• Welcome to Overclockers Forums! Join us to reply in threads, receive reduced ads, and to customize your site experience!

Why Such A Dramatic Drop In CPU Temp?

Overclockers is supported by our readers. When you click a link to make a purchase, we may earn a commission. Learn More.

Tomac

Registered
Joined
Mar 8, 2003
Location
Caldwell, Idaho
I recently added a window sidepanel to my new case which has 1x80mm lower front intake fan, 2x80mm upper rear exhaust fans, 1x92mm blowhole exhaust fan and 1x80mm sidepanel intake fan (plus the PSU fan). Lots of airflow but also negative case pressure. I noticed that the original non-window sidepanel w/92mm intake fan would replace the sidepanel under the mobo w/no mods. So, I attached the fan on the outside of the sidepanel (not enough room under the mobo tray for the fan to fit) blowing in attempting to reduce the negative case pressure. Booted up and what a surprise! My cpu temp dropped by a full 5C!
Anyone got an explanation as to why I've experienced such a dramatic drop in cpu temp from this setup? TIA!...
Tomac
 
If your cpu fans blowing down and the 92mm is blowing onto it, You significantly reduced the lost airflow to the resistance of the heatsink. I put a 120mm above my 60mm copper hs and load temps went from 51-41*C ondie.
 
Thx, but what caused the drop was putting a 92mm intake fan *under* the mobo. I already have the 80mm window sidepanel fan blowing down towards the cpu hs/fan (it was originally an exhaust fan but reversing it also lowered temps). I'm trying to understand why putting an intake fan *under* the mobo would cause such a dramatic drop in cpu temp.
Tomac
 
it isnt lower because of the spot it is in. it is lower because you reduced the negative airflow, even if it were up higher it would still decrease the temps dramtically. negative airflow=not so good.
 
schismspeak said:
it isnt lower because of the spot it is in. it is lower because you reduced the negative airflow, even if it were up higher it would still decrease the temps dramtically. negative airflow=not so good.

While I agree that neg airflow isn't good, I disagree with the fact that changing this affects the CPU temp. Neg airflow is said to be bad, because you get more dust in your case, thats it.
I think the reason the temp dropped is that with the blower under the mobo you're cooling the temp probe that is most likely mounted on the mobo itself (no sticking up to the cpu, right?). I bet that your CPU is still close to the same temp as it was, its just the probe that is colder now.

A.
 
Ange said:
While I agree that neg airflow isn't good, I disagree with the fact that changing this affects the CPU temp. Neg airflow is said to be bad, because you get more dust in your case, thats it.
I think the reason the temp dropped is that with the blower under the mobo you're cooling the temp probe that is most likely mounted on the mobo itself (no sticking up to the cpu, right?). I bet that your CPU is still close to the same temp as it was, its just the probe that is colder now.
A.

That would make sense except that the fan under the mobo is actually blowing onto the mobo tray and not on the mobo itself. Perhaps reducing negative case pressure is allowing more airflow over the cpu via the exhaust fans or maybe this new source of intake air is reducing case turbulence??? The mystery continues...
Tomac
 
the only way to know would be to - oh
make a cardboard sidepanel for your standard side
and cut an extra hole for your extra fan.

If the results are still 5 degrees down - then it is the
negative pressure you have fixed.
 
I've basically already tried that. I untaped the vent holes and unplugged the fan so it should have equal air access and temps went up again. Taping the vent holes shut again and plugging in the fan on the sidepanel under the mobo tray resulted in the same 5C temp drop. BTW, this is an actual cooling benefit as internal cpu temp drops 2C for the 5C surface cpu temp drop. Right now I figure it's probably a combination of reduced negative case pressure, reduced turbulance and cooling of the mobo's underside. Any other thoughts?...
Tomac
 
Back