Question for Aero guys

[AZPete]

My donor radiator has a Main fan and a Sub fan. The Main fan has 5 blades and the Sub fan has 7 blades. Why the difference?
My friend with an ME degree says there must have been an argument between 2 engineers. ;)
My fans work fine but I'm curious about this.

[UnhipPopano]

Are the two fans made by the same company? Most engineers would use the same fan for both and save money.

[AZPete]

Both fans are from my donor, a 2006 WRX, so I assume both are made by the same company. They are in the same car and the factory service manual says they are 5-blade and 7-blade fans, so it's not like one has been replaced before I cannibalized it. Just wondering why 2 OE fans, side by side, are different. They are controlled by the ECM and both seem to go on and off at the same time.

[RM1SepEx]

Both of my fans are the same and they turn on sequentially. So far only my left fan has turned on but it's never very hot here

[904svo]

From what I remember the 5" fan only runs when the AC is turn on and the car is standing still ( no VSS signal) and engine is running (signal from PS pump). When the engine gets hot they both run.

[Bob_n_Cincy]

Here is the fan rules out of the 06 manual that I have.

55921

[AZPete]

Thanks, guys. I understand low speed and high speed and when the ECU runs the fans. That all makes sense. But, I wonder why the two fans have different blades - one has 5 blades, the other has 7 blades. There must be a reason and it bothers me, though my fans are doing their jobs just fine.

[Bob_n_Cincy]

Thanks, guys. I understand low speed and high speed and when the ECU runs the fans. That all makes sense. But, I wonder why the two fans have different blades - one has 5 blades, the other has 7 blades. There must be a reason and it bothers me, though my fans are doing their jobs just fine.

Hey Pete,
I will put my engineering hat on and pull an answer out of my A**.

With 2 identical fans the audible wave form of each fan combine making them loud.
With 2 different fans the audible wave form of each fan will not combine and may even cancel, making them quieter.


Bob
My motto: If you can't fix it, feature it.

[skullandbones]

Hey Pete,
I will put my engineering hat on and pull an answer out of my A**.

With 2 identical fans the audible wave form of each fan combine making them loud.
With 2 different fans the audible wave form of each fan will not combine and may even cancel, making them quieter.

Bob
My motto: If you can't fix it, feature it.

I like that explanation even if you are pulling out of your A**. It is believable.

My best guess is that when the temp is in the acceptable range the 5 blade fan should move the air adequately and quietly. When there is a movement to and above the normal range, the second fan will activate and move more air than the same size 5 blade fan so it will recover the in-range temp faster. If the computer isn't doing it then you could install a set of switches to manually/ auto the process as needed. Also, maybe the ECU is following the rules stated below and you are not observing at the exact moment of that particular event. That should cover everything (ha)!

Good luck. I know you will eventually figure it out, Pete.

WEK.:cool:

[Bob_n_Cincy]

OK, this settles it.

http://www.freepatentsonline.com/5681145.html


Quote: The unequal spacing angles minimize noise produced by the fan assembly.

[AZPete]

Bob, thanks! The engineering report was very interesting and quenched my curiosity. Thanks.
Pete