While Dennis is correct, most of us do not wish to do that.
To provide a small amount of further information for Mac128, even when my stock Elina fan was in my SE/30 (which moved slightly more air than my existing Silenx fan replacement), if I left the computer on for a couple hours and then touched the very top of the case with my hand, it was very warm. And again, this is with a fan in there, pumping out hot air continually. The reason is because the amount of heat flowing up is still faster than the fan can pump out. And so the residual heat flows up into that pocket at the top of the case and stays there. I've not left my SE/30 turned on 24.7, but I can imagine that the top of the case would get even hotter if I did leave it on that way.
Remember that the fan on back is fairly low down vertically on the machine. It's not right up there at the top of the plastic. But anyone can do a test, with or without a fan on the SE-series machines. Just turn on the machine and feel the top, then re-test with your hand every 30 minutes for a few hours. You will feel that the top heats up.
"Yes, but isn't it true that the electronics are at the bottom and the fan would cool them sufficiently such that they won't overheat." Well, if you have a fan, then yes perhaps. But again, even with a fan I yanked my internal drive because I have a Diimo 50MHz 030 card along with an Ethernet card that produce more heat than a stock machine. The 7200rpm hard drive was always too hot for me to touch, and it alone became a heat problem, resulting in my removing it. But if you don't have a fan, all that heat up there in that Heat Pocket is going to build at a fast rate, faster than what would be pushed out the little fan hole in the back, so your overall machine is going to get hot, very hot, in a couple hours.
So to the individual who opened this thread I would say this, just test it. Kill the fan and turn it on a few hours and touch your machine and let us know how hot it gets. We can then use common horse sense to make a decision on what you should do. (But I can tell you right now, you really need a fan.)
To provide a small amount of further information for Mac128, even when my stock Elina fan was in my SE/30 (which moved slightly more air than my existing Silenx fan replacement), if I left the computer on for a couple hours and then touched the very top of the case with my hand, it was very warm. And again, this is with a fan in there, pumping out hot air continually. The reason is because the amount of heat flowing up is still faster than the fan can pump out. And so the residual heat flows up into that pocket at the top of the case and stays there. I've not left my SE/30 turned on 24.7, but I can imagine that the top of the case would get even hotter if I did leave it on that way.
Remember that the fan on back is fairly low down vertically on the machine. It's not right up there at the top of the plastic. But anyone can do a test, with or without a fan on the SE-series machines. Just turn on the machine and feel the top, then re-test with your hand every 30 minutes for a few hours. You will feel that the top heats up.
"Yes, but isn't it true that the electronics are at the bottom and the fan would cool them sufficiently such that they won't overheat." Well, if you have a fan, then yes perhaps. But again, even with a fan I yanked my internal drive because I have a Diimo 50MHz 030 card along with an Ethernet card that produce more heat than a stock machine. The 7200rpm hard drive was always too hot for me to touch, and it alone became a heat problem, resulting in my removing it. But if you don't have a fan, all that heat up there in that Heat Pocket is going to build at a fast rate, faster than what would be pushed out the little fan hole in the back, so your overall machine is going to get hot, very hot, in a couple hours.
So to the individual who opened this thread I would say this, just test it. Kill the fan and turn it on a few hours and touch your machine and let us know how hot it gets. We can then use common horse sense to make a decision on what you should do. (But I can tell you right now, you really need a fan.)