Thinking about UNDER clocking my CPU

[Mark Ward]

I've been thinking... I have an Athlon 2400+ in my LMP Case PC, I have a huge thermalright Heatsink and the ZM80A Heatpipe cooler on my Radeon 9500.

Would underclocking my HTPC make the CPU run cooler? Cool enough to not need a fan on the thermalright?

I play no games and very rarely use D-scaler. Almost all use is ThaterTek, Nebula TV card or MediaPlayer so I don't think I'm taxing my CPU's performance potential right now.

I'm running at a constant 50c even when not in use, up to around 56/57c in D-Scaler.

Any thoughts?


Mark.

[JohnS]

Hi Mark,

Much as I think its possible I think you will only sucede if you have at least 1 case fan creating a draught through the machine, on that basis you might as well just stick with the one on the heatsink.

I think you'd have to undervolt the MB and then underclock so that the chip is running around the 1200 mark and then if you wanted to do anything that needed some processor it wouldn't cope.

Quite happy to be proved wrong though.

Regards

[gunrock]

Hi,

It should be easy to have a quick experiment by using the BIOS setup screens to drop your memory and FSB to 100mhz. That's about a 25 percent performance drop. An underclock of this nature will keep you AGP and PCI bus' in spec.

You should hopefully see a drop in temps and test out your rig with some of those activities you normally do.

HTH

Gunrock

[Insp.Gadget]

Are your temp readings for the system as a whole or just the CPU ? I'm running an AMD 2500+ Athlon and even after an hours worth of Dscaler it's only running at 40d according to the BIOS.
Did you use the thermal pad or paste between your heatsink and processor ?
I scraped the thermal pad off and used the Antec silver paste which apparently makes a big difference.

[John_N]

I agree with the remarks about thermal pads. You want to use a tiny quantity of thermal paste like "arctic silver" or similar.

As you reduce clock rate, you can also afford to reduce CPU core voltage a little (not too much!). As said above, if you reduce your FSB to 100mhz and reduce Vcore from say 1.6 to 1.575v (I forget the actual spec Vcore of the various athlon models so you'll have to find out) - you should notice a measurable drop in temperature.

That being said, you will probably need at least one fan in your case to keep the air moving. Whether it is best this fan being the CPU fan, an extra case fan, or just the PSU fan is a matter for experiment.

Maybe you could fit a duct from your PSU vents that directs the airflow past the CPU to the PSU. you can get plastic ducting for extractor fans from B&Q and some silver gaffer tape. That might allow you to get away with using the PSU fan to drag air past the big CPU heatsink.

I'm thinking of going with the Thermalright SLK-900 which is the biggest heatsink that I could find. Before I do that though, I'm doing to experiment with my existing Athlon HTPC in a new case and see what affect airflow, CPU speed, PCI card placement have on performance. I'll keep people posted.
J

[james.miller]

those are some pretty poor temps. im getting 35c dile, 40c load in a case with no fans @ 2084mhz - xp2400 speed.

[gunrock]

Originally posted by james.miller
those are some pretty poor temps. im getting 35c dile, 40c load in a case with no fans @ 2084mhz - xp2400 speed.

Where the hell do you live? Alaska? :confused:

As I type it's currently 30.5 degrees in my study in SE London/kent suburbs and my machine AthlonXP T'Bred B 2200/1.85mhz is currently at 47c idle and 52c full load.

I only have a Akasa Silver Mountain by I have some good airflow in the case.

[james.miller]

ah yes but i have an slk-800. my in-room temps peaked at 35c yesterday:eek: i was at 45c load.

[JohnS]

My XP2100 at stock speeds with a SLK800 and 92mm Panaflo running at 2000rpm is at 47C full load.

What fan are you using James?

[John_N]

Hey John - I thought the SLK800 only took 80mm fans hence the intro of the SLK900 with the extra step for 92mm fans? (I read it in some review somewhere)... In any case - I also read that there was no appreciable difference between 80mm and 92mm fans because a lot of the air bypasses the heatsink and just hits the motherboard in both cases anyway. I would be interested to know your opinion.

I thought about the SLK900 but I've gone off it because of compatability issues with certain boards and having to bend capacitors out of the way. Hence the SLK800 is looking like the heatsink of choice. Hopefully fitted with a panaflow 80mm fan running slowly....

[JohnS]

You can squeeze the 92mm onto the slk800 and it dropped my temps by a degree or 2 but has the additional benefit of blowing air at the surrounding part of the motherboard which certainly cant hurt without any other fans in the case.

[John_N]

Makes sense that.

I'm revising my system spec to SLK-800 with panaflow 92mm fan.

I'm just about to use an XP2200+ thoroughbred 'B' in my new DIGN HV5 case (PSU permitting...).. What case are you using John? I read above you get 47 full load so I guess you're not using a tower case.. :)
Cheers
John

[JohnS]

My is one of the original Digns, even with the lid off it only drops a couple of degrees so I'm not too worried about airflow through the case.

I'm using a 9500 instead of the 9000 that it replaced but it certainly gets a lot hotter than the 9000 so in that respect the 9000 may be a benefit. The motherboard gets quite warm though aboput 42C but I dont know what they should be, I know cooler the better but its been a lot worse previously to the slk800 so I'm not worried.

The only thing to be aware of with the Panaflo's is that most are only 2 wire which means they dont do fan speed monitoring so you wont be able to use the Asus mobo fan speed control if that matters.

The whole thing is not silent but plenty quiet enough that I've stopped worrying about it.

[John_N]

Makes sense that.

I'm revising my system spec to SLK-800 with panaflow 92mm fan.

