Why Static Pressure & Max Flow Specs Are Poor Measures of Fan Performance

Posted: February 18, 2013 in Fans

This is just a quick graphing demonstration of why fan box specs using static pressure and max air flow are generally poor tools in comparing fan performance.  Very much like pumps, fans have a pressure vs flow rate relationship that is not at all linear.  Some are flat some are curved, some have big bumps and dips and it’s the area between these two points that ultimately matters, not what is at the ends.

For example, let’s compare these two fans using box specs:

Cougar Vortex PWM


Max Airflow = 70.5CFM

Static Pressure = 2.2 mm H20

Gentle Typhoon AP-15


Max Airflow = 58CFM

Static Pressure = .08inwg = 2.03mm H20

Using these specs alone you would assume the Cougar would produce more air flow on a radiator.  Both max air flow and max pressure are greater, you can’t go wrong with that can you?

Well, luckily both of these fans do provide P-Q curves(Most do NOT), so I spent some time cropping out their P-Q curves, converting units and overlaying them.  First here are their native P-Q curves in different units of coarse making a comparison difficult.


So I brought both into Excel and plotted them out the best I could, here is how that looks:


In addition noise box specs are “Open Air” with no restriction and no adverse “Real World” affects when mounted to a radiator.  Since most fans out there don’t bother publishing P-Q curves, you essentially don’t know what there real world performance will be and the noise specs don’t mean much on a radiator either.

So, just goes to show you…don’t rely too much on fan specs, they are not very good measures of real world performance.  Just like pumps, it’s the pressure somewhere between these two points that matters in the real world and that isn’t static pressure and it isn’t max air flow, it can only be represented by a full P-Q curve.

And my intent here is not to pick on any one fan. I would like to try some cougar fans out myself considering they are supposedly producing this curve at only 1500RPM, but the curves were available so I wanted to see for myself how the PQ curves compared.  Sure enough it produces a very contradicting conclusion vs. looking at only static pressure and max air flow.

Hope this muddy’s the waters for the box spec discussion out there…:)


  1. Steve says:


    You do some amazing work when you start wondering about something (Cougar fans) to get to the bottom line. Who would have figured a little square fan spinning around could be pretty interesting to read about what makes it tick and how it performs in the real world vs. what the box says. You are indeed a valuable resource to everyone!

    Where did you get the Restriction Pressure Drop line, from a radiator test? And why did you pick between 30/40? Just wondering, no big deal as you are a bit over my head anyway, I did get the AP-15 was a better radiator fan though.

    One other thing since we are talking about fans, read through both pdf pages, pretty cool stuff. About that PWM signal they showed. I read that a 4-pin fan will work on a 3-pin connector, just at full speed if the 12v is not varied, much like a 3-pin fan will run at full speed on a 4 pin connector. They show the duty cycle of the PWM signal high @ 5v in the pulse. If a PWM fan wants to see 5v on that pin to pass the 12v and fire up, then how would a 4 pin fan work on a 3 pin connector if no signal was there?
    The only thing I could think of is the duty cycle is really the bottom of the pulse at 0-.5v, effectively ground or not their perhaps. I guess if I had an old fan I could give it a ground and 12v then hit the PWM wire with 5v and then ground it and see what the fan did after all the smoke cleared :).

    • Martinm210 says:

      The pressure drop curve was just hypothetical but it is a pretty good guess. On my round 6 testing I measured the GT15 producing around 36CFM through an MCR120 plus my test rig. It is difficult to get exact CFM values though since the air speed varies across the port opening a little bit.

      4 pin fans do undervolt like 3 pin fans, but I don’t understand PWM at any detail. I do know that you need a scope to measure PWM signal which are expensive and a tool I don’t have..:)

  2. Steve says:

    Never mind that stupid PWM thing, that logic in the is probably pretty smart and if the line is a float then it just passes the 12v line to the motor and if it sees .5 then it knows to look for 5v to control the motor. Sometimes I type faster than I can think.

    I read your review of that Sunbeam Rheosmart, very cool. Too bad it does not work so good with a high current draw (your pump). I read a review at New Egg on it and another person was having the exact same problem with the one with 5 controls. You could replace the 3 power transistors with something rated for 60 watts then throw a small fan on it to keep it cooler maybe.

    • Martinm210 says:

      Yeah, I’ll have to tinker with the sunbeam when I get these kit tests all done. I want to PWM control the NZXT 140mm case fans I have on the switch 810.

