how much fan performance loss by going from 12v to 7v?

[Shadowgate]

is it fairly save to scale fan rpm by a percentage if you move from running them at 12v to running them at 7v? say something like

12v = 2000 rpm
7v = 1500 rpm or 75%?

trying to figure out how to get best performance/noise from my fans, and i don't have rpm monitoring capability or i'd check myself

 

[MedicBob]

Your systems BIOS should have a monitoring setup, just use the monitor wire off the fan to the appropriate pin on MB. Cheap, but kinda ugly to do. I don't think it will matter much to change the RPM, other then to hurt preformance. At least with 80mm and 90mm fans, 120 might be worthwhile. Just depends or what exactly you are trying to do.

 

[Shadowgate]

according to some charts i've looked up and a little bit of experimentation, doint the 7v mod to your fans will reduce rpm and airflow by approx 40%.

not positive about rpm but according to sanyo denki's website at 7v you will have approx 60% the airflow of 12v.

all this in the name of moving to bigger fans and then slowing them down to quiet down a system.

 

[WangButter]

7volts / 12volts = 58.3 %

 

[Anubis]

ALOT

thats all you kneed to know

 

[FenrisUlf]

Originally posted by: MedicBob
Your systems BIOS should have a monitoring setup, just use the monitor wire off the fan to the appropriate pin on MB. Cheap, but kinda ugly to do. I don't think it will matter much to change the RPM, other then to hurt preformance. At least with 80mm and 90mm fans, 120 might be worthwhile. Just depends or what exactly you are trying to do.

Actually, this won't work. You're using the +5v as the ground for the fan. The fan sensor is expecting a voltage relative to the normal computer ground. At best you'll have an open circuit, at worst you'll be giving it a 10v signal instead of 5v (Fan sensor uses TTL logic). I've never gotten one to work using the standard 7v trick. I have gotten the monitoring to work by using an LM317 variable voltage regulator to adjust the speed, this keeps the fan ground at computer ground. Fan speed at lower voltages isn't linear - there are way too many factors involving fan design, electricity, magnetic flux, and aerodynamic drag. For a theoretical fan, doubling the voltage will double the RPM, and assuming constant resistance, will double the current draw, resulting in 4x the power usage. It will also double the fan tip speed which will give you 4x the drag and 4x the lift (airflow). However, this is just theoretical - brushless fans have lots of design quirks and it's best to just test them rather than rely on theory for specific data.