Do I even need an exhaust fan? - Corsair 650D pos. pressure

[Celoth]

UPDATE: No I don't need an exhaust fan

As the title says, do I even need an exhaust fan in a decent positive pressure system, as long as metal grills are cut?

Currently working on a Corsair 650D (case pics) for a new positive pressure build (overclocked Sandy Bridge i5-2500K). Measuring and cutting two-layer 12mm sound-dampening foam for the entire case, and will also remove all the metal grills by intakes and exhausts. All intakes will use DEMCiflex dust filters, which are rated for 15% CFM loss. Top intake will also sport a wire grill for extra protection, but other than that nothing but the dust filters.

The picture below is not my setup, it just gives you an idea of the case.




In my setup I will be using the following fans:

• 200mm Front Intake: CoolerMaster Megaflow 200 - rated as 700 RPM, 110 CFM, 19dBa
• 120mm Top Intake: NoiseBlocker Multiframe S-Series MF12-S1 - rated as 750 RPM, 34 CFM, 8 dBa
• 150mm PWM CPU Fan: Thermalright TY-150 - rated as 500~1100 RPM, 38~84 CFM, 19~23 dBA

The layout I am considering is this:


By using that barrier to section the bottom of the case off, the entire airflow from the front intake should be better directed at the GPUs and CPU. The AX850 PSU runs its own little airloop anyway. The GPUs are unknown still, so I don't know how much warm air they will be putting back into the case, or if they push it out back.

My original plan was to put a temperature controlled Noctua NF-P14 FLX (140mm PWM, 750~1200 RPM, 42.5~66 CFM, 10.1~19.6 dBA) as rear exhaust fan, but with all the CFM coming in, and the TY-150 on the CPU cooler, I am thinking perhaps it isn't necessary, and might even be detrimental because it can't keep up with the airflow. I know the rated CFM numbers from companies aren't always accurate, but assuming they are to compare intake and exhaust, I'll be getting over 100 CFM into the case, taking the dust filters into account as well, and the Noctua will struggle with that at low RPMs.

The CPU cooler (Thermalright Archon) is doing just fine with a single fan in push, also at low RPM (those results are with the TY-140), rivaling even the Silver Arrow that uses 2 fans, so the Noctua as exhaust won't do much good there either:

[Celoth]

After more reading, I am indeed going to try without exhaust fan, but will add another intake fan in the 5.25" bay:

[Nakkor]

How is this build coming along? I'm considering the 650D for a workstation/gaming rig, and I'm very interested in hearing about your results and findings!

How are you planning to mount the 120mm 5.25" bay fan? I'm thinking about perhaps getting a Scythe Kama Bay.

My planned hardware: ASUS P8Z68-V Pro/GEN3, i7-2600K OC @ 4.x GHz, 8GB 1.25V RAM, MSI N580GTX Lighting, 120GB SSD, WD Green 3TB HDD, Corsair AX850 PSU. I haven't decided on which CPU-cooler to get yet, but I'm leaning towards one of the top performing/silent tower coolers, or perhaps the Prolimatech Genesis.

[Celoth]

It's working out great!

I have a work log at the overclock.net forums, and I am currently doing the whole overclocking thing. Still only using the onboard GPU, but I can pretty safely say the positive pressure thing without exhaust fans can work really well.

I have a stable 4.5GHz overclock, where the CPU fan doesn't spin above 800 RPM at full load with 20C ambients. All I can hear at night with low ambient noise is the faint whoosh of air that pleasantly fades into the background. It's not silent, but it is very quiet.

Specifically about the 5.25" bay intake, I did this (copy/paste from work log and sorry about bad picture quality. My ancient, crappy camera can't focus up close):


5.25" Bay Intake

For the 5.25" bay intake, I used some of the separate 2mm bitumen mats that came with the Cooltek kit, and made an insert for the front panel. The fit is so precise I can wedge it in there with slight pressure and not have to stick it on. That makes removal in the future a lot easier. The brown-ish frame is the magnetic frame that comes with the standard 120mm DEMCiflex filter. The blue stuff on the wings is just the plastic film covering the adhesive side:

Without filter


With filter


I've put the insert in the bay and lined the bay with the sound dampening material. This way I can place the intake fan further inside the bay to reduce noise, and prevent it from sucking in the air already inside the case. Note, I haven't actually stuck any of the pieces on there yet, I'll do that after I am sure of the cable management.

Front:



Back: If you look closely, you can see a little cut-out in the bottom foam. This is where the SSD goes. I am not gonna have a regular HDD in the case at all.



Here I've added the Noiseblocker Multiframe S-Series MF12-S2. It's wedged so tightly in there I don't need anything else to hold it in place. I can't even hear it at 1250 RPM when it's in there unless I put my ear right next to it. At 50cm the sound is gone. It is, simpy put, the best damn 120mm fan I've ever come across.

Front:



Back:



With the filter:

[Celoth]

I got my 4.6GHz stable overclock as well, with decent temps (peaked at 65C with a vcore of 1.408v). The TY-150 (CPU fan) mostly hovers around 800 RPM at 20C ambient at full Prime95 load. Sometimes it goes up past 820 RPM. Very quiet system. I ran IntelBurnTest as well to test max temps, and I peaked at 75C with the fan going 900 RPM, which is still in the fairly quiet zone.

So bottomline, no, I don't need no steeenking exhaust fan! Less noise for me!