Crossfire 6950 questions.

[454xalucard]

Hello, I currently have a reference HIS 6950 2GB, and I'm interested in getting a second one.

A couple of questions first though.

I know it doesn't matter what card you get as long as it's another 6950. From what I've heard, I'd also want to get another 2GB card, because a 1GB card will force my 2GB card to run at 1GB.

I'm wondering how clock speeds work when using crossfire. For example, if I were to get an IceQ X Turbo 6950, would that increase my performance while using crossfire? Or would the speed be limited to what my reference 6950 is clocked at.

IceQ X Turbo http://www.amazon.com/HIS-Radeon-256bit-Mini-DisplayPort-H695QNT2G2M/dp/B004WZ0HQ6/ref=sr_1_1?ie=UTF8&qid=1310956703&sr=8-1

Finally, is an 800W (Cyberpower ) psu enough to handle two 6950s if I overclock them? I currently have my reference 6950 clocked at 920 core, 1375 memory, up from 800 core and 1250 memory.

 

[bystander]

Contrary to popular belief, or rather, someone's assumption spread like wildfire, crossfired cards will each run at the clock they are set to. If you were to open up MSI afterburner, or GPU-Z or even CCC and watch what each card does, you'll see they maintain their clocks. The faster card does not down clock to the slower card. The same may be true with memory (I have no evidence to this).

However, when in crossfire, and each card is rendering a different frame, one card will hold back performance a bit while the other is working on it's frame faster. I imagine if the difference is enough, it could cause a bit of a stuttering effect, but I doubt it would be possible to get to that point due to clocking limitations. If the games crossfire method has each card generating the same frame, but one works on the top half of the screen with the other on the bottom half, then which ever card is slowest will force the other card to wait, so in effect, you can only go as fast as the slowest card.

If the same is true with the 2GB vs 1GB card, then when 1GB is not enough, the 1GB card will slow things down. It's best to stick with 2GB if you already have one.

 

[454xalucard]

Contrary to popular belief, or rather, someone's assumption spread like wildfire, crossfired cards will each run at the clock they are set to. If you were to open up MSI afterburner, or GPU-Z or even CCC and watch what each card does, you'll see they maintain their clocks. The faster card does not down clock to the slower card. The same may be true with memory (I have no evidence to this).

However, when in crossfire, and each card is rendering a different frame, one card will hold back performance a bit while the other is working on it's frame faster. I imagine if the difference is enough, it could cause a bit of a stuttering effect, but I doubt it would be possible to get to that point due to clocking limitations. If the games crossfire method has each card generating the same frame, but one works on the top half of the screen with the other on the bottom half, then which ever card is slowest will force the other card to wait, so in effect, you can only go as fast as the slowest card.

If the same is true with the 2GB vs 1GB card, then when 1GB is not enough, the 1GB card will slow things down. It's best to stick with 2GB if you already have one.

So then, if what you say is true, the only actual reason to get an IceQ card would be to help with heat issues? My motherboard jams the cards together when running crossfire.

 

[bystander]

You should also be able to overclock your existing card to match the new one, although it will run hotter, but still cooler than the jammed up one. I'd advise testing both configurations to see what gives you the best overall temps.

 

[454xalucard]

After testing my card a bit, I found my max stable core clock is around 930 mhz.

In reviews for the IceQ the average max clock seems to be around 955. However, this varies by individual cards I understand, so there's no way to know what your card can actually reach without testing it.