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Old 03-07-2013, 11:01 PM
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ztom ztom is offline
I gotta have more cowbell
Location: Thousand Oaks CA
Join Date: Sep 2007
Posts: 892
Mein Auto: 1997 528i
Originally Posted by doru View Post
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.


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.
This seems good information. If combustion gas isn't getting in, then the cap doesn't make a difference, it's only a pressure release if a problem. If combustion gas is getting in i.e. cracked head, then the pressure will go the limit regardless of engine or coolant temps, and then a lower bar cap may help by not overpressurizing the system and getting a burst.
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