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Detailed Analysis of the i7-8700B & i5-8500B - Part 1

A project log for Mac mini 2018 Hexa Core Cooling Analysis

Analyzing the stock cooling in a 2018 Mac mini

michael-obrienMichael O'Brien 02/08/2019 at 08:530 Comments

After a bit of voodoo with Excel, I was able to make some dynamic graphs and simultaneously extract temperature, wattage (power), and absolute thermal resistance from the logs I created. I reached out to a YouTuber, PixelPoison, and asked him to run the same cooling test I did and pass me the data. He was nice enough to do so. I wanted data for the i3 variant, but didn't get a response, though after stepping through the i5 data, I'm not terribly concerned about it.

The 'B' designation of the CPUs is insignificant in this testing, but is included for accuracy.

Overview

Here is what I did. After smoothing the data with Savitzky-Golay methodology, I was able to calm the sporadic nature of the power reporting by Intel Power Gadget but then also smooth the windowed data I generated to identify patterns much more easily. The data is heavily convoluted, but due to how S.G. filtering works, I does an excellent job at extracting the real signal. In a couple of the following screenshots, there is still some temporal aliasing, but it isn't hardly a concern. The following windowed data will be presented as follows:

Average temperature, power, and thermal resistance will be reported for each of the 5 windows at 0.1 °C temperature, 0.01 W power, and 0.0001 °C/W thermal resistance resolutions. Additionally, the data will be ordered in the i5-8500B in the stock configuration, i7-8700B in the stock configuration, and the i7-8700B with a lapped heat sink and MX-2 thermal paste. Overt and semi-obvious observations will be included too.

Due to how tight the graphs get, I'm included the smoothed temperature data, "RAW" and smoothed power and thermal resistance data. Frequencies and fan speeds were recorded, but not included in these graphs. Thermal resistance is the higher of the two plotted data sets and it's y-axis is the one of the right. Temperature (white) and power data are both plotted on the secondary y-axis on the left.

First 12 Seconds

i5-8500B Stocki7-8700B Stocki7-8700B MX-2
Average Temp72.3 °C92.2 °C88.2 °C
Average Power62.53 W97.5 W99.6 W
Average °C/W0.8064 °C/W0.7297 °C/W0.6683 °C/W

The i5 turbos up to 3.9 GHz with no problem and due to the thermal mass of the system, albeit how little it is, no throttling or immediate issues arise. Now, due to the simple nature of calculating absolute thermal resistance, temperature delta relative to ambient divided by [heating] power, you see that the same cooling solution produced different results for all three setups. This will be discussed as a separate topic.

If you look closely, you'll see that with MX-2, the i7-8700 had a longer sustained Turbo Boost level by about 2 seconds relative to stock, which increased the average power by 2 watts, coincidently. If you look closely though, you'll see throttling of Turbo Boost happen about 0.25 seconds in due to temperatures. This data also confirms that the TDP of 65W for the i7-8700 is erroneous.

First 60 Seconds

i5-8500B Stocki7-8700B Stocki7-8700B MX-2
Average Temp87.3 °C94.1 °C94.1 °C
Average Power64.7 W79.0 W88.3 W
Average °C/W1.008 °C/W1.0167 °C/W0.8771 °C/W

This is our first glimpse of things beginning to slow down. The i5 levels out and then the thermal throttling of the i7 begins and shows us the difference the paste makes. THe MX-2 allows for a bit more Turbo Boost and that is pretty evident when looking at the power figures, but even still, the temperatures match each other.

First 15 Minutes

i5-8500B Stocki7-8700B Stocki7-8700B MX-2
Average Temp92.9 °C95.9 °C96.0 °C
Average Power65.6 W72.2 W75.3 W
Average °C/W1.0813 °C/W1.0368 °C/W0.9859 °C/W

Between 5 & 6 minutes, the i5's fan is in full swing and brings the cooling solution into thermal equilibrium as it doesn't heat up or cool down any further. It is pretty much smooth sailing with no interruptions from then on. Amazingly, this CPu never throttles, but it does run damn hot.

However, with the i7, the opposite seems to happen. I don't have time to pull up the fan data at the moment, but since the i7 is overpowering the cooling solution, the thermal behavior is different. After ~3 minutes the thermal resistance of the cooling solution begins to rise under stock TIM and about a minute later for the MX-2. Both begin to plateau @ ~12 min, but aren't fully normalized as you'll see shortly. It goes like "I'm coping, I'm coping, I'm coping" and then is just too fatigued and it just gives out slightly. The effect is subtle, though definite and might be dealing with a slight drying out of the heat pipes since we're just on the edge of their inferred limit, QMax. 

Speaking of heat, when it comes to thermal transfer, it either happens or it doesn't When it does happen, heat is conducted. But when it doesn't that heat is insulated. That's what we're seeing here. The better conductivity of the MX-2 is allowing the thermal energy move into, and thus dissipated, the cooling solution. It isn't by much, but it is noticeable. The dissipated power is aliased directly due to the multiplier, the frequency at which the CPU is running, and the MX-2 is allowing it to be faster for longer, but only just.

Middle 40 Minutes

i5-8500B Stocki7-8700B Stocki7-8700B MX-2
Average Temp92.3 °C96.8 °C96.1 °C
Average Power65.3 W71.6 W72.0 W
Average °C/W1.0784 °C/W1.0455 °C/W1.0308 °C/W

After about 5 minute into this window, the stock TIM vs MX-2 TIM difference is moot. This is because the problem isn't specifically the TIM, but the cooling solution. Here is the funny part: the single, 250 mm heat pipe cooling solution for my HP ProBook 450 G3, which is made by Foxconn, is stated to have a thermal resistance of 1.26 °C/W according to its label. Apple's huge-by-comparison solution is netting ~82% of that. The fin stack radiator is several multiple times the ProBook's fin stack and Apple is using 2 shorter heat pipes (shorter is generally better) too.

Last 10 Minutes

i5-8500B Stocki7-8700B Stocki7-8700B MX-2
Average Temp92.2 °C96.6 °C96.3 °C
Average Power65.2 W71.2 W71.3 W
Average °C/W1.0787 °C/W1.0485 °C/W1.0425 °C/W

So if you're doing some intensive and long duration taxing work, then it doesn't matter if you run the stock TIM or upgrade it a touch. There is less throttling of Turbo Boost happening, but averages and filter show that it's really insignificant. As demonstrated by the i5-8500B, Apple really did design this cooling solution around a 65 W TDP.

Whether in firmware or just the standard limiter behavior, the average temps will not go above ~96 °C for the entirety of the package. With Apple putting in the i7-8700B but not validating the performance of the CPU under sustained load is irresponsible. You get the privilege of spending an extra $200 for a CPU that doesn't even give you a full extra core's worth of performance. More on that later.

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