Home » Public Forums » GMCnet » Re: [GMCnet] Fan Shroud questions
| Re: [GMCnet] Fan Shroud questions [message #163160] |
Fri, 16 March 2012 12:27  |
enate98690
Messages: 23
Registered: January 2012
Karma: 0
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Junior Member |
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Steve,
I don't know the "official" reason for the tight, metal venturi ring on the GMC's. I always assumed it was due to potential fan disintegration. Having the fan blades fly off of a Corvette where the engine is well forward of the driver is one thing.
Having the fan blades fly off of a GMC where the driver is almost literally right above the fan is quite another. And, let's face it, fans are most likely to go through disintegration in high RPM situations where the centrifugal force would be immense.
Just my thoughts; the "official" reason might be something completely different.
Thanks,
Keith Lee
-----Original Message-----
>From: Steve Southworth <midlf@centurytel.net>
>Sent: Mar 15, 2012 6:20 PM
>To: gmclist@temp.gmcnet.org
>Subject: [GMCnet] Fan Shroud questions
>
>
>
>I'm pulling the radiator and fan shroud off my front clip and am somewhat intrigued by the way GMC did this. On my 75 Vette the fiberglass fan shroud extends back over the fan, with some clearance between the fan blade tips and the shroud, enough to get a fan belt past. On the GMC, as most of you are aware, the fan "venturi ring", is mounted to the engine with a very tight clearance to the fan blades. This is an expensive method, parts and assembly time, compared to some of the alternatives used by GM.
>
>My question(s):
>
>Why did the GMC MH use this method?
>
>Was it the method used on the Toro?
>
>What is the purpose of the tight tolerance between the fan and the venturi ring? Noise reduction? Fan efficiency?
>
>Just curious.
>--
>Steve Southworth
>1974 Glacier TZE064V100150 (for workin on)
>1975 Transmode TZE365V100394 (parts & spares)
>Palmyra WI
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| Re:Fan Shroud questions [message #163240 is a reply to message #163160] |
Sat, 17 March 2012 00:32  |
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John Heslinga
Messages: 632
Registered: February 2011
Location: Edmonton, Alberta, Canada
Karma: 4
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Senior Member |
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I would like to weigh in on the Fan and Shroud discussion. But First: Making the statements I do though,, in no way means that the changes others have made to their units haven’t made improvements, it simple means that the original design was not a bad thing to start with. I have seen many changes to our GMCs that make a lot of sense and improve things a lot. I applaud those that have done the changes and offer us those products, advice and services. I have made many of the changes myself.
I write this to help in understanding.
There really are reasons for the design of the fan, shroud, and cooling system as it is. You see a lot of the same features in todays cars. As the fan is turning it wants to move air through its blades. OK, That's obvious. However: at the perimeter of the fan blades air moves around the blade tips from the high pressure area to the low pressure side. This is a problem behind the radiator because the air needs to go through the Radiator, not around the fan. I would imagine the GMC engineers knew the GMC would need a lot of effective air movement to cool effectively. Especially in such a crowded space under the coach. (Using a large - 7 blade fan and stiffer clutch) The air also needed to be forced out of the engine compartment area, and that fan needs blow a lot too. In order to decrease the “Air Leakage” around the fan tips and increase the effectiveness of the low pressure area behind the radiator, they needed to significantly decrease the space between the tips of the fan and the shroud.
Sounds like a good idea, however, the engine moves (Twists) in the the motor mounts and a shroud mounted firmly to the frame and radiator would cause fan tip collisions with the shroud. The answer to this problem was to mount a ring at the fan tips with very little space and solidly mounted to the engine (venturi ring), and use a flexible (Rag) section mounted between the ring and the shroud to allow the movement needed by the engine. I really does work well. You can see the similar technologies on cooling systems today but the fan is not driven by the engine anymore and the movement is not there. But, Many automotive applications today have electric fans that have rings as part of the the fan assembly and in shrouding with little space, very close to the radiator to move as much air through the radiator as possible.
So: Why then, does the shroud not cover the entire radiator? There is a good reason for that too. Big fans take engine power, use gas, and make noise. While the coach is moving on the highway, vehicle motion creates most of the air movement needed to cool the the engine. The less the fan does, the more gas mileage, less engine noise and less power being robbed. We only want the fan to be turning if it is really needed. BUT, the fan in the middle of the shroud actually is an impediment to that air moving through the radiator. (yes it is moving but not enough while the clutch is free wheeling, and we don't want it to be driven if we can, remember) The area of the radiator outside the shroud allows the air movement, from vehicle motion flow through the radiator with nothing to block it, effectively cooling the engine. If you look at most cars today you see the same kind of thing as well. Fans usually do not cover the entire radiator and vehicles have air dams and ducting in front of the radiator. Later factory GMC designs ducted the air in front of the radiator and others have created air dams to create a greater pressure differential from front to rear of the radiator, thus increasing air movement.
