SLI GPU temp difference

I have a new dual-loop watercooled DS system which, except for a few Win7 hangups, is all I wanted it to be.  Being a die-hard tinkerer, I have already boosted the i7 920 CPU overclock to over 4.1 GHz with no difficulty, and am now working on overclocking the SLI GTX 580s to squeeze a little more performance out of them.  When doing graphics-intensive benchmarks (i.e. Furmark, Heaven), I have noticed a significant difference in the temperatures of the two cards, as much as 20 – 25 deg C.  Although the highest absolute temperatures are nothing to worry about (no more than 78 for the higher of the two), I can’t help but wonder why I’m seeing such a big difference when the cards are under load.  I know they’re sharing the load OK (according to the Nvidia SLI indicator bar), and idle temps are fairly well matched at about 34 and 37 degrees.  Since the cards are connected in series in the cooling loop, I’m wondering if the card closest to the pump would tend to run cooler because of cooler water coming out of the radiator and pump.  You would think the water flow would be fast enough so that this wouldn’t be a factor, however. The only other possibility that I could think of is that the waterblock for one of the cards isn’t making as good a contact as it should.  Although I don’t see this as a real problem and all my games and programs run fine, I’m curious as to why I’m seeing such a big difference.  Any ideas from the watercooling experts out there?

Thanks for the quick reply. 

1. Is there any way to prove/disprove this?  If they were off that much, wouldn’t  there also be a big discrepancy at idle temp?

2. Furmark and Heaven both were run in fullscreen, with the SLI indicator bar showing balanced load for each.

3. My system is about two months old, probably enough time to work out the bubbles.

I think I’m going to have to live with the temp difference, especially since it doesn’t cause any problems.  The engineer in me though says something needs to be fixed, but I really don’t want to tear into the watercooling setup. What would you do?

 

 

 

I suppose I should know this, but what's a poor T.I.M. job?

During DS stress testing, is this be something they would have picked up?  I can't say if I've had the temp difference since day 1, but I remember not long after I got my system I looked at the GPU temps and saw that they weren't the same, but wasn't concerned about it at the time.

Just looking at the cards and the way the plexi waterblock is mounted, there doesn't appear to be any place where I could mount a thermocouple as a crosscheck on the internal sensor. I sure don't feel a big temperature difference between the two cards, so maybe it is just a linearity error in the card sensor. I guess a call to DS tech support might be in order.

UPDATE -- for those who might be interested......

I called DS tech support, and Robert was very helpful but had no new ideas as to what could be causing the difference in GPU temperatures. When my system was initially being stress-tested at DS, they had run Furmark and he had a screen grab showing over 15 degrees difference at that time, with the higher temperature about 80. To me, this seems high for a watercooled system, but apparently nobody was concerned enough to look into it. Robert volunteered to send out a new waterblocked GTX580, but since I had already ordered some Arctic Silver 5 thermal paste, I decided to go ahead and redo the TIM myself. This of course meant draining the system, removing the waterblock from the suspect GPU, cleaning off the old white paste (which was excessive, BTW), and applying the Arctic Silver using the credit card method. It seemed like a good idea at the time to also redo the other GPU, so I wound up doing both. After reassembly and powerup, I was disappointed to see that the temperatures were the same, neither better nor worse with the new TIM job. At this point the waterblock or onboard temp sensor now is suspect, so I called Robert back and asked him to send out the new waterblocked and thermally tested card, which he did yesterday. When I get the card in about a week and get some tests run, I'll post the results.

I don't think a couple degrees difference between the best and the worst TIM (see http://www.tweaktown.com/reviews/3383/arctic_cooling_mx_4_thermal_compound/index.html) will make any difference one way or the other in my case. As far as what's best, it all depends on who you believe. There are plenty of "expert" reviews of Arctic Silver 5 that rate it very highly. The point being, TIM is apparently not the cause of the temperature difference that I'm seeing. It was the #1 suspect, but now I'll have to wait until the new card comes in and hope that solves the problem.

UPDATE #2:

The new waterblocked GTX 580 arrived from DS on Monday, and the first thing I noticed was that they had used a waterblock different from the one used on the original card. The new one was an XSPC Razor GTX580, and the original I couldn't identify. As luck would have it, the fittings wouldn't line up between the new (bottom) card and the original good top card, so I couldn't join the two with the straight connector pipe. Back to Robert at DS, who said he will ship out another waterblocked GTX 580 ASAP. I did install the new card, and am running it by itself until I get the second one from DS late next week. Temps look good -- mid 40's for an extended single GPU Furmark run.

The saga continues.......

what about the fan speeds? do you have a profile for them? my is not LC but ever since i implemented fan speed profiles my 570 sli has been around pretty stable

@supertech,

The temp differences you have now are good.