PCI-E 8x vs 16x SLI

Custom / NA
August 27, 2010 at 19:44:23
Specs: Windows 7, i7 / 6GB
I just built a new PC with two GTX 460 graphics cards setup in SLI. The two PCI-E 16x slots are so close together, that the top card doesn't get very good airflow. I've tried more case fans, but it really just comes down to the lack of clearance. There is a PCI-E 8x slot further down the board. If I put one card in there, both would operate at 8x speed in SLI mode. Would that be a significant performance decrease, or would it be worth it to increase the airflow?

-Ryan Adams

Free Computer Tips and more:http://RyanTAdams.com
Paid Tech Support: Black Diamond


See More: PCI-E 8x vs 16x SLI

Report •

#1
August 27, 2010 at 22:36:48
The fans on video chipsets and cpus always pull air towards the heat sink, and push it through the heat sink. If you have at least, say, 3/8" between the top of the fan and the next card, both card's video chipsets are probably getting cooled sufficiently. You may have current temp readouts in the NVidia software somewhere for each card.

What mboard do you have these cards installed in ? The NVidia video chipset cards can't run in SLI mode unless the main chipset is NVidia, with the exception of a few recent Intel main chipsets that support both ATI's Crossfire and NVidia's SLI. .

If you have a generic system, read the mboard manual. Depending on the main chipset, two PCI-E X16 cards in PCI-E X16 slots may be running in X8 mode in any case. X8 mode does NOT have a max data speed half as fast as X16 mode, as the numbers would suggest.
If the two GTX cards do not require a SLI cable between them on your mboard, then it probably doesn't matter which slots they're in , in order for them to run in SLI mode.


Report •

#2
August 28, 2010 at 00:17:48
Unfortunately, the space between the fans on the top card and the back of the bottom card is almost zero. The motherboard is a ga-x58a-ud3r, which other than being cramped, has worked out well. The temperatures I was seeing during testing were about 100C on the top card (I believe the stated MAX for the card is 104C), hence the reason I think there is an issue.

The two PCI-E 2.0 16x slots run in 16x mode when doing SLI. Alternatively, I can put the cards a bit farther apart, but this would make them run in 8x mode. I know from testing that when PCI-E was new, the cards available didn't have much of a difference between 8x/16x, but with these newer, faster cards, I don't know. I just don't want the PCI-E interface to be the bottleneck.

I'm also considering water cooling the two cards, but even a basic setup from koolance would cost $500+.

-Ryan Adams

Free Computer Tips and more:http://RyanTAdams.com
Paid Tech Support: Black Diamond


Report •

#3
August 28, 2010 at 07:36:15
"The two PCI-E 2.0 16x slots run in 16x mode when doing SLI."

Does the mboard manual state that ?

"I'm also considering water cooling the two cards, but even a basic setup from koolance would cost $500+."

Much cheaper alternatives would be

- should work -

- an aftermarket fan that yields more space between the fan and the card next to it
- an aftermarket fan / heat sink combo that yields more space between the fan and the card next to it and has a better heat sink, such as one with heat pipes.

- WILL work -
- mboards are relatively cheap - get a different mboard that has two PCI-E X16 slots farther apart, which is the usual situation, that is compatible with the same cpu and ram - you may be able to find one with the same, or a similar, superior main chipset.
Sell the mboard you have now, while it's still worth a fair amount.

You could always try one card in a X16 slot, the other in the X8 slot to see what the difference is, though you may need to run some performance tests with the present situation and compare that to the latter to give you some real data.

Why do you want to run two of these cards ? From what I have come across so far, just one has extremely good performance.


Report •

Related Solutions

#4
August 29, 2010 at 00:38:42
1) Yes, the motherboard states that. The two PCI-E 2.0 16x slots both run in 16x mode when doing SLI.

2) As far as I know, there are no aftermarket fans or heatsinks for the GTX 640s. The stock one has heat pipes and two fans.

3) Replacing the motherboard is not only expensive, but seems like a waste, considering the only con of this one is how close the 16x slots are.

However, I have fixed the problem. I ended up making a custom shroud out of Plexiglas and using a 90mm fan to blow cool outside air directly to the two fans on the stock cooler. The temps still get to ~77C under heavy load for an extended period, but it's a lot lower than the ~104C i was seeing before. Thanks all for the input!

-Ryan Adams

Free Computer Tips and more:http://RyanTAdams.com
Paid Tech Support: Black Diamond


Report •

#5
August 29, 2010 at 06:18:01
We're glad to hear you found a solution.

"Replacing the motherboard is not only expensive, but seems like a waste, considering the only con of this one is how close the 16x slots are"

If you sold the original mboard while it's still worth a fair amount, it wouldn't cost you much, net. Most mboards do not have the X16 slots close to each other.


Report •

Ask Question