Basically, let's just say that the scenario is that you're cooling an Athlon XP with a 33cfm fan on your heatsink. For argument's sake, your heatsink's cooling performance is 49°C. Not satisfied, you add a more powerful fan, such as a 68cfm fan, which essentially doubles the airflow of the previous fan you used. Let's say your cooling performance is now 45°C. That is a 4°C improvement with an extra 35cfm added to the mix.
Hearing be damned, you want more performance out of your air-cooling, so why not add another 68cfm fan on top of your current fan? Wouldn't that equal 136cfm? Since we've improved on the original 33cfm fan with an additional 103cfm, does that mean we can expect a 12°C drop in temperatures now?
Well, I'm going to go ahead and flat out say no. If anything, I would expect performance to be worse, but we'll go ahead and test this theory out.
Epox 8RDA+: Athlon XP 2400+ provided by (15x133) 2 x 256MB Crucial PC2700 Ram, Asus GF4 Ti4400, 120GB Western Digital SE 8MB Cache, Windows XP SP1, nForce 2 Unified Driver Package 2.0, Detonator 41.09.
The heatsink used for testing is the Thermalright SLK-800 provided by . For our fans, we'll be using a Thermaltake Smart Fan II (two of them in fact), rated at 75.7 CFM. Nanotherm Ice II is the thermal compound used for testing. To load up the system, we run Prime95 run for 20 minutes, with Folding @ Home running in the background. Ambient room temperature is maintained at ~23C/74F.
As I pretty much expected, performance is actually worse when using two fans stacked on top of each other. Other than that annoying fact, the noise generated by two high-powered fans is horrible.
No doubt, the numbers today may be a bit of a shock to some of you, since we enthusiasts have always had the notion that having more cubic feet per minute equals better cooling performance. The theory holds true, but only when we're talking about one fan.
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The fans spin at a certain speed to achieve their rated CFM rating. The fins on the fan are angled in such a way that it'll draw a certain amount of air into the fan, and blow it out of the other end. We've always banged home the importance of efficient case cooling, and keeping the heatsink area clear to allow the heatsink fan to work close to its potential.
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When you add another fan on top of it, what you're doing is creating more air disturbance than needed. Let's just say that the blades on each fan are not lined up (which will almost certainly be the case), and you turn your PC on. Ideally, you'd want the fan blades to be in sync, like soldiers when they march (left, right, left). The first fan starts spinning, drawing in air, but the second fan is doing the same thing. What is happening is that the second fan is pushing air into the first fan, which just ends up disrupting airflow because the blades aren't aligned.
Therefore, all you end up doing is adding more noise to your PC, as well as putting an unneeded strain on your PSU. You're also probably shortening the life of both fans as well, as both fans are working harder than they should be, and not really doing anything for cooling anyhow.
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