Just last week AMD unleashed their first dual-GPU Radeon HD 5000 series graphics card, and we have to say it was the most exciting launch for us of the year, at least when discussing graphics cards. Known as the Radeon HD 5970, this new graphics card is essentially a pair of Radeon HD 5870’s sharing the same PCB.

However in order to meet certain specifications, AMD was forced to down clock the core and memory frequencies to match the Radeon HD 5850 rather than the 5870. This meant that while the core configurations of the GPU’s are the same as the Radeon HD 5870, they are operating at a lower frequency. This did hurt the performance of the Radeon HD 5970 and it meant that it performed more like a pair of Radeon HD 5850 Crossfire cards.

That said, no one really cared, as a pair of Radeon HD 5850 graphics cards will cost the same amount as a single Radeon HD 5970. The Radeon HD 5970 proved to be the world’s fastest single graphics card, as it easily dispatched of the GeForce GTX 295. However despite being so fast already, many were fixated on the superior overclocking performance, squeezing even more performance from the dual-GPU power house.

When launching the Radeon HD 5970 it was made very clear by AMD that this was an enthusiast product and as such overclocking was a huge focus for the product. Initially AMD told us overclocks of 1GHz were possible, though no one really seemed to achieve this. That said, we were able to push both cores from a modest 725MHz to an impressive 900MHz, where we did see quite incredible performance gains.

However in order to reach this frequency we were forced to use a special voltage tool which was provided by AMD. This tool increased the voltage of both the core and memory to provide greater headroom. The side effect to this was increased heat, while the operating volume became unbearable. Having achieved a core frequency of 900MHz and a memory frequency of 5000MHz, we were expecting serious performance gains and in some games we got them.

Having said that, not all games provided us with impressive or even note worthy gains, and we wanted to know why. It first appeared that the more demanding games were receiving the smallest gains, and to be honest we were expecting them to show the greatest. Furthermore, the longer we ran certain tests, the less impact the overclock had, and scores would slowly decrease. After further testing we think we have an answer as to why this was happening...

So this is not really a practical overclock then since the card will inevitably throttle back. While it won’t happen as fast as it did in Furmark it will happen and that is the point. The Radeon HD 5970 is still a great product and it is the world’s fastest single graphics card but it is not the overclocking behemoth that AMD made it out to be. At least not with the standard cooler which they say is rated up to 400w.

The overclocking issues were first uncovered using FurMark, which is an excellent program for placing full load on both GPU’s while measuring temperatures and performance. For the purpose of this article we clocked both GPU’s at 875MHz, while leaving the GDDR5 memory at the default specification. In order to achieve this overclock the core voltage must be increased and therefore we were testing at 1.1625v.

With the overclock now in effect we ran the FurMark benchmark at 1680x1050, please note that we did increase the run time for the purpose of this test. With the core at 875MHz, the average frame rate was sitting at a fairly constant 115fps. However after just 40 seconds temperatures exceeded 100 degrees and shortly after this the Radeon HD 5970 overclocking problem presented itself.

In order to avoid cooking itself, the Radeon HD 5970 throttles both cores down, negating any positive performance impact the overclock is going to have. However rather than throttle the cores down just a little, or even to the default operating specification, it drops the frequency to just 550MHz, that’s a 24% under clock on each core.

This saw the average frame rate decrease from 115fps before the throttling, to well under 100fps. The screen shot below shows an average of just 94fps after 260 seconds, with the temperature leveling off at 89 degrees. Eventually the average was reduced to 90fps, which is a 22% drop in performance.

What we found interesting was that when testing with FurMark, the Radeon HD 5970 would even back off the card when operating at the default specification of 725/1000MHz. In fact, the only way we were able to avoid the throttling issue was to reduce the core frequencies to 650MHz, at this frequency the throttling never took place.

When overclocking the Radeon HD 5970 we are required to increase the voltage, and in doing so the graphics card is exceeding the PCIe specification. This means that the computer's power supply is being over taxed, and to overclock the Radeon HD 5970 a high-end quality power supply is essential. This is because the power supply is going to be required to deliver more power on the PCIe rails than it is essentially designed for.

This is an issue we spoke to AMD about, and they suggested that users make sure they have at least a 750 watt power supply that can deliver 20A for the rail supporting the 8-pin connector, and 15A for the 6-pin connector. For testing purposes the Thermaltake Toughpower 1500w power supply was used, which is capable of delivering enough power to the Radeon HD 5970, even when overclocked.

Based on our observations we do not feel that power delivery is an issue here, though there could be some kind of compatibility issue with the Toughpower 1500w power supply, and we cannot rule this out till we try another powerful unit. When we manage to get the Radeon HD 5970 back from AMD we will test again with a few more power supplies.

Unfortunately we only had a few days with the Radeon HD 5970 and our review that was published on launch day only featured a very brief section on overclocking. As that review went live we got back to overclocking the Radeon HD 5970 before having to mail it back the next day.

It is also worth pointing out that when overclocking we were forcing the fan to 100%, where it screamed at an unbearable level. Despite this the temperatures were still very high, leading us to believe that the throttling is a result of high temperatures. However what we are most concerned about is the throttling that was seen when operating the Radeon HD 5970 at its default specifications.

This makes us believe that overclockers would in fact be far better off with a pair of Radeon HD 5850 graphics cards, as we were able to push them to almost 800MHz without any kind of voltage mods or throttling. Still, we are not sure if these issues are restricted to early review samples or if all Radeon HD 5970 graphics cards will suffer from them. Furthermore, we do not know if this is an issue AMD can simply fix.

Then finally we began testing the Radeon HD 5970, and while the numbers were truly impressive, given the specifications we were expecting more. However given how time sensitive the review was, we had to go with the results, and as it turns out they were accurate.

Wrapping up the review we decided to include some preliminary overclocking results, and this is where we ran into even more problems. First of all the core overclock of 1GHz that we were told about was just not possible. Having arrived at a stable 900MHz overclock we found the results to be very erratic, with some games providing impressive performance gains while others showed almost no gains.

At the time we had no idea what the issue was, and after confronting AMD with the overclocking issues they said it was likely down to an issue with the beta drivers. Given all the problems the beta drivers had given us we tended to agree with that analysis. However once the Radeon HD 5970 was launched we had one more day to play with it and we spent all that time frantically working on the overclocking issues.

Before long we spotted the throttling issue, but failed to find a way around it. Having messed around with various frequencies, we were shocked to find that the default specifications would eventually result in throttling when both cores were taxed. In fact, we found that unless the cores were both locked at 650MHz, the cores would eventually throttle down to 550MHz.

Clearly Radeon HD 5970 overclocking is not all it’s cracked up to be, and while these problems could be restricted to the card we looked at, it does seem unlikely. Surprisingly, we have not heard any other reports of the Radeon HD 5970 throttling issues, but then having said that, most that were lucky enough to get their hands on a sample were likely forced to shuffle it off quickly, before a second round of testing could be conducted.

Keeping all this in mind, there is still no doubt that the Radeon HD 5970 is the world’s fastest single graphics card. However we are now questioning if it is really a better option at $600, opposed to a pair of Radeon HD 5850 graphics cards in Crossfire. On paper it is certainly a much more impressive product, but with the power and thermal issues that we ran into, we are not sure that it is as practical.

Reviewed By Steven Walton