SLI & CFX PCIe Bandwidth Perf. - x16/x16 vs. x16/x8

Have you wanted to space your SLI or CFX video cards farther apart on your motherboard to allow for better airflow? Do you have a motherboard that will not support a x16/x16 PCIe configuration? We put x16/x16 and x16/x8 PCIe SLI and CFX configurations head to head and show you what sacrifice there is to be made.

Introduction

If you install the secondary video card in an SLI or CrossFireX configuration into a x8 PCI-Express slot, does this degrade your gameplay performance? What if you have a motherboard that has your expensive video cards crammed so tightly together that you have one GPU that runs much hotter than the other? What if you have a motherboard that does not support x16/x16 PCIe bandwidth configurations? These have been questions in gamers’ minds, and we feel that with today’s current GPU generation the time is right to test this in a real world gaming scenario.

The PCIe 2.0 specification with the most bandwidth currently is handled by 16 lanes "x16" per PCI-Express slot. With many current generation motherboards you will have two PCI-Express slots capable of x16 operation. However, there are also many chipsets and motherboards that only support x8 electrical operation on other remaining physical x16 PCI-Express slot. The question is, "Does putting my SLI/CFX secondary graphics card in this x8 slot harm my performance compared to putting it in a x16 slot?" Today that is what we are going to find out.

With the latest generation of GPUs performance is fast. SLI is one of the fastest configurations being purchased by gamers right now. If anything is going to show us differences in performance it will be GeForce GTX 480 SLI, or at least we think so. We are also going to include AMD’s fastest combination it has going for it right now, ATI Radeon HD 5870 Eyefinity6 2GB CrossFireX and ATI Radeon HD 5870 1GB CrossFireX. We are also including NVIDIA’s new mainstream champion GeForce GTX 460 SLI. This wide range of high-end video cards and a mainstream video card configuration should reveal if having x8 bandwidth on your secondary PCIe slot impacts your gameplay experience.

Test Setup

We are going to use the tried and true taping method to reduce the video card to x8 operation. This is simply done by taping up the correct number of pins on the PCI-Express "gold fingers" connectors to reduce it from x16 to x8 operation. We can verify that the video card is running in the correct mode by using GPUz. In the first screenshots below you can see how the video card is running "@ x16" and in the second screenshot, with the connectors taped, it is now running "@ x8". This will allow us to use the same video cards and the same motherboard to make all the tests apples-to-apples.

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The system setup is as follows: MSI X58 Eclipse (two x16 PCIe slots native), Intel Core i7 920 @ 3.6GHz, 6GB DDR3, Dell 3007WFP, Win7 x64. We are using the newer Catalyst 10.7a Beta for the AMD video cards and NVIDIA drivers 258.96 WHQL.

All of our real world gaming tests on the following pages are performed apples-to-apples at the exact same in-game settings for the specific resolution. We are comparing x16/x16 versus x16/x8 on each video card combination in three games, Aliens vs. Predator, Bad Company 2 and Metro 2033. We have chosen AvP because it is very texture and bandwidth heavy, especially with AA enabled. We have chosen Metro 2033 because it is very shader and pixel heavy, but can be quite burdened with 4X AA enabled. We have chosen BC2 because it is a mix of shader heavy and texture heavy. This should let us know if a more shader-heavy game like Metro 2033 is affected or not or if it a more texture-bandwidth heavy game like AvP is affected more.