Monday, August 9, 2010
Experimentation Weekend: Pushing the Thermal Limits of the Intel Core i7 920 CPU
Posted by Jason Dunn in "Digital Home Talk" @ 08:00 AM
I spent a few hours this past weekend experimenting with two things: a new EVGA GTX460 1GB video card, and fans on my media production machine. This post is about the fans and the CPU.
When I built this machine last fall, I listened to the advice of the salespeople at my favourite local computer store (Memory Express). They're all very knowledgeable people, but when it comes to thermals, I've noticed one thing: most people tend to be very conservative. The Intel Core i7 920 CPU, for instance, can tolerate temperatures of nearly 100 degrees Celsius. You wouldn't want to push it to that level and leave it there of course, but the point is, Intel built it to take that heat without damage.
In fact, when it starts to get too hot, the Core i7 920 will actually reduce the core multiplier and down-clock the CPU. That's some very smart engineering - yet if you do an online search, you'll find people in forums saying that if you get over 60 Celsius on this CPU, you'll be in trouble. So people put fast, loud fans in their systems, subjecting themselves to a noisy computer - and I don't think they need to.
I love my Core i7 920 system, largely because I can overclock the CPU from 2.67 Ghz up to 4.1 Ghz, and it was rock-solid stable. That extra 1400 Mhz boost in speed really comes in handy when encoding video, because taking a 1080p AVCHD video and encoding it to a 1080p h.264 video will bring any system to its knees. However, I was really displeased with how noisy the system was. When I built it, I put a tremendous amount of research into which fans were the quietest, which CPU heatsink was the best, and even went with rubber drive mounting systems to silence the hard drives. I took the recommendations from my local computer store at face value though when it came to the speeds of the fans, and ultimately ended up with a fairly noisy computer. In my world, if I can clearly hear that a computer is turned on in a room with no other noise, it's too loud.
In this Core i7 system, I had a 120mm 1300 RPM Scythe fan on the CPU cooler, and it was 26.5 dB in volume. The one and only fan to vent the hot air from the case (the Lian Li case has two front intake fans) was a 120mm 1200 RPM Scythe fan, clocking in at 20.1 dB in volume. Curious about what would happen to the temperature of the CPU and the case if I switched to quieter, slow fans, I started my mad experimentation. And by mad, I mean un-plugging the CPU fan, letting the CPU cooler passively cool it, and leaving the case fan in place to suck the hot air off the CPU cooler. I did this from the BIOS level, so I could quickly power down the CPU if things got out of control. Not surprisingly, they did, but it was an important step in establishing baseline temperatures. With my normal (LOUD) fans installed, the CPU would hit 84 Celsius under full load.
I did some testing with a sound metre that and determined that the system was around 55 dB with the original fans installed at full CPU load. I replaced the case fan with a Scythe 500 RPM fan that's a whisper-quiet 5 dB. The fan on the CPU cooler? I went with a Scythe 800 RPM fan at 8.7 dB. Both fans together are quieter than the original CPU fan - but would they provide enough cooling? I was playing a dangerous game here with my fairly expensive CPU, but I was on a quest for quiet computing and figured it was worth taking some risks.
After much experimenting using CPUz, CPU Burn In, and HWMonitor, I discovered some very interesting things. First, my 4.1 Ghz overclocked CPU (which was also voltage overclocked at 1.425 volts) would start to down-clock itself when it hit 95 Celsius. I'd watch the CPU multiplier of 21x drop in CPUz when it hit 95 Celsius; it would drop to 18x, slowing the CPU speed down by 600 Mhz or so. That was my upper limit; if, when running eight instances of CPU Burn In (and Furmark thrown in for good measure), the CPU started to down-clock itself because the temperature got too hot, I was pushing it too hard. I had to lose some speed it I was going to get things quieter.
I ran several tests, and settled on 3.7 Ghz as the final overclocking speed - it's not 4.1 Ghz, but it's a healthy 1 Ghz overclock from the stock speed. I'm able to hit that speed without bumping the voltage on the CPU or the RAM, and it's stable, surviving a triple-pass PCMark Vantage test. It hits 86 Celsius under full load, but the CPU doesn't throttle back at that heat. I'll be verifying long-term stability over the next few weeks of course. The final verdict on sound? With the computer turned off, the room noise is 32 dB. With the new fans and my Core i7 CPU running at 3.7 Ghz, the volume level at idle is 38 dB - so only a 6 dB bump. Under load, it rises only 2 dB to 40 dB total. That's around 15 dB quieter than it was when I started, and the difference is massive - my Core i7 system is nearly as quiet as my workstation, which has a non-overclocked CPU and a passively cooled video card. The Core i7 system currently has a fan-cooled BFG 9800GT, but only because both of the passively-cooled BFG 9800GT cards I was running in SLI mode died...and this is the second time both cards have died.
So, what do you think? Am I crazy for caring more about quiet computing than the long-term viability of my Core i7 920 CPU? What sorts of things have you done in your quest for quiet computing - or do you even care about how noisy your system is?
Jason Dunn owns and operates Thoughts Media Inc., a company dedicated to creating the best in online communities. He enjoys photography, mobile devices, blogging, digital media content creation/editing, and pretty much all technology. He lives in Calgary, Alberta, Canada with his lovely wife, his wonderful son Logan, and his sometimes obedient dog. He wishes the rest of the world valued quite computers as much as he does.
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