Underclocking v.s. undervolting? Celeron 2.4 w/o fan?

zordex · Post by **zordex** » Sun Jan 30, 2005 12:30 pm

Hi all,

I'm about to set up a server where I don't need a lot of power. I'm aiming for a celeron 2.4 with on-board video, Antec SLK3000B, couple of hard drives. Maybe the ASUS P4SP-MX all-in-one motherboard (I've been an Asus loyalist for a while). For undervolting I understand Asus is a no-go.

[u]My question is:[/u] What are the differences in heat results between underVolting and underClocking? If I underClock my celeron from 2.4 to 1.2 (for example) would underVolting still be recommended? Isn't the CPU the main heat issue? Are there other benefits to underVolting over underClocking? (I could not find this info in the FAQs)

[u]Second question:[/u] How much would I need to underclock (or undervolt) a 2.4 celeron to be able to run without a CPU fan at all? (probably just a good heat sink). Is this mostly tiral and error until the temperature is acceptable? Has anyone done this?

Thank you!

the_smell · Post by **the_smell** » Sun Jan 30, 2005 4:04 pm

Hi Zordex, I'm no expert but so please ignore me if you want...When you say in your first question:

Isn't the CPU the main heat issue?

I should point out that undervolting as it's usually talked about is directly affecting the cpu temperature.
Q1: Underclocking (reducing the frequency your cpu runs at) is typically used in combination with undervolting (reducing the voltage your cpu is supplied with or its vcore) to drop cpu temperatures. I've never done just underclocking as it would seem a bit pointless as you can run cpus at full speed with a lower vcore and thereofre lower temperatures. So yes undervoloting and underclocking together are recommended and are essential is you want to run any moden cpu with passive cooling (appart from specialist cpus like the via C3, AMD geode and maybe Intel centrinos).

Q2: You can get a good guide to what vcore and cpu speed you need to go for using the calculator at:
http://www.benchtest.com/calc.html
Get cpu specs from intel
http://processorfinder.intel.com/script ... ProcFam=49
This suggests a vcore of 1.2v and an underclock to 1.2GHz should be ok for around 19 watts at full cpu useage. This is within the 20 watts which you generally need to passively cool with a good heatsink and maybe a little airflow from a psu fan. But it will need a bit of trial and error when you put the system together, especially since you have to decide on what temperatures you are happy with.

One comment about the components you've chosen, it will probably be easier and maybe cheaper (if you are buying everything new) to use a mobile athlon XP (2400+, 2500+ or 2600+) undervolted and underclocked since they can be used with normal althon motherboards. Or if you've got an old socket 370 pentium 3 motherboard around use a via C3 cpu with it. Sorry this has been such a long post, hope it's useful!

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Mats · Post by **Mats** » Sun Jan 30, 2005 5:07 pm

zordex: Running fanless is not that big deal in general. Since you're going to have more than 1 HD in there it's not that big problem with the noise from the CPU fan. I run my Athlon XP 2500+ at 1.4 V (<45 W) which still won't let me run fanless, but I can run the fan at lowest setting. This makes it the most low noise part in the computer, the HD and PSU makes more noise. The fact is, when I turn off the fan with software (Speedfan) I can't here any difference! So what's the point with running fanless? Besides that, the CPU fan is supposed to make some air flow around the power regulator components which you won't get when running fanless.

Running undervolted is another question. It reduces the heat coming from CPU and PSU (since it will deliver less wattage). This gives lower noise output. On the other hand, I'm not sure that you will notice any difference with that setup.

I couldn't find C 2.4 in that link from the_smell, try CPUPower insted.

You can go for a C 2.4 GHz with an ASUS mobo and it might work out well. You just underclock it to 2.0 or 1.8 GHz and run the CPU fan at minimum. Now to the most important part: CPU heatsink. Since you have such an urge for running fanless I'm wondering if you've heard a good CPU heatsink fan at lowest speed. You should use a Zalman CNPS 7000A ALCu or CNPS 7000B ALCu, you won't get disappointed!

My recommendations:

CPU: AMD Mobile Athlon XP, either rated for 35 W (2400+), 45 W (2400+, 2500+) or 47 W (2600+), nothing else with higher wattage!!! Just grab the cheapest one you can find. Check out Fab51 for the exact model codes (OPN).

Heatsink + fan: Still Zalman.....

Motherboard: Well it's mostly a choice between VIA and nVidia. Don't just look at those with integrated VGA, if you have an old PCI graphic card lying around it will work just fine. This gives you just more to choose from. Make sure it fits the Zalman cooler. Check the undervoltable motherboards list. And don't get a mobo with a fan on the chipset!!!

Mats · Post by **Mats** » Sun Jan 30, 2005 5:27 pm

Some VIA motherboards can change multiplier on the fly and some nVidia can change FSB on the fly, and Vcore. Still don't know any VIA mobo with that latter function, so I guess nVidia is preferred. Check this site for compatibility with 8RDAVcore for nVidia, and here for CrystalCPUID (VIA). Do some more research to find the best mobo. Post again before you buy the mobo want to go with!!

Edit: Remember to get a mobo with locked AGP/PCI frequencies, especially for nVidia.

Tibors · Post by **Tibors** » Sun Jan 30, 2005 5:33 pm

This post talks about some undervoltable ASUS mobo's. They use some windows utility.

widman · Post by **widman** » Sun Jan 30, 2005 7:19 pm

hi,
To reduce the heat you need undervolt and underclock.
Here my case
CPU Intel 2.4B stepping C
Motherboard Gigabyte GA-8PE800(-L)
Ambient 29C (tropical)
Presscot HSF 5200rpm

FSB 133MHz (x18 = 2.4GHz)
VCore 1.50V
idle 39C
load 48C

FSB 133MHz (x18 = 2.4GHz)
VCore 1.20V
idle 37C
load 42C

FSB 100MHz (x18 1.8GHz)
VCore 1.50V
idle 38C
load 45C

FSB 100Mhz (x18 1.8GHz)
VCore 1.15V
idle 37C
load 41C

I can't run stable prime95 @133Mhz, 1.15V.

I think underclocking is much less effective than undervolting. But you can achieve lower volt by underclocking.

I can run passively using presscot HSF @ 100FSB (1.8GHz) 1.15V loaded at 65C, but still with case fan. I'm living in tropical country so ambient temp always around 28-31C. But it's no way I can achived it at 1.5V.

GrahamGarside · Post by **GrahamGarside** » Mon Jan 31, 2005 10:57 am

first you can't just underclock an intel to what ever you wish, you need to have one that runs at the 533mhz fsb and then run it on the 400mhz fsb. I have a celerond d 320 (2.4ghz) and I rewired the bus selection pins to give 1.8ghz and also the vid pins to get 1V, at these settings it doesn't need a fan during idle to moderate loads, and only needs a very low amount of air during full load, it's a 70mm fan and I think it's at less than 500rpm, it is really quiet.
If you want a good budget underclockable cpu then I recomend a socket 754 sempron, a tad more expensive than the celerons but performs better and thanks to cool'n'quiet much more versatile

zordex · Post by **zordex** » Tue Feb 01, 2005 8:57 am

Thanks for all the information everyone!

Looks like I got a little more research to do now... I'll post what I've decided when I do.