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Thermalright SI-128: Evolution of a Past Master

The concept behind Thermalright’s venerable XP-120 heatsink has evolved considerably in the two years since it was released. The idea of a 120mm, top-down heatsink using heatpipes to distribute the heat was quite novel when we first encountered it, and Thermalright has reused the idea in the SI-120, and now the SI-128. The SI-128 is bigger and taller, but the basic concept is not that different from the XP-120.

February 1, 2007 by Devon

Thermalright SI-128
Socket 775 / K8 CPU Heatsink
Market Price

Thermalright is a name that consistently appears on best heatsink lists all
over the web. They’re not cheap, and they make some of the best heatsinks around
— including several that have spent time on SPCR’s recommended list. In
fact, the Thermalright Ultra-120
is currently one of the top-performing heatsinks we know of. It’s so good that
we have to wonder: Why bother selling any other models? The answer is every
product has a finite life cycle, and new products are needed for every company
to maintain or grow sales.

Most current Core 2 Duo chips run cool enough that just about any heatsink
will do. That’s great for users; less heat means less noise and smaller, less
expensive heatsinks. It’s not so good for manufacturers like Thermalright though.
Why buy a US$50 heatsink when the stock heatsink will do the job just as effectively
— and just as quietly — without shelling out any extra cash?

In these market conditions, it’s not so easy to figure out where the market
for the SI-128 lies. The SI-128 costs US$50 — without a fan — but
it’s smaller than the Ultra-120 and is unlikely to match it in performance.
Fortunately for Thermalright, there’s one market sector that has a use for small
heatsinks: The home theater market. Chances, it will be here that the SI-128
has the best chance of success.

The usual plain cardboard box.

Thermalright likes to do things a bit differently from other companies, and
this is apparent even in the way it packages its products. Like all Thermalright
products, the SI-128 came embedded in a nest of custom-cut foam inside a sturdy
brown cardboard box. It is well insulated against bumps and shocks.

Another Thermalright quirk is the stock fan — or lack thereof. Thermalright
is the only company we know of that sells heatsinks alone without cooling fans,
even though they are not designed for fanless cooling (the
fanless HR-01
is an exception). Presumably, Thermalright products are so
exclusive that their customers like to choose their own fans.

Look, ma, no screws! (two fan clips are missing from the photo).

Thermalright SI-128: Feature Highlights (from
product web page
Feature & Brief Our Comment
120mm fan recommended for maximum cooling capability.
Fan not included.
Huge 8mm multiple heatpipes for even more efficient heat transfer
to the aluminum fins.
Are bigger heatpipes better? Thermalright
seems to think so.
Multiple tightly gapped fins for more surface area with longer
and wider wingspan than SI-120.
More surface area is indeed good for heat dissipation… but high
density requires higher airflow (read: noisy) to force air between them.
The best heatsinks feature a good balance between the number of fins and
the space between them.
Vast compatibility across multiple platforms for newest AMD
and Intel CPUs.
Support for older sockets, such as Socket
478 and Socket A has been dropped.
Enormous wingspan gives extra cooling
to MOSFET or NB chipset
This is especially important for passively
cooled high end motherboards that often rely on system airflow for cooling.
Includes adapters for LGA775, K8, and
Here’s where they say exactly what “vast
compatibility” means.
Thermalright: Specifications (from the
product web page
Heatsink Dimensions
L125 x W145 x H91.5 mm (heatsink only)
510g (heatsink only)
Recommended Fan
All 120mm Fans
INTEL: All Intel P4 LGA775 processors
AMD: Athlon 64 / FX / X2 / Opteron all models


The SI-128 is the sequel to the SI-120, which in turn evolved
from the XP-120: One of
the first heatsinks to use a 120mm fan and a longtime SPCR favorite. The basic
idea is the same: A bed of fins suspended horizontally over the CPU socket,
with the fan blowing down. What’s different? The SI-128 increases the depth
of the fins and widens the “wingspan”, with a corresponding increase
in surface area and — hopefully — cooling power.

