Fan Flow for New Overclocked SB i7

I'm putting together a Sandy Bridge Quad Core i7, with 32 GB RAM, on an ASUS P8P67 board, to go in a rackmount chassis. The chassis has two front openings for 120mm fans. The only (pre-existing) exhaust openings in the chassis are in the rear, for two 80mm fans.

I intend to overclock to 4.8 GHz - a stable value - this is not for gaming, but a company computer for a multi-track recording studio. Most of the time, I'll probably never push it to the limit, but when I do, it will likely be for hours at a time.

I need this to be a quiet computer, so I'm using some fans that don't push a lot of air. After reading some of what you guys have suggested to others, I've added an exhaust fan to the top of the case (it's going in a server rack, but luckily, I don't have to cram this in with other equipment right on top of it).

Here's what I computed, based on the fans' specs...

Intake fans:
120mm @ 71 CFM x 2 = 142 CFM

Exhaust fans:
80xx @ 24 CFM x 2 = 48 CFM
200mm @110 CFM
Total exhaust = 158 CFM

Does this sound like a plausible design?
And by the way, is there a CFM computer for this application somewhere?

Thanks, guys.
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  1. You also have to account for exhaust from the PSU...

    It's a plausible design, I ran my old OCed C2D a little negative pressured. You'll be in a rather clean environment, so you don't have to worry about dog hair getting into crevices!!

    I'd say give it a go, keep you eyes on temps for awhile, and adjust the airflow if it becomes a problem. If it's not broke don't fix it (but you do have to try it first).
  2. Ah, yes, forgot the PSU fan. The manufacturer doesn't provide the fan's air flow, but based on their 120mm fans, the average seems to be about 70 CFM. Therefore, I update my example:

    ejbragg said:


    Intake fans:
    120mm @ 71 CFM x 2 = 142 CFM

    Exhaust fans:
    80mm @ 24 CFM x 2 = 48 CFM
    200mm @110 CFM
    120mm @ 70 CFM
    Total exhaust = 228 CFM

    Hmm, seems a bit out of balance, now... by about 38%!

    Maybe I should step down the size of the top fan to 80mm (bringing pressure close to equilibrium) or even 120mm (creating a slightly negative pull)?
  3. Best answer
    I'd go for a 120mm. It's just far more efficient than an 80, especially at the top of the case where heat is rising and pocketing at...
  4. Thanks, Scottie!

    Well, I just found something interesting on - a formula for this calculation. I entered the formula into a spreadsheet to make it easy for anyone to use, but the input temps are in Farenheit. Maybe someone can take this and make a handy little pop-up app out of it, which might be handier than opening a full spreadsheet application every time. Anyway, it's rather simple, (you need to download it, THEN open it)...

    Air Flow Calculator
  5. According to this calculation, I entered the following data:

    I have my sights on a 1000W supply, but don't expect to use more than 850W, really: 850

    Ambient temp of the room (server closet) is usually about 75, but it might top out at 78 F: 78

    Internal case temp should rise above 95 F (is that a decent max target?): 95

    With these numbers, the calculator spits out 158 CFM. So I need to inch it up a bit..

    Intake fans:
    120mm @ 71 CFM x 2 = 142 CFM
    80mm @ 24 CFM (Front bay fan - Vantec HDC-800A)
    Total Intake = 162 CFM

    Exhaust Fans:
    80mm @ 24 CFM x 2 = 48 CFM
    120mm @ 70 CFM (PSU)
    120mm @ 70 CFM (Ceiling)
    Total Exhaust = 188 CFM
  6. That works, and if there seems to be a problem with the pressure, you can always do an exact reverse on the fans and vent through the front as long as there isn't anything else blowing hot air near the back of the case this is in :)
  7. I see. nothing is behind that rack, so it's a possibility. But I saw somewhere else on this thread that it's a good idea to have a bit more "push" than "pull", creating a slight vacuum in the case. Not sure why... perhaps to bring air in through miscellaneous crevices? :??:


    I could create a slight pressure in the case, rather than vacuum, by increasing the bay fan to 120mm (Silverstone CPF51-B) and slightly increasing the overall air throughput in the process. The intake would want to pull in 210 CFS, probably averaging around 190.

    I suspect I ought to stop whipping this dying horse, though. :sol:
  8. You need more Pull than Push to create negative pressure (more air going out than in) and yes, it will pull the air from crevices as long as the case isn't air tight :)

    My old system was negative pressured, now it's positive pressured (more in than out). Just how it worked out on my case when I replaced it.
  9. Thanks for your help, Scott.
  10. Anytime!! Good luck, let me know how it goes!
  11. Best answer selected by ejbragg.
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