[SOLVED] CPU_OPT is slower than CPU_FAN

[PcBuilder845]

I have the be quite! Dark Rock Pro 4 with 2 fans attached. I connected one fan to CPU_FAN and the second one to CPU_OPT (without the Y-splitter). My motherboard is Asus ROG Strix x570-f gaming.

For some reason, the CPU_OPT RPM is much slower than the CPU_FAN. Since they are connected in series I believe that the difference in the RPM reduces the airflow efficiency.

Any idea why the RPM is different? Should I use the Y-splitter and connect both fans to the CPU_FAN?

 

[drea.drechsler]

Did you check the fan custom curve profiles to see how each is configured? On some motherboards the CPU fan follows CPU temp only while a second CPU OPT header can follow any of the motherboard's temp sensors in addition to CPU temperature.

 

[lvt]

Are the fans exactly the same with similar RPM rating?

 

[Phaaze88]

The fans on the Dark Rock Pro 4 are not the same.
https://www.bequiet.com/en/cpucooler/1378

Fan dimensions (mm)	135 x 135 x 22(big fan), 120 x 120 x 23(little fan)
Speed @ 100% PWM /12V (rpm)	1200(BF) / 1500(LF)

If CPU_FAN is running at 50%, then CPU_OPT will run at 50%.

 

[lvt]

The fans on the Dark Rock Pro 4 are not the same.
https://www.bequiet.com/en/cpucooler/1378

Fan dimensions (mm)	135 x 135 x 22(big fan), 120 x 120 x 23(little fan)
Speed @ 100% PWM /12V (rpm)	1200(BF) / 1500(LF)

If CPU_FAN is running at 50%, then CPU_OPT will run at 50%.

If the fans aren't the same so when they are controlled by simply adjusting the voltage, they still turn at different speed based on their own spec.

Simply speaking, 50% speed of a 135mm fan isn't always 50% speed of a 120mm fan.

Even if you control them to exact same RPM, their CFM still are different.

 

[Karadjgne]

Pwm is a % signal. The cpu decides it needs more cooling so raises the signal or needs less so lowers the signal. If the cpu decides it needs 52.5%, cpu_opt and cpu_fan both see 52.5%, as in 52.5% of maximum speed. Which for one fan is 630rpm and the other fan is 787rpm (1200/1500) ± upto 5%, which could mean 1 fan spinning at as little as 485rpm or as high as 863rpm with the same 52.5% pwm.

But as far as efficiency goes, figure that the folks at beQuiet have aero engineers working on fan and heatsink designs, so if they came up with the design for the DRP4, the differences in rpm, fan size, cfm, static pressure, airflow resistance and yes, efficiency too have all played key roles in the design and capacity of the cooler.

 

[PcBuilder845]

Thanks all! My mistake was that I thought that the RPM number is the RPM that the motherboard requests and not the actual fan RPM.

By the way, is there any way to see the percent of the fan speeds that the motherboard requests in the BIOS?

But as far as efficiency goes, figure that the folks at beQuiet have aero engineers working on fan and heatsink designs, so if they came up with the design for the DRP4, the differences in rpm, fan size, cfm, static pressure, airflow resistance and yes, efficiency too have all played key roles in the design and capacity of the cooler.

If this is by design, this is totally correct. I thought the motherboard requests different RPMs...