While servicing items such as the fan belts and clutch, are less convenient for sure. As most of us know, the auto industry never really had “ease of maintenance” as a priority. Easy maintenance adds cost to the manufacturer. The extra cost of maintaining some items are a cost the consumer bears at a later date and that cost is seldom considered in purchasing decisions. Some of the changes some have made to the factory GMC systems identified make maintenance a little more convenient. Others attempt to make the fan do more pulling of the air through the radiator. And others again, do some of the ducting ahead of the radiator. All worth considering, I suppose.
John and Cathie Heslinga
1974 Canyonlands 260
455, Manny tranny and 1 ton, 3:70 LS,
Red Seal Journeyman, DTE, BEd. MEd.
Edmonton, Alberta
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| Re: [GMCnet] Fan Shroud questions [message #163248 is a reply to message #163240] |
Sat, 17 March 2012 07:49 |
mickeysss
Messages: 1476
Registered: January 2012
Karma: 0
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Senior Member |
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great points. thank you. ^.-.^
~
On Mar 16, 2012, at 10:32 PM, John Heslinga wrote:
>
>
> I would like to weigh in on the Fan and Shroud discussion. But First: Making the statements I do though,, in no way means that the changes others have made to their units haven’t made improvements, it simple means that the original design was not a bad thing to start with. I have seen many changes to our GMCs that make a lot of sense and improve things a lot. I applaud those that have done the changes and offer us those products, advice and services. I have made many of the changes myself.
>
> I write this to help in understanding.
>
> There really are reasons for the design of the fan, shroud, and cooling system as it is. You see a lot of the same features in todays cars. As the fan is turning it wants to move air through its blades. OK, That's obvious. However: at the perimeter of the fan blades air moves around the blade tips from the high pressure area to the low pressure side. This is a problem behind the radiator because the air needs to go through the Radiator, not around the fan. I would imagine the GMC engineers knew the GMC would need a lot of effective air movement to cool effectively. Especially in such a crowded space under the coach. (Using a large - 7 blade fan and stiffer clutch) The air also needed to be forced out of the engine compartment area, and that fan needs blow a lot too. In order to decrease the “Air Leakage” around the fan tips and increase the effectiveness of the low pressure area behind the radiator, they needed to significantly decrease the space betw
ee
> n the tips of the fan and the shroud.
>
> Sounds like a good idea, however, the engine moves (Twists) in the the motor mounts and a shroud mounted firmly to the frame and radiator would cause fan tip collisions with the shroud. The answer to this problem was to mount a ring at the fan tips with very little space and solidly mounted to the engine (venturi ring), and use a flexible (Rag) section mounted between the ring and the shroud to allow the movement needed by the engine. I really does work well. You can see the similar technologies on cooling systems today but the fan is not driven by the engine anymore and the movement is not there. But, Many automotive applications today have electric fans that have rings as part of the the fan assembly and in shrouding with little space, very close to the radiator to move as much air through the radiator as possible.
>
> So: Why then, does the shroud not cover the entire radiator? There is a good reason for that too. Big fans take engine power, use gas, and make noise. While the coach is moving on the highway, vehicle motion creates most of the air movement needed to cool the the engine. The less the fan does, the more gas mileage, less engine noise and less power being robbed. We only want the fan to be turning if it is really needed. BUT, the fan in the middle of the shroud actually is an impediment to that air moving through the radiator. (yes it is moving but not enough while the clutch is free wheeling, and we don't want it to be driven if we can, remember) The area of the radiator outside the shroud allows the air movement, from vehicle motion flow through the radiator with nothing to block it, effectively cooling the engine. If you look at most cars today you see the same kind of thing as well. Fans usually do not cover the entire radiator and vehicles have air dams and ducti
ng
> in front of the radiator. Later factory GMC designs ducted the air in front of the radiator and others have created air dams to create a greater pressure differential from front to rear of the radiator, thus increasing air movement.
>
> While servicing items such as the fan belts and clutch, are less convenient for sure. As most of us know, the auto industry never really had “ease of maintenance” as a priority. Easy maintenance adds cost to the manufacturer. The extra cost of maintaining some items are a cost the consumer bears at a later date and that cost is seldom considered in purchasing decisions. Some of the changes some have made to the factory GMC systems identified make maintenance a little more convenient. Others attempt to make the fan do more pulling of the air through the radiator. And others again, do some of the ducting ahead of the radiator. All worth considering, I suppose.
> --
> John and Cathie Heslinga
> 1974 Canyonlands 260
> TC4W "Too Cool For Words"
> Retirement Projects Galore
> Edmonton, Alberta
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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