Hello, I would like to join this thread to share my experience with Silenx fans that from now on replaced the original CPU Papst in my two MDDs (FW400 and FW800).
At first I tried to connect the Silenx to the same plug where the original fan was but I quickly discovered the rotation was so slow that I hardly could feel the air moving thru. Then I tested the same fan using the provided Molex-3pin adapter and the result was radically different, the spinning was fast, the air flow quite intense and the noise just noticeable.
To better investigate this behaviour I picked my multimeter and measured the voltage on the CPU Fan plug on the motherboard (the one in the bottom left corner looking the main board from top) and just after the power on I read 5 volts: after some minutes the voltage slowly start to increase but since the heatsink temperature was going crazy (I did this measure without the fan connected) I decided to stop there.
So the answer was very simple: the Silenx fans does their job properly only when feeded with full 12v, the blade design was done with this in mind. The standard Papst, instead, have a good airflow with just 5v applied and only on heavy load (like running emulators or compiling the MacPorts sources) it increase the RPM but also the noise.
To give some numbers about the temperature measured with the Silenx connected to the CPU fan plug (5-12v) and directly to a molex plug (12v) here are the results after one hour of heavy CPU load (FW800, dual G4 1.5 GHz)
Ambient temperature: 19°C
Silenx driven by logic board: 59.2 °C
Silenx driven by molex: 50.1 °C
I have no numbers with original Papst because my intention was to replace them anyway but the difference is there and I also can touch the copper heatsink without having my fingers burned, I mean, it is hot but not so much to hurt.
After some meditation about the way to follow since at first I would like to use the logic board plug I decided that leaving the Silenx rotating at full speed since the power on was the best choice: the CPUs warms very slowly and the top temperature was far away from the upper limit of 62°C that should be the value where the MDD turns off to protect itself. With the Silenx driven by the logic board the CPUs runs hot very quickly (because the poor airflow at 5v) and the fans run faster only to avoid the worst scenario but the temperature remains very high.
There are also other reasons to have lower temperatures and continuous and consistent air flow since the power on: RAM and GPU: they both are placed over the CPU heatsink and suffer from the heat radiated from below so if the CPU runs cooler also the RAM sticks and GPU card runs cooler (or at least are not more overheated by the CPU).
The silly design of MDD place RAM sticks and GPU just over the CPU heatsink so the GPU cooler ingest the hot air from the CPU: the cooler is the CPU heatsink, the cooler wil run RAM and GPU.
In the end I am happy with my Silenx and I consider them a very good upgrade to the MDD, even at full RPM the noise is very limited and the temperature dropped by about 9°C going from variable to fixed speed.
I know the most of MDD noise come from the two PSU fans and to be honest after the CPU fan replacement it was only a bit quieter but of course this will be the matter of the next (and maybe final) upgrade
