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In this tutorial I will discuss how to install the GTX 1080 Ti on your Mac Pro 4,1 (2009)
You need to get some of the power cables to power your GPU. In Mac Pro there are two 6 pin connectors, unfortunately those are mini 6 pins, so you need mini 6 pin to 6 pin cable. It is pretty easy to find one one Amazon and it is not too expansive. If your graphics card requires two 6 pin connectors then order two of those and that is pretty much it that you need to install it.
However, if your graphics card require the 8 pin connectors it can be little bit tricky. You can find a mini 6 pin to 8 pin cable, but some people argue that there is not going to enough power delivery to the GPU with the adaptor. So, I ended up with using two SATA Power connector to 8 pin connector. Although you have to cut some of plastic pieces from the cable, I found this is the most safe way to deliver the enough power to the GPU.
Plug the mini 6 pin connector into the board
and Plug two SATA Power cable into two of the power connector that you are not using. I used 3&4 bay and it is working fine.
I decided to keep my old GPU in different slot, because I can connect my monitor to my old GPU and I will use my GTX 1080 Ti only for the GPU Acceleration.
So this is how it is look like after upgrade. I have my New GTX 1080 Ti on slot 1 and slot 2 is empty for optimum air flow for 1080 Ti and slot 3 for Original GPU and slot 4 for pcie to SATA III adaptor with SSD.