Home-made shroud question...
#789781
09/01/10 09:11 AM
09/01/10 09:11 AM
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Joined: Apr 2004
Posts: 589
Waynesboro, PA
cuda_tim
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mopar
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Waynesboro, PA
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Guys! A friend of mine constructed a radiator shroud out of aluminum and my first thought was that the fan was inside the shroud "too far". I mentioned this to him and he didn't see a problem with it. I attached a picture. I know that I read somewhere on this forum that you typically don't want the fan to be completely inside of the shroud. That is, it will not be as effective. Does anyone know the physics involved here? I don't want to just blow smoke, I want to give him some straight facts. Will the fan pull more air if the fan is outside of the shroud a certain amount? Is there a "rule of thumb"? And if so, how much of the fan should be outside of the shroud? Thanks!  Tim...
"Robots building robots.... now that's just stupid!" - Will Smith in the movie "I, Robot".
'71 Demon (340/727)/'69 Charger (383/727)/'65 Barracuda (273-2V/904)/'66 Coronet (340/727)/'86 Dodge Aries (2.5L)/'78 Dodge Aspen (318-2V/904)
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Re: Home-made shroud question...
[Re: cuda_tim]
#789785
09/01/10 10:14 AM
09/01/10 10:14 AM
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Joined: Jan 2003
Posts: 24,562
Brookeville, Md
Mr.Yuck
Not enough dumb comments...yet
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Not enough dumb comments...yet
Joined: Jan 2003
Posts: 24,562
Brookeville, Md
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Quote:
Quote:
I think I recall reading that the fan should be 1/2" or so out of the shroud. I'd run it as is and see how it works. It looks like you have a spacer on it so if you have to move it back some it should be that much of an issue. looks nice but paint it black...
Yea, I read the same thing (1/2" or so). I was curious if anyone actually did some testing.
yeah but you can get a 2" or 2.5" if need be. I think you'll be fine.
Especially since my buddy didn't feel that it mattered.
I think that the spacer is about three inches, so IMO that would probably move it back too far.
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Re: Home-made shroud question...
[Re: Mr.Yuck]
#789786
09/01/10 10:43 AM
09/01/10 10:43 AM
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Joined: Apr 2004
Posts: 589
Waynesboro, PA
cuda_tim
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mopar
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Waynesboro, PA
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Great!
I didn't think about the fact that different spacers are available. They have them at Jeg's!
Now my only "remaining" question is the 1/2" or so that we have both read about.
The people at flex-a-lite use the following statement:
"...add a spacer to bring your new fan back into the "sweet spot" in the fan shroud opening"
"Robots building robots.... now that's just stupid!" - Will Smith in the movie "I, Robot".
'71 Demon (340/727)/'69 Charger (383/727)/'65 Barracuda (273-2V/904)/'66 Coronet (340/727)/'86 Dodge Aries (2.5L)/'78 Dodge Aspen (318-2V/904)
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Re: Home-made shroud question...
[Re: cuda_tim]
#789791
09/01/10 04:48 PM
09/01/10 04:48 PM
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Joined: Jan 2003
Posts: 28,096
Irving, TX
feets
Senior Management
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Senior Management
Joined: Jan 2003
Posts: 28,096
Irving, TX
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He took the easy route when building that shroud. it looks okay but don't be surprised if the car runs hot at speed. Those big flat corners block a significant amount of air flow. The air will likely stack up inside the shroud and cause hot spots in the radiator. I'd cut openings in the corners and hang a piece of plastic or thin aluminum sheet over the holes. Mount small zinc plated hinges from Home Depot to hang the covers on the engine side of the shroud. Gravity and air flow from the fan will pull them down at low (or no) speed. At highway speeds the air tacking up in the shroud will push the flaps open and evacuate the shroud. I had a tight shroud that needed those flaps to cool effectively. The auto manufacturers use them on some original applications too. Kinda like the flaps on the side of this fan:  Here's one with the flaps in the corners: 
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: Home-made shroud question...
[Re: feets]
#789793
09/01/10 07:00 PM
09/01/10 07:00 PM
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Joined: Feb 2003
Posts: 21,318
Manitoba, Canada
DaytonaTurbo
Too Many Posts
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Too Many Posts
Joined: Feb 2003
Posts: 21,318
Manitoba, Canada
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Quote:
He took the easy route when building that shroud. it looks okay but don't be surprised if the car runs hot at speed.
Those big flat corners block a significant amount of air flow. The air will likely stack up inside the shroud and cause hot spots in the radiator.
I'd cut openings in the corners and hang a piece of plastic or thin aluminum sheet over the holes. Mount small zinc plated hinges from Home Depot to hang the covers on the engine side of the shroud.
Gravity and air flow from the fan will pull them down at low (or no) speed.
At highway speeds the air tacking up in the shroud will push the flaps open and evacuate the shroud.
I had a tight shroud that needed those flaps to cool effectively. The auto manufacturers use them on some original applications too.
 The way that shroud is flat on the corners instead of cone shaped means you're blocking off air flow thru the rad in the corners. That is wasted cooling capacity in the corners of the rad. 
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Re: Home-made shroud question...
[Re: DaytonaTurbo]
#789794
09/04/10 12:30 PM
09/04/10 12:30 PM
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Joined: Apr 2004
Posts: 589
Waynesboro, PA
cuda_tim
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Thanks for the suggestions guys!
Couple things...
Ok, I talked with a Rep of flex-a-lite and he mentioned that the rule of thumb is half of the fan blade in the shroud and half out. This means I need a spacer of approx 1-3/8 to 1-1/2 to back the fan out.
Now, I did some testing with it "as is" and it appears to be fine on the highway. And if I stop and shut the engine off for a few minutes, then start-up and get moving again, it gets back to normal pretty quickly.
I do however, like the suggestion of the "flaps attached to hinges idea" mounted on the sides, which would close at low speed and open at higher speeds.
And yes, the flat section of the shroud is only about 1/2 inches from the radiator fins. I would prefer if it was farther away. I'll have to see how much effort that would be.
Each of the fan tips is 3/4 inches from inside of the circular section of the shroud. Not sure if this is good or bad.
The fan blades themselves are also 1-1/4 from the radiator fins.
At idle, it doesn't overheat, and it is much better than it was (without the shroud).
However, I would still like the temp to be slighty cooler. Thus my reason for understanding the fan placement "in the shroud".
Originally, I only had a 4-blade fan without the shroud. This seemed to run a lot hotter at idle. Then I added the shroud (just as you see it) and it seemed to run cooler at idle. Then I located a 7-blade fan (on eBay) and this seemed to be much much better at idle.
I never tried the 7-blade WITHOUT the shroud. Is this worth the effort to see if perhaps the shroud is limiting air flow?
Last edited by cuda_tim; 09/04/10 12:35 PM.
"Robots building robots.... now that's just stupid!" - Will Smith in the movie "I, Robot".
'71 Demon (340/727)/'69 Charger (383/727)/'65 Barracuda (273-2V/904)/'66 Coronet (340/727)/'86 Dodge Aries (2.5L)/'78 Dodge Aspen (318-2V/904)
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