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It's likely the top card that is overheating, because they are so close to each other and there is nowhere for the heat to escape. Once the top card reaches 94c the core will begin to throttle to prevent damage to the GPU. It's perfectly safe to run the GPU at 94c or lower, but you may see some reduced performance as the core clock drops.
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Like amdmatt said, this is due to the cards being so close together and the top (primary) card is being fed warmer air dissipating from the lower card, thus why it's reaching its maximum operating temperature so often/quickly. There are multiple things you can do to eliminate this or in the least reduce the primary cards temperatures, but at a cost no matter what the solution.
An ideal Crossfire setup would consist of a few things.
Premium Thermal Compound
Premium thermal compound such as Gelid Solutions-GC Extreme is a must for anyone trying to obtain the lowest possible temperatures. Regardless of whether or not your GPU is brand new or not, manufacturers rarely apply an adequate amount of TIM, nor the highest of quality available on the market. Thus this should always be the first step one should take to obtain better temperatures in single or Crossfire configurations.
When selecting a case, a cube case with the motherboard laying flat is best for temperatures as in the examples via the links below. This allows the GPU's to feed the hot air immediately out of the top of the case, thus lowering temperatures substantially in comparison to standard cases. Now, this isn't the end all be all, but this will reduce the temperatures by roughly 5-8 degrees Celsius.
Sadly though, even after upgrading your case to a cube, when playing graphically demanding games at extremely high resolutions with all settings set to their highest you will once again be presented with your temperatures reaching higher than optimal conditions. Thus, an ideal Crossfire situation is not only for the case to be a cube (flat mobo) layout and to apply premium thermal compound, but also for your cards to be water/liquid cooled.
This can be done easily and for a fairly reasonable cost. You can buy the Corsair H55 and also buy the Corsair HG10 A1 for a combined total of under $100 or to cool both GPU's for under $200. Now, if you decided on going this route, let it be known that the HG 10 A1 would have to be slightly modified to better fit the form factor and dimensions of your R9 390 as it was originally created with the R9 290 in mind. Regardless it can easily be accomplished with a bit of patience by even the least experienced enthusiast.
You can also go with a full water cooling/block configuration from a provider such as EK, but at a much higher cost overall ($350+) with the added benefit of full custom water cooling aesthetics which look fantastic. Although spending an extra $100-$200 ($350 in the case of an EK) on your graphics solution might seem frustrating, it does however allow you to reach your cards true performance potential, all while reaching no higher than 30 degrees Celsius at idle and no higher than 59 degrees Celsius during load, no matter what application you throw at it.
Anyways, I know reducing your temperatures in Crossfire configurations can be frustrating, but it can be done and the results afterwards are truly worth the effort! Good luck and have a nice day.
Thank you all for the replies. For now i cant afford something liquid liquid cooling but thats something interesting.
I solved my problem for now by removing the side painel and mounting a fan right above both cards blowing air into them.
It dropped to 71 72 for now ill keep it like this altough its not good to not have the side painel on.
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There is a less costly way to reduce your Crossfire temperatures which I should have mentioned earlier, aside from the more expensive methods I mentioned previously. This method isn't necessarily intended to be a permanent solution, but it works and well at that. You can create a cooling solution yourself by following the guidelines below quite easily and for less than $30. You'd actually be surprised at the end results as well. Your temperatures under load will drop roughly 20 degrees Celsius.
When viewing the example I created below, please keep in mind that it's simply a basic visual reference created in "paint". Yet, regardless I can guarantee that this very much so works and will reduce your temperatures by a vast margin. I created this for a friend a few years ago, who was encountering temperature issues with his R9 290X's in Crossfire, but his budget was limited at the time. So until he was able to afford a proper water cooling loop, this held him off until he was able to save the amount needed.
What you'll need, if you decide to partake on this journey through wacky makeshift GPU cooling town
Zip Ties: 25 (21 for the primary assembly and 4 for anchoring)
120mm Case Fans: 8 (4 Intake and 4 Exhaust*)
80mm Case Fans: 4 (4 Intake)
Optional: Crazy Glue (To properly seal each fan, reducing air leakage and maximizing positive pressure)
Let it be known that my misspelling of exhaust* below was intentional.....O.k I just lied, it wasn't intentional.
NOTE: The 80mm fans must be lowered and attached with the zip ties at the bottom of each point seen above to enable access for the GPU's power inputs. The visual example above isn't precise as I said earlier, but when assembling this Frankenstein above it will all make sense.
NOTE #2: You will have to locate 4 primary areas to anchor the entire assembly to with zip ties which are stationary near points 1, 3, 6 & 7 to ensure its stability. Yet, it will be surprisingly stable overall once anchored down.
NOTE #3: When people ask why you did this, tell them you never disagree with the voices in your head!
Anyways, although this isn't the most optimal situation (especially in appearance), it will work. So if someone is on a budget and is willing to forego aesthetics in exchange for better temperatures, this might just be the temporary solution they're looking for until they can upgrade to a proper solution.
Although a touch on the crazy side, I hope this helps you out in a bind during the coming summer if needed.