Which fan clutch should I run???

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duster360

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I am going to install an 18" 7 blade clutch fan on my dart this weekend and I am not sure which fan clutch to use. Should I get the regular spring loaded or the thermal type?
 
Advance auto parts sells 3 different fan clutches for '70-'71 A body cars, non-thermal, thermal, and heavy duty thermal. Which would be correct for a 340 equipped A body?
 
That's what I was thinking. Why is there a heavy duty thermal? I saw a picture of it and there were fins in the center part of it also. I can see a difference visually, but what would be the need for one so beefed up?
 
Not bustin ballas here, but, I would have gone to MoPar and grabed there viscous unit and traded blades.
 
What is the difference between the viscous clutch and the others I mentioned?
 
The MP unit is a dyno proven 15 HP gain over others tested. Where is there from, who made them,?
 
I had a problem with the jobber fan clutches.

They are all too long I found. I had a factory 7 blade fan for my 71' 340 Duster with a stock rad and it had an old if not orignal clutch.

I wanted a shiney new one and tried all the parts store ones with no luck they all hit the rad or where within 1/4" of doing so.

The mopar viscous fan package is basically a clutch with a free fan in regaurds to the price compared to the jobber clutches.

I used the mopar clutch and it fit great same size as the original.
 
Not bustin ballas here, but, I would have gone to MoPar and grabed there viscous unit and traded blades.

Yep, Thats the one to get. I have one on my dart and wouldn't trade it for anything.
 
or just go electric and never have to worry about the fan, electric fans draw no power from the engine
 
Use thermal only. If your fan blade is larger than 18" and the blade pitch is greater then 2-3/4" then you may need a HD model clutch. From the factory all Darts came with a standard thermal clutch. Darts just don't have clutch to radiator clearance for the HD type clutch.

No American made car has ever come from the factory with a non-thermal clutch.
 
It's been my experience that the standard fan clutch will not spin the heavier 7 blade fan fast enough at low speed or idle therefore your car will get hot at idle. The HD one works great with the 7 blade fan on my cars. Idle 90*+ with the A/C on and does not heat up at all. I can't comment on the clearence issue though as I have E-Bodies.
 
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