We say “hopefully” because the increase in surface area
has also increased the airflow impedance. The gap between the fins is much tighter
than the previous models, which means a faster fan is probably needed to utilize
the full cooling potential of the SI-128.

It’s big and blocky: You can barely see the base under the fins.

The SI-128 is fairly tall for a “top-down” heatsink, especially when
the space required by a fan and its airflow needs are added in. At a little
over 90mm tall, it probably needs about 130mm above the surface of the CPU to
be effective.

Two support struts give the SI-128 a very sturdy look and feel.

The SI-128 gets by with surprisingly few heatpipes: Four. But, they’re four
extra large ones, with a hefty 8mm diameter. Do larger heatpipes carry more
heat? Only Thermalright knows for sure, but it seems likely enough; the cross-sectional
area of an 8mm heatpipe is almost double that of the more common 6mm variety,
which means the internal volume is probably about twice as much for heatpipes
of similar length.

A glamor shot to make the heatpipes look bigger.

In spite of its exceptionally shiny appearance, the SI-128 is not made of any
exotic materials. It’s an aluminum-copper hybrid, just like almost every other
heatsink out there. So, whence comes the shiny metallic appearance? The copper
base and heatpipes are nickel-plated, smoothing out the appearance and leaving
a higher-end, more finished look.

Fin spacing is quite dense, which makes us worry about low airflow performance.

The base has the painted, lustrous smoothness of a nickel-plated surface —
or at least it appears to. On closer inspection, there are actually hundreds
of tiny ridges that can be felt with a fingernail, but seen only with great

The base looks smooth as a mirror, but it’s faintly textured with tiny ridges.


The mounting system is similar to other recent Thermalright models we’ve seen.
Two mounting brackets are included: One for Socket 775 and one for all K8-based
sockets. A custom retention module is included that allows the K8 bracket to
be used on AM2 systems.

The Socket 775 bracket uses the same pushpins found on Intel’s stock heatsink.
Although the pushpins are designed for ease of use and toolless installation,
installing the SI-128 on our Socket 775 test bed was a bit of a wrestling match.
There were two issues that made the installation process a less-the-ideal experience:

  1. The clip was very tight, requiring a lot of pressure to secure in
  2. The tops of the pushpins were hidden under the bed of fins, making it very
    difficult to generate the pressure required to push them down.

After several brute force attempts to get all of the pins depressed simultaneously,
we eventually decided to get smart by securing the hard-to-reach pins first
and using a piece of scrap metal to gain enough leverage to force the remaining
pins in place.

Given these difficulties, we probably could not have installed the SI-128 if
our test-bed was installed within the confines of a case. For practical purposes,
the Socket 775 installation requires removing the motherboard from the place.
On the plus side, Socket 775’s symmetrical mounting holes meant that we didn’t
have to worry about orientation, so we were able to situate the heatpipes where
they would be least likely to cause compatibility issues.

Thermalright’s bracket uses Intel’s unique pushpin mounting system.

Although most AM2 systems already have retention modules that should work with
the SI-128, Thermalright includes one of their own to ensure compatibility.
Interestingly, Thermalright’s
includes mounting points for Socket 478 heatsinks. The retention
module does not come with either a backplate or mounting screws; presumably
these are reused when the stock module is removed.

A generic retention module is included for AM2 systems.

Installing the SI-128 on an AMD system appears to be quite a bit easier, although
we did not have an opportunity to test the system thoroughly. A single S-clip
fits on to the standard retention module loosely and is then tightened by a
quarter turn of a single lever (no tools required!). The S-clip is designed
so that the heatsink can be rotated up to 90° under the clip, allowing the
heatsink to be installed in whatever orientation is appropriate — even

The K8 bracket is shaped in an S-curve that permits the orientation of the
heatsink to be shifted as necessary.

A simple lever puts the clip under tension.


Wire fan clips.

