Severe vs Heavy Duty Fan Clutches

[oldcarnut]

The 70 RR is starting to climb temps a little too much after awhile.  I checked the old fan clutch right after it got hot and it turned by hand without a lot of resistance so I feel its time for a new one.  Autozone was the only parts place that listed both a severe and a heavy duty clutch but they couldn't tell me the differences. http://www.autozone.com/autozone/catalog/parts/partsShelf.jsp?categoryDisplayName=Climate+Control&fromType=parts&fromString=search&parentId=52-0&currentPage=1&filterByKeyWord=clutch+fan&isSearchByPartNumber=false&navValue=15200118&categoryNValue=15299999&fromWhere=&itemId=118-0&displayName=Fan+Clutch&searchText=clutch+fan

Being that my RR is getting cloned into a Superbird with it's own set of cooling issues and I'll add A/C to it also on top of that, does anyone have any experience with the "Severe Duty" applications or is it about the same as "Heavy Duty" with higher cost? I assume it was for maybe campers, trucks or similar applications but if it's better cooling and fits, I'll pay a little more.  I read some of the posts about Hayden but didn't see this one from Autozone.  Is it hype or does it work better and why?

[lisiecki1]

the one from autozone isn't made by hayden?

[oldcarnut]

Someone at another parts store told me that the spring was bigger and worked at 5 deg's lower temp    Guess I'll find out tomorrow when it comes in.