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Radeon HD 4870 X2 Crossfire (4CF)

Radeon HD 4870 X2: Four Cards Compared

Activating Crossfire

If you have the cash for it and have a compatible motherboard, plus >800 watt power supply, you can combine two Radeon HD 4870 X2 cards using CrossFire. Once you’ve connected the bridge and activated the CrossFire mode in AMD’s driver, you’ll have four RV770 graphics chips running cooperatively (4CF). With the overclocked quad-core CPU, the test system demands 775 watts from the socket under full load. The power supply has to provide 660 watts with 55 amps (12 V). In 2D mode, the demand is still 295 W (at the plug). This is only 15 watts more than a CrossFire combination of two HD 4870s, which can’t currently throttle clock speeds.

We did not overclock the video cards for our four-way tests because the quad-core CPU with its 3.67 GHz frequency can barely handle one dual-chip card. With two X2 boards, the CPU limits performance potential even more, and the test system can’t really take advantage of the additional graphics power. When looking at the overall test result, the additional 5% in performance from the fastest 4-chip combination is somewhat disappointing.

The HD 4870 X2 CrossFire (4CF) really shows its worth in games based on the UT3 engine (such as Mass Effect), with high resolutions, maximum anti-aliasing, and anisotropic filtering. In this situation, with the support of an overclocked CPU, you can gain an additional 76% in frame rate. We’ll show you a detailed comparison between X2 and X2 CrossFire (4CF) after the benchmarks.

GPU-Z clock speeds

For our combination, we used the test cards from HIS and Sapphire—both of the reference-clocked boards. After starting up Vista, the CrossFire combination gets stuck in the same 2D driver error, which doesn’t allow the cards to fully reduce their clock rate. As long as you don’t start a 3D application, the power consumption of the test system remains at 330 watts (at the socket). Once the driver has to change the clock speeds, it drops the 2D power consumption down to 295 watts.

The behavior of the HIS card is interesting. In 3D mode, the graphics chips (GPUs) can easily take 91 to 92 degrees in CrossFire mode. In single-card operation, we saw image errors if the HIS GPUs did not get enough time to cool down. When you look at the GPU load during the tests, it is always at 100% in both single-card and CrossFire mode, and the fan speed ranges between 3,200 and 3,900 RPM.

If you don’t have an angled plug, the wiring for the power supply is somewhat cumbersome. Because the ports sit on the side but the connectors point downward, the second card is in the way. Either the cables bend the board or you have to pinch the cables, neither of which is ideal for long-term use. We saw the same problem with the Radeon HD 3870 X2 CrossFire, which Nvidia solves by having the connectors point sideways, facing toward you as you look down on the installed card.

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