Board analysis (cont’d)

Top of the card

Bottom of the card

As we’ve noted in the past when running two cards in an SLI configuration, one of the drawbacks of NVIDIA’s cooling system is that the primary graphics board on top tends to run about 10 degrees Celsius hotter than the card underneath it. This is because the standard cooling system on NVIDIA’s reference cards relies on drawing in nearby air underneath the graphics card to supply the cooler with fresh air.

This is fine as long as the area underneath the graphics card isn’t obstructed, but once two cards are combined for SLI, the graphics card on the bottom restricts airflow to the card on the top, hence the reason why the uppermost card runs hotter.

To help combat this, for the GeForce 7900 GX2 NVIDIA has reversed the direction of airflow for the uppermost card on the 7900 GX2’s PCB. Rather than taking in air from underneath the board which would be restricted by the second GPU on the PCB, the board up top draws in cool air from above the graphics card. Three vents are drilled into the top of board’s PCB to accomplish this. Air is sucked in through these vents, and then passed across the GPU and memory before exiting the left side of the card’s ducted enclosure (again, air doesn’t exit your system’s case). This helps to keep temperatures down although unfortunately we can’t measure the before/after impact of this change directly.

Air intake on the top of the uppermost card...

...Supplies air for this fan...

...But it does not exhaust here

Another key aspect you’ll notice on the GeForce 7900 GX2 is that each PCB on the card has its own 6-pin power connector. That means you’ll need four of these connectors to run a full Quad SLI configuration.

Technically, while each GeForce 7900 GX2 board ships with 1GB of memory total, keep in mind that each GPU on the GeForce 7900 GX2 board can only access 512MB of memory. NVIDIA’s reference specifications call for the use of 1.4ns memory modules, good for speeds up to 700MHz. Since the modules are only running at 605MHz, that leaves a little bit of headroom for overclocking.

The ABS System

To evaluate the performance of NVIDIA’s Quad SLI platform, ABS sent over one of their pre-release Ultimate M6 Sniper Quad-SLI systems for evaluation. The system was pretty much decked-out, outfitted with an Athlon 64 FX60 dual-core CPU running at 2.6GHz, 2GB of Corsair XMS3200 memory, two GeForce 7900 GX2 cards for Quad SLI, and a Creative Labs Sound Blaster X-Fi Elite Pro sound card. The complete system was housed inside a Thermaltake Armor case while the processor was cooled by a Venus CPU cooler also from Thermaltake.