Guys I have a Antec 300 and a Corsair 750w PSU. Normally in a case with the PSU mounted in the bottom, you turn it upside down. This however is no longer giving me the cable routing neatness that I want. The unused cables have to cross the bottom of the case diagonally to go into the cable nook. So what I want to do is mount the PSU right side up and either drill some holes into the bottom for air flow or, if the 1/3 of an inch of space is enough for the PSU to pull in adequate airflow, I will leave it alone. Here is my rig, ignore the one in the signature.
Core i5 760
eVGA P55 SLI
4GB Crucial Ram
Radeon HD 4870
1 sata DVD
2 sata HDs
Those cables are on the bottom of the case, completely out of the airflow paths. Why is having them run diagonally a problem? You cant see them with the side panel on the case anyway.
Ill try to remember to look in my case tonight, but I dont think you want to "right side up" the PSU without popping a hole in the bottom and putting in a grill with a dust filter. Also maybe get 1inch high feet to give some ground clearance.
All modern power supplies exhaust air out the rear of the psu case.
I have a Lancool Dragon Lord case for my new system. I have a Coolermaster HAF 932 for my emergency backup system. I've also had other cases with bottom mounted cases. You have two choices with a bottom mounted power supply.
First, if your case has sufficient ventilation, airflow, and cooling, then you could mount the psu fan side down so it draws in cool air from the opening in the bottom of the case and exhausts warm air out the rear of the case. The idea is that by drawing in its own supply of cool air rather than drawing in warm case air the power supply will be easier to cool and will last longer. However, do not place the case on a carpeted floor, especially deep pile carpeting. The carpet fibers will block air flow. A few years ago I made my own pc stand with casters.
Second, you could install the power supply with the psu fan on top. The concept is that the psu would help cool the interior of the case by drawing in warm interior air and exhausting it out the rear of the case. This concept goes back many many years when Intel started making cpu's that ran hot. The cpu's did not have cpu heatsinks or fans. In addition pc cases did not have case fans. In the original atx standards Intel specified that psu's should be placed near the cpu so the psu fan could help cool the cpu. Obviously this standard no longer applies. In modern tower cases we now have mutiple exhaust fans near the cpu.
The choice is yours to make based on your own specific ventilation, airflow, and cooling situation.
shovenose - The original spec indicated the psu was to be located near the cpu and exhaust hot air out of the case. That was a very long time ago when cpu's did not have heatsinks and fans. That's also how the power supply wound up located at the top of tower cases.