Thermalright SI-120

Heatsink Test – Joe

SUMMARY: A top choice for P4/K8 CPU cooling – with the right setup, can be both quiet and aggressive when needed.


Size: 140 x 120 x 88 mm; Weight: 400 grams (ex fan).

The good guys at The Heatsink Factory were nice enough to send the latest Thermalright heatpipe, the SI-120 for a test spin (Thermalright link HERE.)

The SI-120 is designed to fit all known Intel P4 motherboards using the stock P4 heatsink retention bracket and AMD K8 motherboards with a supplied adapter. It’s compatible with AMD Athlon64 FX 3200+ (socket 939/940), Athlon64 3200+ (socket 754) and Intel Pentium 4 socket 478, 2.5 GHz and above.

Looking at this from the front

SI-120 Front

shows five heatpipes coming out of the base. A view from the top

SI-120 Top

shows why this large heatsink is also quite light – good fin spacing and elegant design to hold everything together – good job Scott! In use, the fan overhangs the fins by about ½” – airflow from the fan will also cool components around the socket, including RAM – a real plus with this design.

The SI-120 ships with parts for P4 and K8 mounting:


The mounting clips securely engage the P4 retention bracket – a very secure system. For K8, the supplied mounting bracket is a P4 mount for K8 motherboards.

The base is well finished:


When I ran my nail over it, I could not feel or hear any ridges (strobe lighting accectuates polishing marks).


The Thermalright SI-120 was tested on an Acorp 4S845A motherboard with a modified P4 1500 to read CPU case temps. I tested the SI-120 with a Delta’s 120 x 25 mm fan (WFB1212HH).

I varied voltages to get a performance range for various fan speeds, measuring rpms using an Omega digital tachometer. I measured fan noise with a Radio Shack sound meter 8″ from the fan’s intake. These are not comparable to noise levels reported by fan manufacturers; noise is usually measured at three feet from the fan.


Case Temp
Ambient Temp
Die Temp¹
SI-120, 2994 rpm, 63 dBA
32.7 C
24.8 C
SI-120, 2583 rpm, 59 dBA
32.4 C
23.8 C
SI-120, 2006 rpm, 51 dBA
33.1 C
24.3 C
SI-120, 1511 rpm, <50 dBA
34.0 C
24.5 C
SI-120, 984 rpm, <50 dBA
35.9 C
25.6 C

¹Die Temp as measured by the P4 on-die diode, per MBM.

Interpreting C/W: For every watt (CPUw) that the CPU
consumes, the HSF will limit the CPU’s temperature rise to (C/W x CPUw)
plus the temperature at the HSF’s fan inlet. For example, at an ambient temp of 25 C, a C/W of 0.25 with a CPU radiating 50 watts means that CPU temp will increase 50 x 0.25 = 12.5 C over ambient temp, or 37.5 C. The lower the C/W, the better.

Results place the SI-120 in the topmost rank of P4 heatsinks tested to date (Heatsink Ranking). Of particular note is performance at severely reduced fan rpms – the difference under CPU stress is a minimal 4ºC at 100 watts – truly outstanding. Fan noise at 984 rpm is virtually soundless inside a case three feet away.


Thermalright’s SI-120 is an excellent choice for aggressive or low noise P4/K8 CPU cooling. However, the more aggressive the fan used, the higher the noise level – no surprise.

If you want top performance while gaming but almost soundless at other times, I would recommend using an aggressive 120 mm fan with a rheostat – this gives you the best all-around performance capability. For “normal” use, a fan spinning at about 1500 – 2000 rpm will give very good performance at low noise levels.

Thanks again to The Heatsink Factory for sending this our way. The SI-120 is available now from The Heatsink Factory.

Disclosure: Joe Citarella has a financial interest in a company developing thermosyphon products for electronic chip cooling.

Email Joe