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8700K overheating?


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I have a concern that my new i7 8700K based system, that is primarily used for flight simulation, is running too hot and would like opinions.


The system is:

Case: Corsair Obsidian 650D

PSU: Corsair AX760

Motherboard: ASUS PRIME_Z370-A

CPU: i7 870K unclocked

Ram: 16Gb DDR 4



Corsair H100i Top mounted exhaust

120mm Rear fan input

200mm Front fan input


The CPU temperatures that I am getting are around:

30 degrees C - idle

65 - 70+ degrees C when running Prepar3d

Peaks as high as 86 degree C

All temps as per CPUID HWMonitor


A particular concern I have is that the H100i was standing around for about 6 months before it was first installed; could the the pre-applied thermal paste have gone off and lost its thermal effeciency?

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Specifically your coolant delta (H100i Temp from starting value to peak). I also suspect the Coffee Lake series will be like Kaby before it and require some clamping on the Load Line Calibration to keep your Vcore in line, especially when on Auto voltage. Can you get a peak Vcore reading during use?
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On load of CPU usage of circa 15% the CPU temperature is in the mid 60's.

The peaks of 86 seem to be spikes while running Prepar3d.

The 8700K is not overclocked and the H100i is a V2.

I'm afraid that actually doesn't tell much ... 15% overall CPU utilization could still be almost 100% on a single core. This isn't unusual behavior for single-threaded apps or even multi-threaded apps that do a significant amount of processing on a single thread. In that case, a brief spike to 60C for that core and/or the package wouldn't be that unreasonable or unusual.

Now ... when you spike to 86C, does it stay there or does it rapidly drop? And do you have any information on average temps at load over time? Spikes aren't that unusual with current Intel chips, especially with the lousy TIM that they use between the CPU core and the heatspreader. I don't know anything about Prepar3D but if it uses AVX instructions, these kinds of spikes wouldn't be a surprise. Try running a comparison to Prime95, which makes heavy use of AVX instructions. Additionally, use another standard benchmark package, such as Aida and/or ROG RealBench. C-attack likes Aida, I prefer RealBench - it stresses both CPU and GPU at the same time. Temps from these benchmarks will be useful as we'll have a better frame of reference.

Keep in mind, too, that 86C isn't hot enough for thermal throttling. So it may not be ideal but it won't hurt the processor.

It also doesn't hurt to double/triple check the mounting. Make sure that the standoffs are tight and the cooler is tight. You mentioned replacing the thermal paste ... look at this thread for some discussion on thermal pastes: http://forum.corsair.com/v3/showthread.php?t=171620.

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I'm afraid your comments re VCore etc are a bit beyond my skill level.

I have assembled 4 systems of my own but don't have much knowledge of the theory and details behind it. Sorry


That's OK, this one is easy enough to find and my hunch is it will be a big relief when we do. In Link is supposed to list this as "VCPU" (Voltage-CPU) in the first box. However, in my experience you often see a wide range of different things picked up labeled with that. I just noticed Link is telling me VCPU is fixed at 0.95. In fact, that is my System Agent Voltage. As such, lets use something else to measure it. HWMonitor, HWInfo, AIDA, Intel XTU or any of the free monitoring utilities can provide this. I haven't been cataloging 8700Ks yet, but that 6 core on AUTO voltage would like come in around 1.25v at peak turbo. However, the last few CPU models have overvolted aggressively when on the Auto adaptive voltage. That setting is designed to make the board and CPU work with even the worst production chip. As such, when it thinks it sees a difficult load, it pours on the voltage.


Now that I have opened the door to this, I had better take the time to look to find some corroboration. From the Ars Technica Review:


"The problem is the over-zealous VID spit out by the motherboard. In this case, the board pumped in around 1.248V into the CPU to maintain the stock 4.7GHz all-core boost clock. That's much higher than the 1.0V or lower you would see in a quad-core CPU. Indeed, undervolting the CPU greatly reduces temperatures without affecting stability. Naturally, this will vary from CPU to CPU, but I found a -0.090V offset to be a good starting point for undervolting, which brought temperatures down well below 75°C. It's likely that future BIOS updates will fix the issues with Auto settings, but it's something to bear in mind if you jump in at launch."


In essence, the board overcompensates when it see a difficult load (Prepar 3d) and continues to pour on voltage. I am expecting you to report a peak Vcore of around 1.34-1.35 when using Prepar3d. On the bright side, this usually can be countered by setting a manual value (instead of auto) and some tightening of the Load Line Calibration to limit the voltage swing. That last part is very motherboard specific, so you will want to find some advice from other owners on the same board.


