Superquiet Superclocked DIY Core 2 Duo System

Do-It-Yourself Systems
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MOTHERBOARD IDIOSYNCRASIES

The Asus P5W DH Deluxe motherboard is a top-of-the-line product, and is priced accordingly. It has been the favorite of Conroe overclockers, and at the time of this writing (mid-September 2006) has been on constant backorder for over a month due to this popularity.

It does, however, have some oddities, many of them related to cooling and overclocking.

The most glaring problem is the shiny copper-colored covers on the north and south bridge heat sinks (the ones that say "Digital Home" and "ASUS"). These must be removed; they completely obstruct any airflow through what are otherwise pretty good heat sinks. They are very easy to remove.

Second, the stock south bridge TIM is low quality and really should be replaced. This is fairly straightforward. With the board out of the case, use needle-nose plyers to squeeze the push pins on the back of the board, which allows them to slide through the holes in the motherboard. Then use a large flat screwdriver levered on the blue IDE connector to pry up the south bridge heat sink. It will let go with a small "pop".

Pry up the south bridge heat sink
Pry off the south bridge heat sink to replace its TIM (release the push pins first).

a look at the south bridge stock TIM
Here you see why this TIM should be replaced.

Next, replace the TIM. As always when replacing TIM, clean the chip and heat sink thoroughly then apply the smallest amount of new TIM that will cover the chip. The push pins snap through the board and hold the heat sink firmly in place. When done, the heat sink looks much more competent:

south bridge heat sink re-installed on board
South bridge heat sink with cover removed.

If you plan to keep the stock north bridge heat sink, you should replace its TIM as well. The heat sink and related VRM radiator are held onto the board with four push pins. Take care while cleaning the heat sink not to damage the foam spacer that fits around the north bridge chip.

There is a fully illustrated how-to for the above steps here.

The temperature readings on the P5W DH are very unusual. The temperature reported by the supplied PC Probe 2 software, and by SpeedFan, as "motherboard" is actually an on-chip sensor inside the south bridge. The temperature reported as "CPU" is a sensor on the board near the CPU socket, not the on-chip thermal diode or the DST (digital thermal sensor, a new feature in the Core 2). As a result, the reported motherboard temperature is unusually high, and the reported CPU temperature is exceptionally low. Also, there is a bug in Probe 2 (or perhaps the BIOS) that causes it to occasionally report a CPU temperature of 256C.

There are two reasons why the on-board CPU temperature is well below the actual CPU chip temperature. One is that the motherboard has a large heavy copper layer around the CPU socket (called Stack Cool) that draws heat away from the socket very well. The other is that the conduction from the CPU die to the motherboard is quite poor, as shown in this illustration from the data sheet:

exploded view of CPU die, substrate, and socket
Why a sensor in the socket can't accurately report CPU temperature.

The IHS (integrated heat spreader) in the above picture is nickel-plated copper, and is designed to transmit heat to the CPU heat sink, which is mounted above it with an external layer of TIM in between.

The on-chip CPU DST can be accessed with Core Temp, RMClock, or Everest Ultimate Edition, and is highly accurate. However, it is reported as a negative number relative to the (secret) temperature at which automatic throttling starts. Core Temp and Everest use 85C for this temperature, RMClock uses 97C. In some sense, the absolute temperature is not relevant; any good CPU cooling system will prevent throttling. All of these temperatures are well below the 105C junction temperature that semiconductors are designed to withstand.

The remaining issues with this board are with its BIOS. First, very early versions of the BIOS would not POST with Conroe CPUs. This was fixed in release 0801 and all later releases.

Second, many Asus boards overclock the north bridge, a feature called Hyper Path 3 in the BIOS. On the P5W DH, this is done by tying the CPU to the 1066 strap and the north bridge to the 800 strap. This reduces the latency through the north bridge, but causes it to run very hot, and also limits how fast the FSB can be run. To successfully overclock this board, Hyper Path 3 needs to be disabled. There is a detailed explanation of 975X overclocking here.

Third, it is not possible to change the CPU Vcore voltage unless Enhanced C1 and SpeedStep are disabled in the BIOS. This is made explicit in later BIOS versions, but many people had trouble early on.

COOLING THE HARD DISKS AND POWER SUPPLY

The P180 case design separates the power supply and motherboard air flow using two chambers. The hard disks can be mounted in either chamber. The lower chamber has a fan in the middle that pulls air across the hard disk cage and pushes it across the power supply. I put my disks in the lower chamber.

I chose the Antec Phantom 500 "semi-fanless" power supply for my system, knowing that its fan would never come on as long as the case fan was running. The P500 is a revision of the Phantom 350, which is completely passive. This makes the P500 larger than a standard power supply, which requires that the center fan be at most 25mm thick. The stock fan is 38mm thick, and very noisy, so it was destined to be replaced by a Nexus.

To slow this fan down, I installed a temperature-sensitive NMT-3 fan controller from Noise Magic. This controller has an output ranging from 5V to 12V as the temperature ranges from 28C to 42C when connected to a 12V source, and passes the tachometer signal through to the motherboard. Regardless of temperature, it outputs full voltage at startup, to get the fan spinning. This is needed because Nexus fans do not start reliably below 6V. (I also put an NMT-3 on the top case fan.)

Here is what the lower chamber fan and its NMT-3 look like mounted on the mid-chamber baffle, ready for installation.

lower chamber fan and NMT-3, ready to install
Lower chamber fan, NMT-3 controller, and baffle.

The Phantom 500 derives most of its cooling from air flow across its top heat sink. To maximize the effectiveness of this cooling, that area needs to be open, as well as the back part of the case. Consequently, all spare power supply cables should be tucked under the supply.

leave the gap open above the power supply for cooling
Tuck unused power supply cables in the bottom to keep the top open for cooling.

The power supply fan activates when the top heat sink reaches 51C. However, the center fan keeps it well below this, even when spinning as slowly as 400 RPM. The result is silence.

The case has two sliding panels that allow cables to go between the lower and upper chambers. The main one for the power cables has a rounded notch that seals air flow quite well. The second one for the disk cables does not. This is simply rectified with a notch so it can completely isolate the two chambers. Here's what the slider looks like after being notched:

small slider with notch for disk cables
A notch in the smaller slider lets the SATA data and power cables through.

So with just one fan and some cable management, the bottom third of the system is taken care of.



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