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Discussion Starter #1
What is the ideal gap around the perimeter between the end of the blades and the shroud ?

For example, a fan that is 18.5 wide- would a shroud with an 18.75 opening work or is that too tight ?

Thanks-
 

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Too tight. As the engine moves on its' mounts due to torque the fan will hit the shroud. The opening in the shroud needs to be 1"-2" larger than the fan.
 
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Just as important is that the fan blades sit about 1/2 way into the shroud.
 

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In addition to what PetesPonies mentioned, for good if not best efficiency, the fan should be positioned within 1" of the radiator surface. Also, 50/50% coolant is still a popular recommendation.
 

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In addition to what PetesPonies mentioned, for good if not best efficiency, the fan should be positioned within 1" of the radiator surface.
Not when using a shroud. The 1" rule is for no-shroud installations, which by the way IS a rule I learned from a Ford engineer. When using a shroud, the fan could be a foot from the radiator with no negative effect. Look under the hood of a Corvette sometime. Any Corvette.

 

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Not when using a shroud. The 1" rule is for no-shroud installations, which by the way IS a rule I learned from a Ford engineer. When using a shroud, the fan could be a foot from the radiator with no negative effect. Look under the hood of a Corvette sometime. Any Corvette.

That's not exactly true, but the key point is that the distance from the radiator to the fan is not critical as long as the whole distance is covered by the shroud and you have the shroud properly placed 1/2 in 1/2 out of the shroud. Ideally the fan blades should be as long as possible to almost touch the shroud for maximum efficiency, but as was earlier noted, in a car you have to leave space for the fan to move as the engine flexes on its mounts.

The shroud must also fit well to the radiator. The goal is the ensure close to 100% of the air moved by the fan comes through the radiator, not around it.
 

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Hmmm, a foot? While that may work in the case of early vettes. I've had good, if not excellent, success following the rule of "within and 1" of the rad surface and with the 50% of the blades within the shroud".
 

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Engineering info

Here's an old engineering document on the subject. I've had it forever and I think it was put out by Caterpillar in the 70's.

There are probably a lot of necessary exceptions due to mounting and spacing constraints of engine / fan / shroud / radiator. Particularly to account for relative movement between them. But you could take these as general best practices and increase spacing where needed.
 

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Alright, so attaching the image didn't work so well. Too blurry. How do I get it to appear in the post?!?! Using an iPad. Thanks!
 

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Hmmm, a foot? While that may work in the case of early 'Vettes. I've had good, if not excellent, success following the rule of "within and 1" of the rad surface and with the 50% of the blades within the shroud".
Sure, but on some cars it's just not possible.

As far as "may work", well, it worked on every Corvette made from 53-82. No alternative, as the radiator was slanted.

But as I said, with the fan centered both ways in the shroud opening, the distance from the radiator means nothing. There's a lot more than 1" here:

 

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Discussion Starter #13
Related Question...

Thanks for all the useful info !

One more related question...How much does it raise cooling efficiency by increasing the number of blades on the fan ?

I'm helping my Dad work on his 1953 Ford and the fan only has 3 blades...is it worth the trouble to switch to a 4 blade ?

Thanks again.
 
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More blades will move more air. But understand that a fan is only functional to about 25-30 MPH, after that, nothing.
 

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More blades will move more air. But understand that a fan is only functional to about 25-30 MPH, after that, nothing.
Not disagreeing with you, just a clarification.

The number of blades won't do anything for you if the pitch of the blades is not optimal. Ever compare the pitch of the fan blades of the 4 blade HiPo fan againt the stock A code fan ? They are different. The HiPo fan will move more air even though it "appears" to be less substantial.

Many, if not all, propeller aircraft run at a steady rpm. The pitch of the propeller(s) is variable to speed up or slow down the airplane. The pitch of the blades is the single most important factor when determining how much air needs to be moved.


Z
 
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