Fan Modding

how do you mod fan voltage? what is the ratio for the CFM's DB's when you drop voltage?

There really is no direct ratio since there are so many different variations in fans in regards to fan RPMs and CFM output.

There are several ways to modify fan voltages.

One method is to control the fan speed/voltage is to use a fan controller, or make your own using a proerly sized potentiometer/rheostat this gives you the most flexibility and allows you to speed up or slow down the fans at any given time based on your needs or hearing tolerance limits.

Or you could used a properly sized inline resistor or zenor diode to make the fan always run at a constant fixed speed. However the easiest and most often used method to accomplish this is the 5 or 7 Volt mod refer to this excellent sticky for all the details.

Guide to the 7v Fan Mod

To get voltage mod, usually the simplest way is to use +5v rail instead of the ground rail. With fan between +12v and +5v, you get a total of 7v. You could also try between +12v and +3.3v to get 8.7v total.

To get an estimated CFM, use the formula v/12*cfm where v is the low voltage rail and cfm is the fan's rated cfm at it's rated voltage. For example you want to mod a 150 cfm Delta to 7v, you use this formula 7/12*150 which gives you estimated 87.5 cfm.

You can use other voltage rails ie +5v to -5v to get 10v or +12v to -12v for 24v but overvoltage will shorten the life of fan. Another thing, some fans may not want to spin a really low voltage and requires higher voltage to "start" the fan. Variable speed controller will work on this, turn it up to start it then down to get really low volt/quiet.

pump em up to 24V baby YEAH!!!!
just kidding. but i just have to do that to an old fan i have lying around one day...