As long as you’re using a fan with open flanges, mounting the fan is a simple
process. Two fan clips fit into into holes in the fins and snap over the inner
frame of the fan, effectively clamping the fan to the heatsink without requiring
screws. There’s even a couple strips of silicone strips that be laid down between
the fan and the heatsink to help cut down vibration noise.

However, if you have a fan with closed flanges, as in the photo below, the
flanges need to be cut open to give the clips something to hold on to. This
is easy enough to do with a
hacksaw and a pair of pliers
, but those who aren’t handy are advised to
choose their fans with care.

Make sure you use a fan with an open flange!


On the test bench…

Testing was done according to our
unique heatsink testing methodology
, and the reference fan was profiled
using our standard fan testing
. A quick summary of the components, tools, and procedures follows

Key Components in Heatsink Test Platform:

Test Tools

  • Seasonic
    Power Angel
    for measuring AC power at the wall to ensure that the
    heat output remains consistent.
  • Custom-built, four-channel variable-speed fan controller, used to
    regulate the fan speed during the test.
  • Bruel & Kjaer (B&K) model 2203 Sound Level Meter. Used to
    accurately measure noise down to 20 dBA and below.
  • Various other tools for testing fans, as documented in our
    standard fan testing methodology

Software Tools

  • SpeedFan
    , used to monitor the on-chip thermal sensor. This sensor is not
    calibrated, so results are not universally applicable; however,
  • CPUBurn
    , used to stress the CPU heavily, generating more heat that most
    realistic loads. Two instances are used to ensure that both cores are stressed.
  • Throttlewatch
    , used to monitor the throttling feature of the CPU to determine
    when overheating occurs.

Noise measurements were made with the fan powered from the lab variable DC
power supply while the rest of the system was off to ensure that system noise
did not skew the measurements.

Load testing was accomplished using CPUBurn to stress the processor, and the
graph function in SpeedFan was used to make sure that the load temperature was
stable for at least ten minutes. Every fan was tested at four voltages: 5V,
7V, 9V, and 12V, representing a full cross-section of the fan’s airflow and
noise performance.

The ambient conditions during testing were 19 dBA and 20°C.



Thermalright SI-128 with Reference fan
Fan Voltage
°C Rise
22 dBA@1m
~19 dBA@1m
<19 dBA@1m
<19 dBA@1m
Load Temp: CPUBurn for ~20 mins.
°C Rise: Temperature rise above ambient (19°C) at load.
°C/W: Temperature rise over ambient per Watt of CPU heat, based
on the amount of heat dissipated by the CPU (measured 78W).
Noise: SPL measured in dBA@1m distance with high accuracy B &

The SI-128 performed acceptably well at every level except 5V, when the temperature
began to jump up. A cooler processor (including most Core 2 Duo chips) would
probably be fine with good system airflow, but hotter chips will probably require
more airflow.

Besides, there is little acoustic difference between 5V and 7V when using our
reference fan, so there is very little benefit to running the fan this slowly
anyway. At 7V, the cooling performance was good enough for the majority of mainstream
processors. Bleeding-edge overclocking chips like the QX6800 or the FX-74 will
probably require more airflow — but these are exceptions, not the norm.

Performance at higher airflow levels continued to improve temperatures a bit,
but the improvement came at the expense of increased noise from our reference
fan. Given the level of performance already achieved at 7V, there’s probably
not much point in running an increased fan speed.


Heatsink Comparison: Heavyweight heatsinks &
SPCR’s 120mm Reference Fan
Fan Voltage
°C Rise
°C Rise
°C Rise
°C Rise

A comparison against the current high-end heatsinks and Thermalright’s old
XP-120 confirms the SI-128’s position. For most of the lower-airflow points,
the SI-128 performed no better than the original XP-120 that is two generations

The one point where the SI-128 did perform significantly better was with the
fan at 12V, which suggests that the SI-128 was held back by our slow reference
fan. The densely spaced fins are not ideal for high performance with low airflow,
and it’s quite likely that we did not have enough airflow to reach the SI-128’s
“sweet spot” during our testing.

