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Pressure drop when reducing a fan outlet

grrl

grrl

Diamond Member
I removed the loud 60x60 high speed Delta fan from my Taisol HS and replaced it with an 80x80 Sunon fan. According to the specs, the Delta puts out 30 cfm and the Sunon 40, yet my CPU temp has risen 3 degrees with this mod.

Can anyone provide an educated guess as to how much of a drop in cfms I should expect when reducing the output of an 80x80 fan into a 60x60 hole? I took an old 80x80x25 fan, removed the guts and fared the inside down to 60mm with clear silicon caulk, so there are at least no right angles or hard edges to create excess resistance.

 
I read a review on one of those "size adapters" and an 80MM delta with the adapter didn`t fare any better than a stock 60MM delta, the only advantage was that the whine was lower in pitch.. strapping an 80MM fan onto a 60MM heatsink without an adapter means that most of the valuable air (produced at the edges of the fan) spends more of its time cooling the motherboard and doesn`t benefit the heatsink, also take into account that the fan-hub on an 80MM is larger than a 60 so there is LESS air getting to the H/S

This is no doubt why 80MM heatsinks like the thermalright AX7 and Swiftech MC462 arrived on the scene, they`re a great idea (and about the best two air-cooled heatsinks from all accounts). they also allow for a slower fan to be fitted reducing the noise level.. Even the Volcano-7+ (another great unit) has a 70MM fan on, but I`d be wary of getting one as 70MM fans aren`t easy to replace and it uses one-lug mounting for the Athlon..

 
Do you have a link for that review? Did they simply put an 80mm on top without an adapter? Because of the design of the collar I made, none of the air simply blows down the outside of the HS, it all gets funneled into the 60 mm hole on top of the HS. Even if the specs on the Sunon are optimistic, I wouldn't expect a 3 degree rise in CPU temp, even with the added resistance and compression caused by the collar.
 
I was dissapointed trying the adapter route as well. Unless the fan is a delta bad boy, it just can't overcome the backpressure created by the adapter. It's like trying to funnel a garden hose through a straw. You need some good pressure to make it work.
To solve my problem I hacked a 120mm hole in the side of my case and put in a Panaflo running at half speed directly in front of the CPU. Temps dropped 7 degrees. Looks b!tchen too!
 
WitchFinder:

Interesting article. But I have a question: Has anyone tried using one of these fan adapters on a shrouded HS/F (Such as a PAL 6035/8045), and then set that
80mm fan to "suck"? This would relieve the back pressure problems, and would allow the fan to work a bit harder to pull air through the HS/F, I would think.
 
I had the stock Athlon HSF on my XP1800+

I ordered the 80-60mm adaptor from Directron (its machined aluminum, anodized blue)

I also added an Enermax adjustable 80mm fan.

I tuned the fan speed to the least audible speed and tried it out.

Before, my max temp was maybe 43 degrees - now the very max is 46 - after hours of 100% usage

Thats still very reasonable and I like the reduced noise.

Most of the time it idles at about 39-41 and when it does go up, it drops within a minute of going to a reduced load.



 
Thanks for the link Witchfinder. I'm not surprised by their results. Even so, like davidrees I may keep the 80mm since I want a quieter computer.
 
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