  3. Steve says:

    Just wanted to follow up on my dumb questions on PWM, looked into it this morning and sort of know more about it but not about our fans we use. There are many types of PWM, not just one. The duty cycle could be either the high state of the cycle or the low, or even just the leading edge in a saw tooth wave. All of our fans work the same, but it still did not answer my question if they use the high or low as the duty cycle.

    In any case the advantage of PWM over voltage control is less lose of power in the controller. So the motherboards lose less power controlling PWM fans than 3-pin fans, it’s all good. Guess that is why most new MB are using 4-pin controllers for fans, less power consumed and less heat.

    I don’t think the motor cares either way, all it knows is 12v so it’s efficiency is unaffected I assume. But the controller in the fan must use more power for the PWM circuit. So I am guessing here of course, but the power used to spin the little fan may or could be just about the same either way. PWM is just shifting the power loss and heat from the MB to the fan. And maybe PWM control has better/finer adjustments than voltage control, who knows.

  4. Luca says:

    pwm is not stupid (pulse with modulation), with pwm you have about full torque power from the engine across the speed variation. The problem is that speed variation is not linear since friction is not linear and when relative speed is low there is also static friction that is far more high than dynamic friction.
    pwm diagram http://c1.neweggimages.com/BizIntell/item/35/553/35-553-002/PWM-Function.png
    voltage diagram https://koolance.com/files/products/diagrams/ctr-cd10bk_o2.gif

    pwm is not linear, so you have never accurate speed regulation, but can have lower rpm (due to higher torque) and higher efficiency.

    • Martinm210 says:

      I don’t think he meant PWM was stupid, he was just exploring a reduced voltage idea in regard to one of the fan documents. They refer to something about 5V but I think it just has something to do which the voltage used in the PWM signal or something. I think we all appreciate the benefits of pwm.

      I really like PWM particularly when it comes to pump control. This is what I got on the fans and pump for PWM response on the H220:

  5. Martinm210 says:

    FWIW I have found that PWM usually has a larger RPM range for pumps, but not fans. Fans seem to vary a bit. My Titan Kukri fans for example that I used for rad bench testing would go no lower than about 900-1000rpm using PWM control. With voltage control I could get them down to a out 550rpm so I ended up using voltage control instead.

    Pumps have really shown the greatest range. Something like the 35x has a much lower RPM than taking a normal DDC3.25 and voltage conrolling.

    In the end it must depend on the PWM controller built in which I don’t understand much at all.

  6. Steve says:

    Luca, I did not say PWM was stupid, was referring to my dumb assumptions on just how they worked as apposed to what they did. Perhaps knowing this would answer some questions about what we are seeing while testing them.

    I found the PWM Specification for the fans we use on our systems, it is real good in simple terms and answers some questions about low speed operation.


    It sure answered many of mine anyway. The duty cycle is indeed the high state of the pulse, if no PWM signal is present the fan shall operate at 100% (which should be greater than PWM control as fan specs are PWM specs, not the motor in the fan, so with no PWM wire it should spin faster than spec as PWM delivers 98% of power at 100% signal).
    What is interesting on this specification is how they handle low speeds. I think it explains why you can get lower speeds with voltage than with PWM control. There are 3 ways a fan handles minimum speeds with PWM, of course voltage is straight forward.
    Another thing I was wondering is if the controller in the fan averaged the PWM signal and fed the motor a linear voltage, it does not. It gives it a sawtooth 12v and lets the motor average it out.

    So PWM fan performance depends on 3 things, blade design / how well the motor can use the sawtooth 12v and the design of the PWM circuit itself. This may explain the difference in the “P-Q” chart of the GT and Cougar, maybe anyway .

    Still trying to figure out a simple solution to make that Sunbeam work for you. Ebay has some cool PWM controllers but they are in China and Hong Kong, too far away. Found many circuits you could build yourself but that would eat up a lot of time.
    Thinking now perhaps adding a power amplifier to each channel voltage out wire may do the trick, but it would change the speed profile, and not sure if the rpm feedback would mess things up. Will work on it some more.

  7. MorroW says:

    Good reading 🙂

  8. cfus says:

    I just built a standalone controller for my pump and fans. A four wire PWM modulated fan or pump uses the fourth wire as a control signal. It is a 5v open collector source comming from the board in the fan. The motherboard grounds this pin with a frequency of about 25kHz from 0-100% duty cycle. Connecting the pwm pin to ground is essentially 0% duty, and the fan will run at minimum speed. Disconnect the wire for 100% duty and maximum speed.

  9. Sounds Cool! How are you using it?