Ducting ... out
I made a duct for my computer since I was tired of hearing the whine of the CPU fan.
The duct exhausts hot air directly from the CPU using the case fan.
At first with the duct I was using a generic heatsink, and standard clock, and no CPU fan, and overall everything was much quieter but at the expense of about 4C temp rise at the CPU.
When I wanted to overclock my Thunderbird 200 MHz to 1600 I got a better heatsink (Vantec Aeroflow) and decided to use the CPU fan included with the Aeroflow, a nice TMD fan.
Currently the CPU fan is running at 3100 RPM (full power would be 5500 RPM) and it is reversed (upside down) so that it draws air thru the heatsink and blows it into the duct, where the case fan helps blow it out.
I think that reversing the heatsink fan makes it *slightly* less efficient, but on the other hand, the case stays much cooler so the CPU can much better shed heat through the socket, so that's slightly better. All in all, I think it's a wash - the only real difference between ducting-in and ducting-out is that with ducting-out you don't dump CPU heat in the case and your case can stay much cooler without a lot of case fans going (I am using x2 80mm low-speed fans for intake.)
With this setup, 3200 RPM fan, under "normal" use (browsing, etc) the CPU temp is read as 47C and under 100% load it is read as 52C.
5500 RPM fan, at 100% CPU load CPU temp is read as 48C.
This is with the socket thermistor (no diode in Tbird) and Asus Probe, however accurate they are (not very.)
"Motherboard" temp (whatever that means) shows as about 2C above room temp. 24C at the moment - 75F.
In any event, I am overclocked, stable, and quiet.
I also discarded the chipset fan - this is only a 133 MHz chipset, so its not really necessary.
So, overall, using the duct for "exhaust" has worked fine for me.
I don't know how you'd get an exhaust duct around the base of the heatsink - that sounds like tricky placement.
Anyhow, I'd like to hear other opinions about reversing the heatsink fan to enable blowing CPU air directly out of the case.
By the way, Dell p4's sometimes have no CPU fan and just use a case duct over a rather large heatsink, suckng air out from the CPU ...
The wesson