Crossfire Overclocking 7870 and 7850

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__Turbo__

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Dec 9, 2013
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Hello,

this is my first time publishing anything on here so please bear with me if I don't provide enough information.

I recently upgraded my rig to accommodate a crossfire configuration. I purchased a Rosewill 750 watt Hive PSU and I am now running an HD 7870 and a 7850 in crossfire. Yes, that's right a 7870 and a 7850. My motherboard only has one PCI 3.0x16 slot (where my 7870 sits) and my 7850 is in the PCI 2.0x4 slot. My issue: I overclocked both cards to 1050MHz/1350Mhz and I was only getting roughly 40FPS with AA on (this is worse than my single 7870), and some crazy artifacts. However, when I down-clocked my 7870 to 990MHz/1300MHz and changed my 7850 to 1050MHz/1350MHz I am getting a stable 60 FPS with AA on and no artifacts. Why is this? My temps are fine (monitoring everything with MSI afterburner), and both cards are now being used. Also, my voltages are a bit strange. MY 7870 had a stock voltage of 1166Mv (changed to 1188Mv, I don't know what's safe or if this even helps) and my 7850 had a stock of 1210Mv. Why does the lower card have a higher stock?

Thank you for reading and for any help. I truly appreciate it!
 
Solution
quilciri
The overclock ceiling drops because it's paired with any card. It doesn't matter that the other card is a 7850, you would have a lower OC ceiling if both cards were 7870's.

The 7870 lowers it's performance to 7850 level when you pair it with one, but that is a (mostly) separate phenomenon from overclocking paired cards.
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quilciri

quilciri

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Mar 16, 2012
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When you crossfire two different model cards, the faster card will reduce it's performance to match the lower card. This can mean disabling shader units and/or lowering the clock speed. That means the 7870 will use less voltage when crossfired with a 7850 than it would by itself.

Most people will tell you that while it is possible to xfire two different model cards in the same 100 series, it's not a good idea because you lose the performance of the higher card. You're getting the same performance as someone with 2 7850's would.

When you use multiple cards, the ceiling for effective overclocking will be lower. A card that could get a stable 20% overclock on its own may only reach 5 or 10% in a multi-card configuration.
 
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__Turbo__

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Dec 9, 2013
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Just to make sure I understand, this mean although I normally would be able to OC my single 7870 to around 1150MHz, because it's paired with a lower card my OC ceiling drops. Now if I try for above that ceiling I get worse FPS and artifacts?

 
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quilciri

quilciri

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Mar 16, 2012
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23,965
The overclock ceiling drops because it's paired with any card. It doesn't matter that the other card is a 7850, you would have a lower OC ceiling if both cards were 7870's.

The 7870 lowers it's performance to 7850 level when you pair it with one, but that is a (mostly) separate phenomenon from overclocking paired cards.
 
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