Some readers might balk at the above comments and ask why a higher airflow
fan was not used. The answer is that in our heatsink reviews, we’re always interested
not in ultimate cooling performance, but performance at low noise levels. With
a high speed fan, the SI-128 may provide the best cooling of all the heatsinks
mentioned, but this is irrelevant for SPCR, because the cost will be too high
a level of noise.


Reference 120mm fan: 5V-7V-9V-12V, 5s Ambient between
: One
, One Foot


Scythe Infinity: 5V-7V-9V-12V, 5s Ambient between
: One Meter,
One Foot

Arctic Cooling Alpine 64: 5V-7V-9V-12V, 5s Ambient between levels: One Meter,
One Foot

Scythe Mine w/ stock fan: 5V-7V-9V-12V, 5s Ambient between levels: One
, One Foot

Big Typhoon:
5V-7V-9V-12V, 5s Ambient between levels: One
, One Foot


These recordings were made
with a high resolution, studio quality, digital recording system and are
intended to represent a quick snapshot of what we heard during the review.
Two recordings of each noise level were made, one from a distance of one
, and another from one foot away.

The one meter recording is
intended to give you an idea of how the subject of this review sound in
actual use — one meter is a reasonable typical distance between a
computer or computer component and your ear. The recording contains stretches
of ambient noise that you can use to judge the relative loudness of the
subject. For best results, set your volume control so that the ambient
noise is just barely audible. Be aware that very quiet subjects may not
be audible — if we couldn’t hear it from one meter, chances are we
couldn’t record it either!

The one foot recording is
designed to bring out the fine details of the noise. Use this recording
with caution! Although more detailed, it may not represent how the subject
sounds in actual use. It is best to listen to this recording after you
have listened to the one meter recording.

More details about how we
make these recordings can be found in our short article: Audio
Recording Methods Revised


With a properly chosen fan, the SI-128 is capable of cooling most mainstream
processors very quietly. The K8 clip is elegant and quite easy to use; the issues
with 775 socket mounting have more to do with Intel’s choices than Thermalright’s.

However, in a market where the top-dog processors consume no more than 65W
and most heatsinks are designed to cope with 130W loads, the low noise, low
airflow performance is no longer as impressive as it once might have been. The
fact is that there’s no shortage of stiff competition, especially at US$50.
When the cost of the fan is taken into account, the SI-128 doesn’t look like
a great deal.

The heatsink market as a whole has evolved past the point where performance
is the deciding factor. The SI-128 has good build quality and
a convenient K8 mounting system, but that may not be quite enough to set it
apart in a crowded marketplace. It’s quite good — like a dozen other heatsinks
we’ve reviewed.

As noted in the introduction, it will probably find most of its buyer in the
home theater market, where price is no object and tall, high-rise heatsinks
are counted out by their size. The SI-128 is just short enough to fit in a HTPC
case that supports full-size expansion cards. And, given how poor the airflow
is in many HTPC cases, the SI-128 may have a real advantage over the more compact
heatsinks out there.

Enthusiasts, overclockers and gamers who don’t mind noise may also have fun
using a high speed fan with the SI-128. There’s little doubt it will perform
better with a higher speed fan.


* Performance with slow, quiet fan good enough for most CPUs
* Elegant K8 bracket
* Provides VRM and MOSFET cooling
* Socket 775 bracket is awkward and too tight
* No fan included
* Poor value-for-money for silencers
* Not compatible with closed-flange fans

Much thanks to Thermalright
for the SI-128 sample.

* * *

Articles of Related Interest

Recommended Heatsinks
SPCR’s Unique Heatsink Testing

SPCR’s Standard Fan Testing

Thermaltake Big Typhoon
Heatsink / Fan

Scythe SCNJ-1000
Ninja Heatsink
Thermalright XP-120:
First 120mm Fan CPU Heatsink
Thermalright Gets
Back on Top with the Ultra-120

* * *

on this article in our Forums

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