Earlier this month, AMD laid months of speculation to rest when it confirmed that it would be releasing two new AM3+ processors based on its Piledriver core. The new chips, the FX-9370 and FX-9590, are both 225W parts. Sampling and retail availability are both uncertain at this juncture; AMD initially implied that the chips would only be available from boutique manufacturers, but that position isn’t set in stone.

Still, the announcement got us thinking — why not take a shot at hitting the FX-9590’s 4.7GHz base/5GHz Turbo clock with the FX-8350 (Piledriver core) we reviewed last autumn? That turned out to be something of an adventure, but we got it done thanks to an old cryo-cooler, some bread ties, electrical tape, foam insulation, and a lot of creative verbiage. This article will explain the process and our findings.

Cooling a 225W CPU: Phase-change cooling not required

AMD has already stated that the FX-9370 (4.4GHz stock, 4.7GHz Turbo) and FX-9590 will both have a 225W TDP. That’s far above the standard 115-130W TDP that AMD and Intel have generally stuck to in recent years, but that limitation has been largely self-imposed. AMD and Intel both pivoted away from higher TDPs when the size and weight of newer heatsinks would have caused problems for mainstream desktop shipments, not because it’s impossible to cool a CPU.

We asked premium heatsink manufacturer Noctua if its products could handle CPUs with a 200W+ dissipation. According to company spokesperson Jakob Dellinger, the Noctua NH-D14 and NH-U14S are both capable of dissipating that much heat, though these are top-end air solutions.

Unfortunately, in our case, air cooling alone was not enough to hit the overclock frequencies we were looking for. So we stepped it up a notch.

Setting the stage

When it comes to overclocking headroom, every CPU is different. The FX-8350, unfortunately, wasn’t a particularly great sample. Even high-end air cooling wasn’t enough to hit the clock speeds we wanted. Fortunately, I’ve got something a bit more esoteric — a Cryo-Z phase change CPU unit from OCZ, circa 2007.

Phase change cooling works on the same principles that keep a refrigerator cold. A compressor is used to pressurize R507 (the refrigerant) into a liquid, which is then pumped into the condenser. Excess heat is bled off, and the liquefied coolant is pumped into the evaporator, or coolant head. This is the section of the phase change unit that makes direct contact with the CPU. As the coolant expands, it draws heat off the processor. As it absorbs heat it expands, turning from liquid back into gas. Now in a gaseous form, the coolant is drawn back to the compressor, where the cycle begins again. A single-stage phase change unit using R507 will hit a head temperature of around -50 degrees Celsius.

That said, the Cryo-Z isn’t a shining example of phase-change capability. It’s loud. It’s noisy. I’m not sure it ever shipped commercially in any great volume, and to be perfectly honest, mine has a bit of a short — I had to take the front off to fiddle with the wiring, at which point the temperature readout fell off. It was also designed in an era when CPUs drew less power and is spec’ed for a maximum of 120W. An FX-8350 running at 4.7GHz dissipates about twice that.

My first test got the chip down to -15 Celsius, but keeping the CPU at that temperature under load was a different story. Once the core was fully loaded, system power consumption went from 75W to well over 250W and the Cryo-Z’s temperature went with it. The situation was further complicated by the fact that our FX-8350 wasn’t a particularly good overclocker. The chip wasn’t stable with all eight cores clocked at 5GHz, even when cooled well below freezing.

I cannibalized a second mounting system from an older phase-change unit that had used hard plastic for mounting rather than the semi-flexible plastic on the Cryo-Z. I attached the hard plate underneath the flexible one, secured them together, added additional foam to insulate the socket, and fired the system up again. All eight cores at 5GHz would have been nice, but we ended up backing off that goal and matching the FX-9590’s 4.7GHz base/5GHz Turbo.

All these changes got the CPU down to -23 Celsius. More importantly, it increased the time it took for the CPU’s temperature to push the cooler head out of subzero territory. Even so, we were rather limited in tests we could run. While the CPU was stable at sub-zero temperatures in any workload we threw at it, once temperatures rose past the freezing point, the CPU would crash in short order. This made comprehensive benchmarking impossible.

Consider this a preview of what AMD’s 5GHz CPU is likely to offer — not a comprehensive review or a final word on the product.

Next page: Testing at 5GHz…