So I don't know how many C6100's are still in lab use, but I just turned my 4 year old C6100 into my new homelab system a couple of weeks ago and needed to quieten it down a bit. It's an improvement over my older lab...
Anyway, to that end and thanks to this thread I purchased the Evercool EC8038HH12BP from Amazon and a set of the 4 pin to 5 pin adapters, also from Amazon; https://www.amazon.com/gp/product/B005G50C6Q/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1
I knew that the pinout was wrong but it didn't take long to figure out how to re-wire the 5-pin end so that it carried through the right signals. Basically you just make them "straight-through" adapters as the proper wiring is in the same order as PWM wiring on the fans.
All four are spinning, but only 3 of the 4 are reporting RPM's back. I am not 100% sure if it's the fan or the adapter cable that's wonky but I intend to do a bit of troubleshooting when I get some time (at the weekend). Anyway, these adapters gave me plenty of wire to reach the controller.
Now, I CAN report that my CPU's are running about 10C hotter than before on average, and the "MLB TEMP" are showing about a 15-20C hike... not great but certainly still well within limits of the components. With the PIC16 controller I can also report that the fan speeds are highly variable, which leads to much greater temperature variance than I was getting with the stock fans... again though I consider it acceptable for my lab use. Besides, I only have 2 of the 4 blades actually powered up, but even a full test with all four blades fired up made zero difference to the temperatures or fans on the utilized nodes.
As far as noise though? Wow, this made all the difference in the world. I now can't hear my server stack over the noise of the 10GBase-T switch they're attached to (Dell 8132 I flashed to an N4032). I measured the noise in front of the rack at the drives before the upgrade as around 60db... now I am getting a pretty good 48db but I suspect some of that's from the switch. I will measure it again with the switch off this weekend after I've troubleshot that dodgy fan.
For reference, my CPU's are dual E5530's, so not the coolest of the bunch... with each blade rocking 8 sticks of RAM (with all the baffles in place). Yeah, cooler CPU's might bring my temperatures down a bit but I'm not overly concerned about it.
Hope that helps anyone else still planning to do this mod... it's worth it, but anyone should also be aware that you might end up cooking your server... or reducing its lifespan. Since it's a "play system" for me anyway and is actually just extending my timeframe to buy a new homelab system anyway it's all good. If I kill it, I kill it