked up to the cooling system in the following conditions:

Idling when cold

Idling at operating temperature

at 2500 rpm with fan blowing air through rad to simulate highway cruising

I'd be curious to see all the pressure readings under these conditions.
And another thing that hasn't been clarified, is the 2 bars of pressure gauge or atmospheric? I assume it's gauge since that's how we measure everything else (tires, fuel pressure, etc.).
 

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Discussion Starter · #20 ·
people here dont understand the cap doesnt cause pressure, the heat does. heat is produced and maintained by thermostat opening and closing to route water to the radiator. rad cap is simply a safety device to allow pressure to escape if necessary. venting in a car ejects precious coolant and introduces air into the system; this is NOT the solution to lower pressure. lowering temperature is the ONLY way to reduce pressure of a ethylene glycol/water mixture. this total failure of people's understanding may be especially bad for the 540is(as they run hotter) and hit that 1.2 bars sooner then the i6 guys.
Never said it reduces pressure in the cooling system.
 
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