Catching Two Tahitis Behaving Badly

This little write-up is predicated on the notion that dual-GPU cards are best-suited to four-way arrays. After all, why bother with a $1000 GeForce GTX 690 if you can snag two (faster) GeForce GTX 770s for $800? And why bother with a $1000 Radeon HD 7990 if you can find two (faster) Radeon HD 7970 GHz Edition cards for $900?

Of course, when I wrote AMD Radeon HD 7990: Eight Games And A Beastly Card For $1000, I only had access to one. Now there are enough cards to pair them up the way these things were meant to be used. Performance wasn’t even my top concern when I got my hands on an additional two Radeon HD 7990s. We already know that dual Tahiti-based configurations run into issues with dropping and truncating frames in a number of titles. AMD knows this too. The company has a driver in development intended to achieve better pacing between frames. I previewed it in the Radeon HD 7990 review, and it looks promising. Today is not the day that driver becomes available.

Rather, I wanted to know how 7990s in CrossFire coexist, both acoustically and thermally. It’s a particularly important question given the three axial fans and sink orientation AMD employs, which exhausts most of the heat out the top of the card and some down toward the motherboard (but none, really, from the I/O bracket’s cut-out exhaust).

Best-case scenario: Two-slot separation and lots of cooling

Setting Up The Experiment

The ideal test setup, then, becomes a case with enough airflow from the side to cope with two 375 W cards pushing all of their waste heat out the top, a power supply able to deliver the 1000+ W this platform pulls from the wall, and a motherboard flexible enough to give us one and two spaces between Radeon HD 7990 cards.

At launch, AMD was recommending two enclosures to support the 7990, one of which was Cooler Master’s HAF X. It’s a testament to Cooler Master that the HAF comes to highly recommended, and the company was kind enough to send one over for my experiment. In it, we installed Gigabyte’s X79S-UP5 motherboard, a Core i7-3960X, Corsair’s AX1200i power supply, and Noctua’s NH-U12S heat sink. Using an Extech 407768 sound level meter and TM200 dual-K thermometer, we tested the original press sample on its own, a retail card on its own, both retail cards together, and the press sample with a retail card for verifying the findings.

What we discovered was that two 7990s behave quite a bit differently than one, and adding space between them only prolongs the time it takes for them to get there. While we typically see Tahiti GPUs top out in the 84-degree Celsius range, whether they’re on single-chip boards like the Radeon HD 7970 GHz Edition or dual-GPU boards like the Radeon HD 7990, three or four minutes in Unigine’s Heaven sees CrossFire’d 7990s slamming up against the processor’s 102-degree protection point. Far Cry 3 outright crashes after heating up to 98 degrees (or, if you stay in the game’s menu too long, it’ll jump up to 102 degrees as well). And 10 runs through Metro: Last Light’s benchmark has the top card’s GPUs at 97 degrees.