Cooling fan...

Mike Mulligan · 01-29-2001, 10:09 AM

This might sound kind of dumb, but does anyone know about how many CFM of air the stock cooling fan moves? I know it varies with revs, but I'm just interested in ballpark figures here.

Any help appreciated!

------------------
Mike
'73 914 2.0

DK914 · 01-29-2001, 07:48 PM

Hey Mike,
I'm afraid I'm no help at all answering your question, but wanted to tell you that the engine's out of my car being rebuilt(gonna go 2055 cc and 8.5:1 compression). It should be back together, with enough break-in miles, to make the March 10 auto-X at Qualcomm. Hope to see you there!
Doug Kott

MCLEODRE · 01-30-2001, 03:55 PM

According to the Tech Spec information on this site, the cooling system on the 2.0L delivers approximately 800 Liters of air per sec at 4600 rpm. That equates to about 1700cfm

MCLEODRE · 01-31-2001, 10:09 AM

Last night I was going through one of my Engineering hand books and came across the "Fan Laws". The "Fan Laws" gives the theoretical relationship between a fan's CFM, RPM and Horsepower. This "Law" says that CFM is directly proportional to the fan's RPM. So, If the 914's fan moves 1700 CFM of air at 4600 rpm (per the Tech Specs) then we can calculate:
CFM = 1700 x (rpm/4600)

RPM CFM
1000 370
1500 555
2000 740
2500 925
3000 1110
3500 1295
4000 1480
4600 1700

The "Fan Law" also states that the Horsepower required to drive a fan varies with the cube of the RPM, which means that it will require eight times the HP to double the speed of the fan.
This probably more than you wanted to know about fans.

Mike Mulligan · 02-02-2001, 04:48 PM

Actually that's exactly what I was looking for. Thanks! I guess I could have checked the tech specs myself, but I didn't think of it.

I'm using the info to design a duct, but it occurs to me that it woyuld be quite easy to find an electric fan that flows much more than 1700 CFM. I've seen as high as 2900 in a Summit catalog. It would require some funky duct work, and the wimpy alternator could cause problems, but...

Anyone ever done this?

SteveStromberg · 02-03-2001, 08:42 AM

Laurie Manifold(thats his real name,over 200 wins in the UK) used that on his race car it was a VW-AUDI racing team factory built item they found that after a 30 minutes the engine temp would climb up and get hot and wouldn't go down.If the factory work's team couldn't get it to work and they had really deep pockets,I would try but, don't spend a bunch of money on your dream motor,then bolt your fan and over heat it try it on a cheap motor first.Good luck Steve
www.914underground.com

neilca · 02-03-2001, 01:27 PM

Mike,
I have seen a flat electric fan set up on a 914. Made by some guy in Knoxsville, Tenn. I saw it at a vintage race in Atlanta on not a real hot day but nice. The guy was having problems with head temperatures. I was looking into cooling systems and decided to go with the 911 fan. I have also seen ram air systems from the headlight opening to the stock fan housing used successfully on EP cars in the SCCA. Works great as long as your moving!
Good Luck,
neil

Mike Mulligan · 02-04-2001, 01:40 PM

One of the ideas I was playing with in my head was using two squirrel-cage type blowers that fed directly into the front of the stock engine tin where the stock fan housing normally joins.

I've heard about that flat fan thing somewhere before, Neil, but It sounded a bit marginal to me so I didn't really pursue it. From what you saw it seems that it is a bit marginal since he was having cyl head temp problems.

Yeah Steve, I agree. Run the tests on that old 1.7 before you risk the good motor!