For now, we need to confirm this is the case. Basic cooler functionality you can see by watching the H100i Temp in Link. In a bad cooler, it will go up right in from of your eyes at a brisk pace. Normally, it would only go up 1-2C a minute even at max load. Use a monitoring program besides Link to confirm the Vcore when using Prepar3d or any game. There will be a lot of things in the monitor, but core voltage will be listed something like Vcore and have a value between 1.20 and 1.40 when loaded. Likely 0.70-0.80v when idle and cores stepped down.

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In reading c-attack's post, it also occurred to me to check the motherboard that you are using ... and, sure enough, it's an Asus. Now, that's not a bad thing but they do typically ship their boards with "Asus Multicore Enhancement" enabled by default. This runs all cores at the top boost frequency, instead of just one core (as per the Intel spec). It provides better performance, sure, but it's technically an overclock. It may well explain a higher VCore and the temperature spikes that you are seeing.

Still, if it's just a brief spike, it may not be out of line. Certainly, comparing this to more standard benchmarks will give us a better picture. And yes, you might be able to shave a couple of degrees off of your max with a different TIM on the cooler but I wouldn't expect miracles. As I've never actually used the stock TIM on the Corsair coolers, I would be curious what your results would be compared to something like Noctua or Gelid.

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I have run stress tests in both Prime95 and Real Bench.

In both the results are similar.

All cores are at 100% utilization and clocking at 4710 - 4720 MHz


In Prime all the cores generally fluctuated between 60 - 75C with max levels hitting up to 87C.

In Real Bench all cores were running consistently >71C with max levels in the range 81 - 88C


From LINK, with the H100iV2 running in balanced mode it's own temperature was around 32.7C; don't know if this was relevant.


Voltage data I picked up from HW Monitor is:

VCore 1.392V

VID 1.432V (all offsets 0V)

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OK, your cooler is working just fine. I think we can confirm that based on those numbers. Both the Prime and the RealBench temps make sense.

You do have "Asus Multicore Enhancement" enabled. According to Intel's spec, only 1 core should run at 4.7 Ghz. This is absolutely typical for Asus.

Your CPU temps do not indicate that you are overheating ... none of those are high enough to trip thermal throttling and both Prime95 and RealBench are going to stress your CPU more intensely than actually usage likely will. That said, your temps may be a little higher than you'd like. This is due to the VCore at 1.392V. VID is, essentially, the amount of voltage that the CPU "requests" while VCore is the voltage actually delivered and it's VCore that matters when it comes to heat. If you look back a c-attack's post, he specifically mentions that, in at least some reviews, the VCore was higher than it needed to be and that the reviewers got better temperatures by reducing the VCore by setting an offset in the BIOS. You could also go into the BIOS and simply disable the Asus Multicore Enhancement and see how that impacts your temperatures; this would run the CPU at the straight Intel spec.

For a good discussion on VCore, VID, adaptive mode and offsets, see http://www.overclock.net/t/1621347/kaby-lake-overclocking-guide-with-statistics. While this is for Kaby Lake, the concepts still apply to Coffee Lake, even if the Coffee Lake voltages may be a little higher due to the 2 extra cores. But you shouldn't need almost 1.4v ... my Kaby Lake is running @ 5.1 Ghz across all 4 cores with a max of 1.36 volts.

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Thanks to both DevBiker and c-attack for your very constructive and supportive input.

I will certainly look at the impact switching off the Asus Multicore Enhancement and a VCore reduction has on temperature and performance. Especially should I wish to ramp up the clock speed.

Your willingness to share knowledge and experience is what keeps the community strong. Thanks again.

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Also ... have you tried updating the BIOS? It's not uncommon for the first revs of a BIOS for a new CPU to have Auto VCore values set a but too high. We certainly saw this with both Skylake and Kaby Lake. It wouldn't surprise me in the least to have it with Coffee Lake as well ... and there has been a long list of BIOS revisions for your motherboard. Your current BIOS version is 0430; initial release was 0408. https://www.asus.com/us/Motherboards/PRIME-Z370-A/HelpDesk_BIOS/
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E-Z update has never worked well for me. Go directly to the support page for your motherboard and OS version. Regardless this is both a combination board and cpu behavior, so a BIOS update alone will not solve it. That is a crazy amount of voltage for that clock speed or any other. No matter what frequency you want to run, you need to set a specific voltage in the BIOS for core. Auto is not going to work. Find an overclocking guide. This is going to be a universal problem so the topic will come up.
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