Mac Pro 5,1 PCI GPU power 6/8 pin confusion

Question

I have a Sapphire HD 7950 from eBay I got to go in a Mac Pro 5,1. It has dual 8-pin sockets. Being second hand it has no instructions/data sheet.

The one I already own [different Mac Pro] has dual 6-pin sockets, which was the perfect fit for the motherboard's 6-pin PCI power. Plug & go.

Google gives far too many opinions over far too many models for me to know what the solution is. There seems to be cable adapters for dual 6-pin to single 8-pin, which really doesn't seem optimal; single 6-pin to dual 6 & 6+2, which sounds a bit optimistic.

Simplest start point… can I use my two existing 6-pin cables in the 8-pin sockets, or will something go boom?

I can't seem to find a single 6 to 8-pin cable, of which I could buy two.
In terms of power requirements/stress, this Mac is never going to be used for anything more than video playback. I just want to get the Mac to run Mojave, rather than be stuck on High Sierra [Metal2 requirement].

Answer 1

The 2 pins more in the in the 8 pin plug are 1x ground and Sense1. Sense1 is only to inform the card whether a 6 or 8 pin plug is plugged in.

I think the big difference in your usage is that 8-pin plugs are able to pull 150 W, where 6-pin plugs only provide 75 W at max.

As far as I see the Sapphire HD 7950 needs 80 W at idle.

So I see 3 Possibilities:

1. Buy a 6 to 8 Pin Cable and risk your power supply (amazon link) (worst idea)
2. Use the card with only 150 W max (75W from PCI Power Supply + 75 W directly of the mainboard)
3. Get another Power Supply

Answer 2

Well, in the end it came down to "what works"…

Using two 6-pin cables, Apple standard as supplied with the machine from new, it simply refused to register to the PCI bus at all. It would light & the fans would spin, the machine would start up but register nothing in the PCI at all. I could test this by remoting into it with Apple RDC.

I got a couple of 6 to 8-pin cable converters, 4 quid[bucks/euros] for three with next day delivery on Amazon Prime [they were cheaper than buying just two] & it immediately works as advertised, right from the boot screen.

Slight update - well, for some reason it would hang the first Mac after a few hours, so I moved the card to my other. Now works fine. Older 6-pin 7950 moved to the first Mac.
I wonder if there's a difference in the PSU spec. Crashy Mac is a 'real' 5,1 3.33GHz 6-core. Stable Mac is a 4,1 with firmware upgrade, 3.46GHz 12-core. Maybe the dual CPU required a heftier PSU?

Anyway. Done. Next, Mojave… then who knows, Big Sur, now this is a spare 'experimantal' machine ;)