Fan behaviors Nexus and Slipstream 800 (impedance effect )

Control: management of fans, temp/rpm monitoring via soft/hardware

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Fan behaviors Nexus and Slipstream 800 (impedance effect )

Post by thejamppa » Thu Jul 23, 2009 1:02 pm


I ran intresting thing today. With my new Scyte KazeQ fan controller 5v~12v fan range +/-10% I started testing fans.

Scythe Slipstream 800 rpm @ minimum 450~420 rpm. Around 5v. Slightly over. Nexus in free Space 545 rpm's around. Well with-in 10% tolerance. Nexus in heatsink and minimum speed rose around 620-640 rpm's. Quite intresting. When I closed case door Nexus fan speed rose into 660 rpm's +/-10 rpm's. (Sample variance difference was 13-24 rpm's between two nexus 120mm rs )

Similar thing happened with Another Nexus fan too. Heat sink and case closing seemed to rise minimum rpm's around 100 rpm's. I wonder why.

I have top Down Noctua C12P heatsink and case is Solo.

All Scythe fans seems to operate well with-in the 10% margin but Nexus fans seem to... operate a lot higher than they ought. Maybe heatsink has something to do but I expected maybe 600 rpm's maximum with 10% error if Nexus 120 RS' fan speed is around 490 rpm's @5v.

its intresting that Scythe fans are just few dozen rpm's what they ought to be compearing to SPCR fan testings. Nexus fans are nearly 200 rpm's higher. This makes me puzzling.

As mentioned Nexus fans were around 50 rpm higher in free space what SPCR tests @5v were: 490 rpm's. Against dop town heatsink and closed in case minimum rpm still grew nearly 120 rpm's...

Just wondering if Back pressure has anything to do with this... Or what.


I made tests with 800 RPM Slipstreams which ran in air 810~824 rpm so variation was 13 rpm's.

Now on heat sink fan stayed about 810 rpm's
Exhaust was 815-824 rpm's when side panel was off. When side panel was on Heat sink is 807-813 rpm's
exhaust is 880-890 rpm's.

Simply closing the side panel in solo put about 60-70 rpm's more on exhaust fan without touching the manual control's. That is quite leap just closing the side panel...
Last edited by thejamppa on Sun Jul 26, 2009 12:18 pm, edited 1 time in total.

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Post by Sylph-DS » Thu Jul 23, 2009 3:00 pm

Are you one hundred percent sure that the power input did not change? (for example if you perhaps plugged something into the PSU on the same line that might change it slightly.

If you are sure, then the only thing that can be different is the air resistance on the fan blades.

I'm guessing that by "in heatsink" you mean in the kind of heatsink that sandwiches the fan between two towers like this one for example? If this is indeed the case then I can imagine that the fins of the heatsink straighten out the airflow, decreasing turbulence and with that the air resistance on the fan blades.

As for closing the case, do you have any air intake fans? (perhaps you filled up the two 92mm fan spots in the lower front of the solo?) Because if that's the case then the fan might be pushed ahead a bit by the positive pressure the two 92s generate.

Still, it puzzles me as well, am quite surprised that it has such an enormous effect..

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Post by thejamppa » Thu Jul 23, 2009 3:09 pm

I have two Nexus real silents near 5v running ( lowest on KazeQ ) but I have no rpm sensoring them. Only real explanation ( that I can think with my poor physics knowing ) is that resistance air resitance / impedance changed. But I would have expect them to slowdown fans and not increase exhaust fans speed.

Since they are manual controlled and I did not touched case I simply closed the case its intresting.

I doubt closing side panel simply increases voltages of fan controllers one channel...

Edit: at minimum speed heat sink fan runs 504 rpm's and Exhaust runs 550 rpm's when side panel is closed. When side panel is opened exhaust fans speed drops to around 500 rpm's.

2nd edit:

When case is closed and two front fans are minimum rear exhaust is 880-890 rpms.

When both front fans are full, the rear exhaust drops in 830 rpm range.

With Single front fan full Exhaust is around 855 rpm range.

2nd edit:

with case open I make Hand test. I place my hand very close or rear exhaust. Before hand its 813-824 rpm range at full tilt. With hand close and covering most of the fan and blades it rises over 850 rpm. I get it temporarily 900 rpm's simply putting my hand close to it... So as impedance grows, Slipstreams spin faster... But I wonder how as I haven't touched my controller...

3rd edit 26.7.2009.

Today I discovered what was causing my fans spin more thaought to should have:

I had placed sorthobane foam blocks in empty 5½" spots to prevent dust being sucked in from the front bezel ventilation slits and act as extra noise barrier. When I removed sorthobanes case exhaust fans drop immediately 120-150 rpm's falling perfectly into their respectable fan speeds 5v-5,5v in +/-10% margin.

Impedance in case affects greatly at least low impedance fan like Slipstream while having lesser impact on higher impedance fan like Nexus 120 Realsilent Basic.

Solo does benefit empty 5½" slots somewhat it seems, especially when using Slipstream's or similar low impedance fans.

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Post by Rebellious » Sun Jul 26, 2009 5:38 pm

Could be turbo effect from pressure change when you close the door. I noticed that in my TJ08.

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Post by RoGuE » Sun Jul 26, 2009 6:14 pm

this is textbook fluid dynamics.

Have you ever sucked a rug up into a vacume cleaner and heard the motor spin up really fast? That's the same thing thats going on in your a lesser degree.

When the airflow is restricted in any way, the fan has to do LESS work, believe it or not. It has less air to move..and with less air to move, the power has to go somewhere, so as a result, the fan RPM is speeding up. This is of no benefit to you however, because now, not only is the fan moving faster, and louder, but you have a lower airflow than before.

It is no mystery that fans speed up when they are more restricted, and it is just a matter of reducing or eliminating that restriction.

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Post by thejamppa » Mon Jul 27, 2009 1:47 am

Thanks guys! That makes perfect sense.

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