Our guide below covers those interested in setting up Nvidia SLI or AMD CrossFireX on an Intel Z77 motherboard such as the ROG Maximus V series (we’ve used the Maximus V Formula), equipped with a 3rd Generation Intel Core CPU with PCI-Express Gen 3.0 capable CPU (such as the Core i5-3570K or Core i7-3770K).

Installing the first graphics card

• The Maximus V Formula has three full size (16x) PCI-Express slots, colored in red
• The two we are interested in for 2-way multi-GPU is the top slot and middle slot. Using just the top (primary) slot will give the graphics card PCI-E 3.0 16x bandwidth, while using both slots splits the lanes such that both have PCI-E 8x. Despite losing half the potential bandwidth per card, the PCI-E 3.0 standard provides more than enough data throughput, leaving your graphics card performance unaffected.
• Ensure each card fits securely, with the retention clip popping into place and that you connect the necessary PCI-E 6/8pin power connectors to the graphics card.

Installing the second graphics card

• When selecting a second GPU, you should ensure it’s as similar as possible to the primary GPU. For Nvidia cards, you must match the same GPU; so a GTX 670 with a second GTX 670. The driver will synchronize clock speeds if the two cards run at different frequencies. With AMD cards there is more lee-way; you can, for example, match an HD 7950 with an HD 7970, although for optimal performance scaling it’s again better to match both cards like for like.

• It might be obvious, but it’s worth saying you can’t mix Nvidia with AMD GPUs and you cannot mix two different generations of product: a GTX 500 series won’t SLI with a GTX 600 series for example.
• Fitting a second graphics card to the Maximus V Formula is easy; just slot it into place in the second (middle) PCI-E 3.0 slot. The board is laid out to allow a clear gap between the top and bottom GPU even if you’re using a graphics card with a triple slot cooler, such as some ASUS DirectCU II models.
• While the added space between the two cards might seem helpful for cooling, DO NOT fit the second card into the third (lowest) PCI-E slot. This will result in the top slot with 8x lanes of bandwidth, however the bottom slot only offers 4x bandwidth, limiting performance.
• With both graphics cards fitted and powered, it’s time to fit the SLI or CrossFire bridge cable (depending on whether you’re using Nvidia or AMD GPUs respectively). On both AMD and Nvidia graphics cards you can plug in the cable to either set of pins, with the cable aligning in a straight line from top to bottom. The SLI and CrossFire connectors do not match, so you can’t accidentally fit the wrong bridge.

  CrossFire cables are provided in AMD graphics card boxes, whereas SLI cables are provided with the motherboard instead.

The SLI/CrossfireX Bridge

Just pop on the bridge between the two cards as follows. Both will require just one connector. The second connector is used when more than two cards are added in 3-way and 4-way multi-GPU.

Connect your displays

If you’re using a single display, connect it to the output of the top-most card. However, be advised that certain outputs will be disabled when using multi-GPU setups; refer to Nvidia or AMD’s requirements pages to make sure your setup will work. If you plan to use three monitors, be aware that AMD requires you to connect the third monitor via DisplayPort, whereas Nvidia’s model favors three DVI connections.

For Nvidia configurations, refer to this guide.

Bios Tweaks

With the cards fitted into the right slots, powered up and connected via the required SLI/CrossFire bridge, there’s still a couple of things left to check.

Head into the Maximus V Formula’s UEFI BIOS – you should be in Advanced Mode by default. Navigate to the Advanced tab, and select System Agent Configuration, followed by the NB PCIe Configuration. You should see a screen which matches that below, with both the top PCI-E slots running at 8x. You can force these slots to use PCI-E 3.0 (Gen 3) here if you wish, as this can sometimes improve stability.

Install the driver

With the hardware in place, it’s time to head into Windows and install the latest Nvidia or AMD driver. You may want to grab the latest beta driver, as these will often have the most recent SLI or CrossFire profiles included.

• AMD driver page
• Nvidia driver page

SLI – SLI should automatically enable if the setup is correct. You can double check this though by entering the Nvidia Control panel and selecting the Configure SLi, Surround, PhysX tab. Here you can check that SLI is enabled, adjust Nvidia Surround if you’re using multiple monitors, or assign one of the GPUs as a PhysX processor. 

CrossFire – CrossFire should also automatically enable once the driver has been installed. To double check it, open the Catalyst Control Center, select the Gaming tab and select AMD CrossFire. Here you can enable, or disable CrossFire. Also noteworthy is that if you’re using a 7-series AMD GPU, don’t be alarmed if your second GPU’s fan switches off when idle. This is part of AMD’s ZeroCore power technology; the card will power back up when needed.

A word on performance scaling

Doubling up on graphics power does not always grant matching improvements, as there’s a significant reliance on the driver. However, as you can see from our example benchmarks in Unigine Heaven 3.0, given the right circumstances you can expect huge gains even at lower resolutions.

Below are some Unigen Heaven 3.0 benchmarks run at 1920 x 1080 for both 1x (left) and 2x (right) Geforce GTX 670:

And for and 1x (left) and  2x (right) Radeon HD 7970. It’s not exactly twice the performance, but at around 90% increase is pretty accurate for most multi-GPU setups now.

Driver Note:

Switching to use a multi-GPU setup does not always guarantee a significant performance increase inevery game. Certain titles will offer better and worse multi-GPU scaling, and new releases will likely require a driver or SLI/CrossFire profile update to ensure optimum performance. If you encounter such issues, you should refer to Nvidia’s or AMD’s latest drivers – even beta (pre-official release) drivers – which typically receive multi-GPU updates before the WHQL drivers which have to be certified by Microsoft.

Power Supply Note:

Adding a second graphics card will also dramatically increase your PC’s power consumption in 3D gaming; ensure you have a PSU that’s up to the task. For a dual-GPU setup as detailed here, we’d recommend no less than a 700W PSU.

Did you follow our guide, how did you get on? Let us know in the forums.