R1 Fan Testing PQ

Posted: March 26, 2009 in Fans

Ok, I rebuilding this thread and making it easy on myself this time around.


The purpose of this thread and intent is to have fun and try doing some fan testing a little differently than has been done in the past. I’ve done enough pump testing to know that a pump’s maximum flow rate and maximum pressure alone are NOT enough to tell the whole story on how well a pump performs. So my intent here is to attempt testing a full PQ curve of the fan to more fully understand how it performs under more than one condition. To do so I’ve built my own little flow chamber and it’s not at all perfect as I’ve tested with it I see several areas where I would improve upon it if I had planned to build another one. Regardless, I plan to test everything with the exact same setup, so regardless of errors…the data should at least be relative to other tests as I’ve learned that’s generally much more important than the numbers themselves.



The test rig consists of an anemometer permanently fixed to the inlet side of a split flow chamber. I’ve then arbitrarily calibrated it to one fan the yate loon slow speed. To calibrate it, I can adjust the anemometer’s flow chamber area and it gives me a direct CFM reading. In addition I’ve installed a Dwyer Magnehelic 0-.25″ pressure guage and installed two pressure tubes aimed and nearly touching the hub of both sides of the fan to measure a pressure difference. This gives me the ability to measure both pressure difference and air flow. To adjust restriction, I simply move a box closer or further from the chamber outlet. In essence this gives me a means to measure pressure difference and flow rate while adjusting restriction to plot out the full PQ or Pressure vs. Flow rate curve like is done with water pumps.

I’m performing three tests:
All tests are done after the fan is allowed to warm up at least 10 minutes.

1a) Low Voltage Startup Test – A quick little test to see what the minimum voltage to start the fan is to the nearest 1.0V.

1) Pressure Vs. Flow rate test – In this test I plot out the full curve, usually at a fixed 12.00V +-.03V. I first start with the chamber flowing freely. I zero out my guage at startup of each test. Then start recording, first at maximum flow (no restriction) and read RPM from my crystalfontz. Then I slowly start adding restriction by placing a box at the outlet and strive for reducing flow rate by about 5CFM increments. I then make another recording of CFM, Pressure, and RPM. I continue this until I begin having CFM read errors. This occurs when the CFM starts to climb as I increase restriction and it due to my flow chamber being too small, but thankfully it’s generally off the curve enough that it’s data that’s not as important. I skip over this area, stop flow completely and get the last reading which is completely blocking the air flow and measuring static pressure. This is the scientific big picture test.

2) Test 2 is done to see affects of voltage on an open air condition. I simply leave the end wide open, adjust voltage and make measurements of amperage, voltage, RPM, and CFM. This test just gives you an idea how voltages changes would affect the fan in an installation like a case fan and it’s more of a practical results test.

3) Same as test #2 except I place a heatercore radiator against the outlet. This is also more of a practical test seeing actual CFM on an dense heatercore radiator giving you some idea how voltage affect results of CFM on an actual radiator type of application.


It does not present noise AT ALL. Let me be clear than this is strictly performance only testing. CFM is however extremely important to radiator performance and this testing shows just how much difference there is between performance levels. In the testing done so far, I’ve seen some fans produce as little as 10CFM on the radiator and some as much as 80CFM. That’s 8X the performance difference.

Is 8X the CFM going to mean 8X the radiator performance?…NO…but it can mean up to 5X the performance difference on a dense radiator. I would estimate the radiator gain is somewhere between 50%-70% of the CFM difference.


In order to make any sort of “Degrees” estimate, you absolutely have to know what your water vs air difference is, I’ll call this the Water/Air Delta. If the air entering your radiator is 20C and your water temperature is 30C, your water/air delta is 10C. On average I suspect 10C is about average, 5C is extreme performance levels, etc.

OK, so now lets use an example. Lets say your water/air delta is in fact 10C with the fans you currently have. Now lets assume that the fans you currently have are producing around 20CFM through a radiator. What would happen if you were able to double the CFM amount you might ask and produce 40CFM. If you did gain 100% performance your water delta would be exactly 10C/1+1 or 5C. But assuming it’s 70% gain, it would be 10C/1+.7 or 6C for a 4C gain.

Lets do it again…say you had 20CFM and increased that by 10CFM. Your result is 10C/1+.7(10CFM/20CFM) or 10/1.35 = 7.4C, so that’s about a 2.6C gain.

Now do it again, say you have a really high performance setup (ie extra large radiator or already higher speed fans). Assume it’s a 5C water/air delta. Assume you have 30CFM (Rad producing) fans and increase fans to produce 40CFM on a radiator. Your gain would be 5C/1+.7(10cfm/30cfm) = 5/1.233 = 4.05C for a net gain of only 1C.

So as you can see, if you’re already very strong in the radiator and/or fan department, extra fan power does little for you….BUT if you’re on the other end, you can really see some decent gains by using stronger fans.


More than anything I’d like people to better understand the power differences between fans. For example, a 900RPM 25mm fan may produce as little as 10CFM on a radiator and a 3000RPM 38mm fan can produce over 80CFM. So that’s only triple the RPM for 8X the performance. It is an unfortunate reality that fan performance falls on it’s face HARD as RPM go into silent territory. Fans are many times more efficient running at obnoxious RPM levels and you really need to understand the give and take. People that want to run sub 1000RPM fan need to face reality and install MANY TIMES the radiator frontal area than someone willing to even put up with moderate noise level. The fan and the radiator are a package performer. If you go on assuming a radiator will perform the same with any fan, you are sorely mistaken, the differences are many fold.

I personally don’t like much noise, but I also run a dedicated quad radiator for my CPU all by itself. If you want high performance water cooling and silence, you simply need to understand that you need alot more radiator. At the same time, someone that has no issue with noise, can be happy knowing that a single 120mm sized radiator with a 3000RPM 38mm fan can have the capability of dissipating more heat than a quad radiator 800 RPM fans.

Bottom line, radiator performance differences are HUGE depending on the fans used, do not overlook the fan component. The FAN USED IS FAR MORE IMPORTANT THAN THE RADIATOR BRAND AND EVEN THE SIZE ITSELF!

Now don’t take this as a “Go buy the strongest fan available”. You won’t like that, just understand how the fan is part of the dissipation package, and it’s going to be a noise give and take. Unfortunately it’s a pretty major give away performance for a take in silence.



  • 38mm fan’s extra thickness doesn’t seem to help until about 2000RPM +
  • Curved fan blades generally produce more CFM per RPM than straight blades
  • Fans with a higher number of fan blades produce more CFM per RPM
  • 38mm fans in general seem to have trouble with undervolting motor noise more than 25mm fans
  • Fan RPM VARIES with restriction and it’s not at all linear
  • Some fans are a fair amount OFF on the specified RPM, hard to know what you’re buying.
  • Fan efficiency significantly increases at higher RPM on all fans.
  • Fan performance is NOT proportional to cost.
  • There are alot of GOOD fans that work well, many times very hard quantify how one is necessarily better than the next. Many times it will simply come down to personal preference on things you just can’t quantify.

Here is the same charts expanded to see the higher speed stuff:

Comments are closed.