Ultimate Underclock & Undervolt Project

Do-It-Yourself Systems
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July 17, 2002 by Leo Velikovich

In this article I share my experiences building a near-silent PC with an Athlon XP and GeForce4 Ti4200. The entire system including power supply is cooled by two under-volted 80mm Panaflo fans, which combined produce around 18 dBA No exotic or expensive cooling methods are needed. My goal is to show you that quietness CAN be accomplished even with hot-running, high-performance CPU's or graphics cards. All it takes is determination and a little ingenuity. (Editor's note: A key aspect of Leo's project is the successful modification of a GeForce4 Ti4200 video card with underclocking, undervolting and heatsink replacement for fanless cooling.)

GeForce 4 Ti4200: Now cool and silent

Why did I do this? The cost of noise.

Before we begin, I have a confession to make: I was once on the Dark Side, an overclocking speed addict. I would run a Swiftech Air Peltier just to eke out an extra 10%, and its horrific hair-drier fan noise didn't hold me back. But one fine day, God condensed water on my CPU. I was forced to replace the Peltier with a normal heatsink/fan, and funny how my hearing suddenly improved! Just because I was "used" to the noise didn't mean it wasn't costing me. My costs included a less comfortable environment, difficulty of concentration, and reduced sound quality. I am convinced that many PC users who tolerate noise would be much happier without it. Imagine reading a book with a whiny fan strapped to it; that's the sad present state of computing.

What is underclocking and why do it?

Many PC enthusiasts harbor a vague hostility towards the idea of underclocking (Who in their right mind would SLOW DOWN a fast processor?!) It's a prejudice, which I fight with information:

Yes, underclocking means you run your CPU or video card at a slower frequency. The goal of underclocking is to reduce heat output, resulting in less need for noisy fans. But the beauty of underclocking is that only a TINY reduction in actual performance gives you a LARGE reduction in noise. There are many reasons for this:

1) Massive heat reduction from lower voltage. Any processor requires a certain core voltage; for most Athlons it is 1.75V, for newer Pentium 4s it is 1.5V, and for a GeForce4 chip it's around 1.65V. If not given sufficient voltage, the processor may become unstable at its default MHz. However, if you underclock you'll be fine at a lower voltage. The reason this helps is that total heat output (in Watts) is proportional to voltage SQUARED. This means if you halve the voltage, heat output should be cut by a factor of 4! Simply because I run my 1.75V Athlon at 1.6V means it's only producing (1.6/1.75)squared = 84% of the original heat.

2) Heat reduction from lower speed. Lower MHz automatically means lower wattage. For example, the approximate formula for an Athlon XP/Palomino's maximum wattage is: 20 + .03 x MHz. Try it yourself: a 1600 MHz model produces 68 watts max, an identical model at 1400 MHz only produces 62 watts max per AMD's specs. By underclocking my 1466 MHz Athlon XP to 1400 MHz, I'm decreasing heat production from 64 to 62 watts = 3% less heat (and it adds up). (Editor's note: The relationship between CPU clock speed and power dissipation is linear - Leo's underclocking-related CPU heat drop is actually closer to 4.5%.)

3) Higher temperature tolerance at lower speed/voltage. Consider the Intel Coppermine Pentium III: as they pushed the P3 all the way to 1 GHz its temperature tolerance fell drastically. The 600 MHz version should endure around 80 Celsius, the 1 GHz could fail at 60° C (Intel spec). This is why overclocked processors need so much extra cooling. Underclocked processors may work fine even past their ceiling temperature. This means you have a greater safety margin. At 57-67° C temps, my AthlonXP is well below its 90° C spec, and further below its true limit @ 1400 MHz.

4) Better clock-for-clock performance at lower speed. This is the law of diminishing returns, only working in your favor. The processor may run slower, but the memory and bus still run at the same speed! So the real performance loss isn't as great as the loss in frequency. In reality, Athlon performance loss equals roughly 80% of the frequency loss (average calculated from Anandtech benchmarks). My AthlonXP is running 4.5% slower, but I should only be losing about 3.6% in performance. This phenomenon is more profound in memory bandwidth-limited video cards; frame rate losses might be less than half the clock speed losses.

Now, for a concrete illustration of all this:

EXAMPLE #1: My Athlon XP 1700+ allowed the following possibilities:

Athlon XP 1700+
Clock speed
1466 MHz
1400 MHz
1266 MHz
1000 MHz
Core voltage
Maximum heat(*)
% performance(**)
% original heat

(*) Editor's Note: Leo arrived at the wattage figures using calculations based on his comments in 1) and 2) above:

  • voltage_ratio = (1.60V/1.75V)squared = 0.836
  • wattage_at_default_voltage = (20 + 0.03*1400 MHz) = 62W
  • actual_wattage = 0.836 x 62W = 51.8 Watts

They correspond closely to results obtained via the utility Radiate, which has the AMD MP in its database. MP is identical to XP in its electrical characteristics.

(**) Using the formula: Performance loss = 0.8 x Clock speed loss (average from Anandtech benchmarks for AthlonXP)

At speeds within 1 GHz, the "hot" Athlon XP could run fanless altogether! And with respectable performance, likely far better than VIA's low-power C3. How hot your Athlon runs is completely up to you. (Editor's note: Well, maybe not quite fanless, but close, with a massive powerful heatsink. The C3s run 10W max, typically way less, and even they can't be run fanless in all cases. )

EXAMPLE #2: My GeForce4 Ti4200, which I modified for lower voltage and fanless operation -- I'll explain later. I don't know its true wattage, but here are the remaining numbers:

GeForce4 Ti4200
Clock speed
250 MHz
225 MHz
Core voltage
% performance(*)
% original heat

(*)Using Direct3D 8.1 SDK GPU-stressing benchmarks at 800x600x32 resolution with Quincunx FSAA.

Is underclocking starting to make sense? At a price of 4-7% performance I gained 20-33% cooler components; in combination with highly efficient cooling, this permitted the near-silent system I now have.

On to the mod details...

Contributor Leo Velikovich is an undergraduate computer science student at the University of Maryland, College Park.

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