Cooling Fan RPM Wacky

Running a 2.4 C P4 with a stock Intel (P4 3.06, copper base inset) HSF. Mobo is an IS7. Installed in an Antec case with 2 80mm inlet fans in the front, 2 80 mm exhaust fans in rear, and 1 80 mm intake fan on side right at AGP slot. The Antec power supply has two fans also. With everything running , the CPU fan averages about 3300rpm. Full load temps run at 50 C on the CPU and about 34C System. Fan control is disabled in the BIOS. If I take the side panel off, the CPU fan speeds up to 4600 rpm. And , of course, CPU temps drop. If I put the side panel back on and plug in the side fan, the rpm's drop back to about 3300 rpm. What's up with this ?? If I remove the side fan and put the side panel on, the rpm's still run 3300. If I block the hole ( where the fan would go) the rpm's again rise to 4700. Can anybody tell me what they think is happening here ??

Thanks

It is probably causing a reverse vaccuum. before i had watercooling, I had 3 intake and 1 exhaust, and my heatsink fan did the same thing. You might want to check the CFM on the fans you are using, cause if the intakes are a ton higher than the exhaust, that might be your issue. You could just rotate some of you intake to exhaust and you problem might be fixed.

What are you saying ? Should the fan normally be running at 3300 or 4600 rpm ??

Anyone know what the rpm, on the fan , on a Intel stock HSF for a 3.06 P4 (copper base) should run normally ?

4500 RPM, probably.

I googled "Intel stock fan pentium RPM" and this was the third hit:

http://www.frostytech.com/articleview.cfm?articleID=719

The situation you describe implies that the side panel hole impedes the fan somehow (rather dramatically.) I can only suppose that it sets up an air current opposite to what the HSF is trying to create (?!?)

I don't see any indication that the fan uses sensors, and you claim that the mobo fan speed control is disabled, so that rules out the most likely notion: that air coming in through the side hole cools off a sensor which controls the fan speed by temperature.

the wesson

http://www.intel.com/support/processors/pentium4/sb/cs-007999.htm

"
In addition to the Thermal Monitor feature, the boxed Intel Pentium 4 processor fan heatsink uses a high quality variable speed fan which allows the processor to remain within its operating thermal specifications by running at different speeds over a short range of internal chassis temperatures. The processor fan operates at a low speed while internal chassis temperatures are low. If internal chassis temperatures increase beyond a lower set point, the fan speed will rise linearly with the internal chassis temperature until the higher set point is reached.
"

My best guess is that the your side panel opening is putting cool air on whatever the fan uses for a thermal sensor.

With some fans, you can short circuit the thermal sensor - if it is visible. I have an antec case fan I did that for - I wrapped some copper wire around the two arms of a little thingey sticking out near the hub. Now it runs at full speed all the time.

the wesson

Thanks guys. It appears that the fan itself has a thermistor to control speed relative to temp. 32C and below it runs low speed. Above 40 C it runs high speed. In between these temps it responds linearly. Makes sense. I don't think I want to mod it to run high speed all the time. High speed is ~6000 rpm and that gets quite loud.

You're welcome.

I was a little surprised to learn the stock Pentium HS was thermally controlled, myself. Maybe they had to - a 70mm fan at 5500 RPM will make something of a racket.

the wesson