I'm just about to use an XP2200+ thoroughbred 'B' in my new DIGN HV5 case (PSU permitting...).. What case are you using John? I read above you get 47 full load so I guess you're not using a tower case.. :)
Cheers
John

[Mark Ward]

My System currently idles @ 49Degrees CPU Temp and goes up to around 56 or so with reasonably heavy use, I've not yet got around to a 2 hour movie with DCsaler.

I have a Themalright 8000 which currently has a PAPST 80mm fan running at full speed on it. I've removed my 92mm Panaflow as it appear faulty. I used Artic Silver 3 Thermal Compound but may try removing, cleaning and re-seating to see if any improvements can be made.

I also have a Zalman ZM80A-HP Heatpipe GFX Cooler (man do THEY get hot!)

I think a lot of the problem is due to the lack of airflow through my LMP-PC case.

Tell you what.. I'll remove the cover and leave it a couple of hours and see what difference that makes.

Currently 49DegC doing nothing.

Will report back later.

Mark.

[Assesears]

Under clock why, loads of good h/s & fans on the market now.For £150 you can get a good watercooling kit and silence is golden

[Mark Ward]

Me earlierI think a lot of the problem is due to the lack of airflow through my LMP-PC case.

Tell you what.. I'll remove the cover and leave it a couple of hours and see what difference that makes.

Currently 49DegC doing nothing.

With cover off 42DegC now doing nothing. Obviously I can't leave the cover off 24/7. Is Mid-50s potentially damaging to the CPU?

As I said before I think I will attempt to re-seat the HS, perhaps I'll try a little less Thermal Compound. I don't have a chance in the next few days so I'll just have to accept current temps.

Under clock why, loads of good h/s & fans on the market now.For £150 you can get a good watercooling kit and silence is golden

I have a good HS & Fan, not looking to spend too much more of heat disipation to tell the truth, won't rule it out though, thanks.

Mark.

[james.miller]

remember that the sensors on just about every motherboard know to man are off by a good few degrees. you really cant compare them. what you can do is feel the heatsink - is it hot? if it is then you have problems.

you can try lowering the cpu voltage if the bios allows it - my xp1700 can do stock speeds (1466mhz) at 1.1v - that drops the temps by quite a margin

[Assesears]

First off I would download Motherboard Monitor from here http://mbm.livewiredev.com/.
Check out amd.com the temps being on the high side are although well inside amd specs.Some boards report temps about 10 deg too high, especially asus depending which bios revision you are using.
Get hold of some Artic Silver 3 and use a tiny amount!.

[james.miller]

motherboard monitor is unreliable. its fine if you ALREADY know what the temps are, but since you first have to set it up its basically usless for the beginner.

the onlyt way to do it is to use the bios.

[Assesears]

Thermalright SLK-900 is the King with a Delta 80mm Fan
Real loud though

[gunrock]

Originally posted by Mark Ward
Me earlier

With cover off 42DegC now doing nothing. Obviously I can't leave the cover off 24/7. Is Mid-50s potentially damaging to the CPU?

Mark.

As someone else here said, the temperatures can often be way off. I have an ECS K7s5A motherboard and it doesn't use the on-chip diode of the Thoroughbred B cpu's and is roughly 10c or so too low.

Now if that's the same case with your machine, I would say that running at 62c under full load is not really a problem. Excess heat in your case is not great for any component, but the CPU will not be damaged or degrade in any way over years of excess heat. The only problem is that if your CPU fan died, your CPU would die a lot quicker and with less chance to save it.

Gunrock

[John_N]

........
As a rule of thumb, your CPU half-life will double for each decrease of 10 degrees C in temperature. The relationship is non-linear so this is only rough but if your CPU has 50% chance of failure in X hrs at 90 degrees you can do the maths yourself. The difficulty is knowing when..

Silicon junctions tend to fail at about 180 degrees C. The smaller the junction, the more likely it is to be damaged by electron migration and other effects and the lower the failure temperature. This is why power amplifiers (using large BJT and FET devices with gate lengths measured in millimetres) can run at higher temperatures than submicron devices found in CPUs.
Hence it is important to keep the temps at a reasonable level to get reasonable life from your CPU.

I personally would be uncomfortable running at 62 C. I try not to build any system where the CPU temp exceeds 50 degrees max.

Don't forget though it depends on the time you operate the machine. It's more likely to fail operated 24/7 at 62deg than 24/7 at 52 deg. You halve your CPU life. But if the life was 10yrs anyway then it's probably not going to bother you. Getting these absolute figures are very hard because it's all a game of chance and quantium physics as to when the junction fails.
J

[John_N]

PS. The trouble with the above is that it's very rough. I've just looked at an Arrhenius plot of Mean time to failure of an electronic device against junction temperture.

For this particular device, we have 10,000hrs at 410K and 1000hrs at 460K.
This equates to 10,000h at 137c but only 1000h at 187c. Don't forget that this was not for a CPU but a device with relatively large and robust junctions.

Also, don't forget that the thermal diode of a motherboard measures the case temperature of the CPU at best and sometimes not even that - it measures the air inside the socket. Even when the CPU has an onboard thermal diode, where it is on the die is important. One would hope the manufacturer would position it in a hotspot in the core since some parts of the core run hotter than others.

In either case, an individual junction temperature inside the CPU will be much hotter than the device case temp and hotter than that indicated by the on-die thermal diode.

The manufacturers know this and build in some safety into the specs which is why when we see a safe max operating temp of say 80 degrees for a CPU this builds in some latitude for the junction temperature to be higher than this by some margin.

Therefore the rule is...cooler is better. I personally would try to operate my CPU at T<50 most of the time, if for no other reason than to keep case temperatures down and hard drives in particular tend to have safe operating temperatures of about 50 degrees.