LM fan - cut down, reason? racing?

[carolinefx]

Hi,

I came across a cut=downfan - see photo - it's only cut a little and the cut out matches the bottom side of the turkey roaster pan pretty well. My only guess is that someone cut it to raise the fan closer to the shroud - for better airflow???? or?? Is this a racing/ high performance hack?

It looks like a quality job, not just that the fan rubbed and wore down. Just curious what the reason might be.

Thanks for any ideas!!

[erco]

Pretty sure I saw that as a potential performance mod mentioned either in HTHCE or HTKYCA.

Google found this at both http://www.corvairs.org/Corvairsation/00Aug.pdf and http://autoxer.skiblack.com/fan.html

HiPo Operation.
For higher RPM use, you start to run into the limits of the stock system, but there are some things that
can help:

• A cut down fan will reduce the inertia, and the airflow won't stall at high RPMs.

[Nashfan]

That fan in the picture is an example of what happens when the fan bearing moves upward in the housing and hits the bolts in the shroud. The notches were cleaned up... theres one blade on that fan that is not straight and is for sure cracked. Real cut down fans for racing look nothing like that... they reduce the outside diameter, in some cases the diameter is at the "posts" (ejector pin posts in the blades). The reason they do that is as follows... it was noticed early on in racing that there was a fan noise at high rpms, incorrectly called cavition. It is a bad fan stall or surging (another type of stall). Wrong terminology aside, it was clear that the fan was not flowing well at these high tip speeds, so the easy thing to do was to reduce its diameter. It had the side benefit of reducing inertia and lowering the hp usage quite a bit from what it would be at 7k engine speeds. I did a math study of this not to long ago and it was really interesting. I compared a hypothetical cut down mag fan that was 10" in diameter, weighed .95 lbs at its stock 1.58:1 pulley ratio vs a stock mag fan using a 1.2:1 pulley ratio, and analyzed the 2 set-ups for both "flywheel effect" (hp usage to accelerate ) fan hp usage, and air flow for both. The gist of this was that both fans were at nearly identical power to accelerate the fans, and both fans used around 18hp. For the cut down fan though, if there was no belt slippage, it would be using 24hp and would still be down almost 6% on airflow, but factoring in the belt slippage, it was down something like 16% on airflow to use the same amount of power! In both cases, little fan spun fast vs big fan spinning more slowly, the power required to drive the damn things was 10 TIMES greater than it was to accelerate either of them!! moral of the story I guess is that the cut down fan solved a problem, but it isn't ideal!

[erco]

Oh Jeez, I missed that, Nashfan! I was only looking at the outside edge, which I assumed was cut down slightly from stock, which is the recommended race mod. Those topside notches are definitely not good.

[carolinefx]

Thanks very much for the input. Funny thing: one person made the comment about a bent fin (and probably cracked) - it's actually an optical illusion in the photo I took. I checked that fin, and all others with a straightedge and they are all perfectly straight. weird.

Anyway, thanks for the Knowledge.

;-)