The only positive thing with any of them would be that a positive pressure (if a filter is present at the intake) prevents dust from being sucked in through small cracks and holes. Generally you will see much more difference from a planned airflow than from pos/neg pressure. The pressure difference in itself is so very small anyway, so the difference in temperatures due to different pressure wouldn't be noticable.
Negative I think is easier.. Which ever you'll use, make sure you'll fill unwanted holes. They usually ruin the system.
i did some experimenting with that when i built one of my chassis. Overpressure, combined with filters on intake fans, will stop most dust, but filtered fans have a very low throughput, so it will get hotter and/or more noisy. Underpressure is very effective for cooling, one single 120mm 24V standard fan at 12V in the chassi top, combined with the PSU fan, will cool an entire computer as long as the harddríves are mounted in front of holes in the case with a good space between them. But your computer's inside will end upp looking like a low light picture of a winter forest (grey "snow" on everything). But since most computer parts take no immediate damage from dust, i run underpressure and just drag the computer down to the basement once a year and blow it out with compressed air. Don't forget CPU heatsink. Note on doing that thou, even if it looks fun, don't blow high pressure air directly into your fans, you can break them very easily since they can get up to very high speeds quickly.
Positive is better for the fans, especially quiet ones. Generally you won't have enough so it matters, but it's worth keeping in mind. You also avoid having dust getting sucked in cracks.
personally I prefer negitive presure. that way, if an area needs a bit more air-flow, I can just add a duct in. But yeah, dust is a problem, water cooling here I come...
Hate to throw a spanner in the works, but you can't really pressurise a PC case, either positively or negatively. All you do is cause the fans to work against each other and for the airflow to stall. This causes higher temperatures (and more noise, incidentally). Dust in your case is inevitable. The only way to prevent it is to have no air movement through your case at all. And given that your PC gets toasty inside, there will always be air movement, even if only through convection. So best thing to do is to balance those in and out fans, and use dust filters.
Actually, there will be a slight overpressure if you have more fan power blowing in than out, not a lot by the BAR scale, maybe 1.001 BAR, but enough to be messured at least. If there was not a higher pressure inside the case, no air would come out of the cracks and vice versa. So effective dust filters on intake fans and no exhaust fans would prevent dust from entering the case.
Im with you on this one, its just sounds like people are trying to sound techie to make themselves look cool you need to balance you intake with your outake for optimal air flow, more important than pressure is fan placement and figuring out the "dead spots" in your case
negative pressure ALL The way. I run negative presssure on all my cases yes it is true you'll never have a sealed case but that's not the true point its more of how most modern cases are built and the parts inside setup. I always run negative pressure. it aids in convection and flow because the case is underpressured (even by a small amount) it helps the air flow better as the little bits coming in help alleviate hot spots. if your running positive your trying to jam air in to the case (again its not a lot of pressure buts its signficant) the real point is to try to get the air to flow across components if to many fans are pointing in your going to disrupt that current and the air will have more disturbed exhaust paths. on Modern cases usually exhaust fans are located directyl behind the mobo while intakes are several inches forward and infront of the HDD's (which is a good thing). That back fan will draw more air across the mobo than the front one pushing will. (am I making sense?) It is true that its more the design of a case (lots of constriction or fans in the wrong place will be worse) but I still run negative pressure. Also think of OEM computers from my experience Most dells, IBM, E-machines etc are NEgative pressure I don't think they do this on a whim. the only OEM computer off the top of my head that might be positive pressure ( I don't remember) are the G5's but they have an extremely well laid out compartmentalized case that gets plenty of flow. you can always try the experiment where you just switch fans around and look at the temps its not that hard to do.
Intake Ducts and effective fan placement. A minute pressure variance has little to do with the effectiveness of your cooling setup. It's volume and turbulence that matter.
As nexxo pointed out, case fans really don't have enough power to do any measurable pressurization. However I've been told by people that do nothing but design fans that positive is better if it's not equal. Chances are it'll be negligible in a case though. And OEM cases all have openings somewhere to let air in towards the front bottom. They only use a single fan at the back because it's quieter. If you've never checked, they'll run right on the limits of the thermal allowances. Yes they plan them well, but they also design them in a way that's a) cheap to make b) quiet and c) "if it's in spec, it's good enough" Just planning your airflow will give a much better impact to cooling. Generally you want in at the front bottom and out at the rear top.
Same here, i dont think you can talk about pressure when most cases have 3 or more 80mm minimum holes in them even if you have a fan in it and most chassis have openings, cracks and breather holes in the back etc. You can always add a bit more intake than out since you mostly want fresh air in your case as much as possible and always have them blowing on things that makes the most heat (video card Hard drives, and of course the cpu. 1 bad point about the fact that you put more fans out your power supply will start to send his hot air inside the case or worst, it'll stay in the psu.
