Waterblock Buying Hints

Like it says – Joe

SUMMARY: It takes three to tango.

After testing a fair number of waterblocks, I thought I’d share some thoughts on what factors you might want to consider when making a purchase decision.

The natural tendency is to go for best performance – however, there are other factors to consider before taking the plunge:

First, let me reiterate what’s going on here – no matter what the system, the simple fact is that the objective is to move heat from a tiny surface (the CPU core) to a larger surface (the radiator) to dissipate it. Of all the factors impacting performance, the radiator is where size (surface area) does matter.

The design tradeoff, just as in heatsinks, is surface area vs air flow – if you want a compact radiator with good performance, increase air flow over the fins. The problem, of course, is noise – more air flow, more noise. For less noise, go for more surface area and lower air flow – I would suspect that a car radiator with a large house fan at low speed would be a very good performer, although a car radiator is a pretty bulky item to have around the house.

We have shown that water flow is a factor in determining performance; all other things being equal, more flow is better. However, some waterblocks do better at lower waterflow than others – in general, waterblocks that accelerate waterflow over the CPU will generally degrade more as flow decreases than others. Low pressure pin fin blocks seem to be less sensitive to flow variation than others.

However, a look at the ratings show that the variation among waterblocks tested to date is relatively narrow – 8ºC. That difference is evident under 100% stress – in normal use, the CPU is fluctuating between idle and full stress, with the average most likely a lot closer to idle than stress.

This means that at the desktop, the temp variation you most likely will see will be fairly tight – maybe something like 3-5ºC for any waterblock tested to date.

So here’s the balancing act: Radiator size vs Waterpump rated gph vs Waterblock pressure drop. All other things being equal:

• The larger the radiator surface area, the better the performance
• The larger the waterpump, the better the performance
• The lower the waterblock’s pressure drop, the higher the flow rate

It would seem the ideal is low total system pressure with a large radiator and high flow waterpump. It gets a bit more complex when you consider all the possible options:

Put it all together and it comes down to what you expect out of your system:

• Power User: For someone who pushes a system hard – let’s say someone doing lots of sound or video editing – All factors, the waterpump, radiator and waterblock, focus on performance. In this system, going for the best performing waterblock makes sense as the CPU will be in a stressed state a high percentage of the time.
• Low Noise: This is me – if you sit at a PC for hours at a stretch doing “thinking” work, noise can be a big distraction. In my case, writing is simply not a high CPU stress activity; even when I’m using Photoshop, the amount of picture editing I do is minimal and not very CPU intensive. I’m very happy with the original Koolance case powered by an Eheim 1046 as my work system – I never see CPU temps more than 3-5 ºC over ambient temps.

• Good Performance, Moderate Noise: Probably where most systems wind up. For the DIYers, the challenge is choosing the radiator-waterblock-waterpump configuration among the myriad choices available.

I think the first decision is radiator size – the larger, the better. If you want a fully internal system, most likely the radiator will be mid-sized. The next factor I would consider is the waterpump – the larger the better (note that you have to play gph vs head – some waterpumps might have lower flow rates but higher head, which means they may perform better with a high psid waterblock).

The last then is the waterblock, which I think should be driven by waterpump and radiator size.

My own personal bias is to go for a low pressure drop waterblock. A moderate power waterpump (300 gph, 5′ head) and the largest radiator you can tolerate should yield a good performing system with very tolerable noise levels.

Lots of choices when assembling a system – take it one step at a time, start with the radiator and evaluate each component’s impact on total system pressure drop.

Email Joe