I had an extra 78 fan shroud off our first 78. I put it on Barb's 73. That 78 shroud is a lot better. Perfect fit. How much air a fan pulls into the radiator. Is based on fan RPM's. When any fan clutch is engaged fully. They all pull the same amount of air. Bob Dunahugh
I just got a new fan clutch for Barb's GMC. I checked several souses. In the parts business there seems to be some combining of cross reference numbers. Not sure. I didn't dig deep. As I just didn't have the time to do that now. So I just got a Hayden heavy duty. As I know that clutch is sized to our stock fan. It's working perfectly on her GMC. As every time I've found that a new unit was engaged too long. It's been a Hayden sever duty. The SD units aren't going to last longer in an equivalent use. It's the fan load/size. We as gear heads are of the mined set that a sever duty item has to be better. NOT in this case. If you insist on a SD. Go get a bigger fan in dia, pitch, and number of blades. Bob Dunahugh
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The coolant sensing fans have been on GM trucks for many years. When Jim at Applied has them available for our GMC's. I'm getting one. Since my 78 -403 Royale GVW is always between 17,000 to 21,000 lbs. Bob Dunahugh
The heat starts building during the climb in the radiator. It takes time to heat up the fan clutch sensor from the radiator's heat. Thus the fan clutch has no idea that you're climbing a hill when your doing it. As to the fan clutch size. The standard duty unit is designed for a smaller fan then we have. Heavy duty unit is designed for our fan's dia, pitch, and number of blades. Sever duty is for a larger dia fan with more pitch, and blades. Thus the SD unit will have a tendency to be engaged for a longer time then it needs too. As our blade can't put enough load on the clutch to disengage the fan. Bob Dunahugh
I just got a new fan clutch for Barb's GMC. I checked several souses. In the parts business there seems to be some combining of cross reference numbers. Not sure. I didn't dig deep. As I just didn't have the time to do that now. So I just got a Hayden heavy duty. As I know that clutch is sized to our stock fan. It's working perfectly on her GMC. As every time I've found that a new unit was engaged too long. It's been a Hayden sever duty. The SD units aren't going to last longer in an equivalent use. It's the fan load/size. We as gear heads are of the mined set that a sever duty item has to be better. NOT in this case. If you insist on a SD. Go get a bigger fan in dia, pitch, and number of blades. Bob Dunahugh
________________________________
The coolant sensing fans have been on GM trucks for many years. When Jim at Applied has them available for our GMC's. I'm getting one. Since my 78 -403 Royale GVW is always between 17,000 to 21,000 lbs. Bob Dunahugh
The heat starts building during the climb in the radiator. It takes time to heat up the fan clutch sensor from the radiator's heat. Thus the fan clutch has no idea that you're climbing a hill when your doing it. As to the fan clutch size. The standard duty unit is designed for a smaller fan then we have. Heavy duty unit is designed for our fan's dia, pitch, and number of blades. Sever duty is for a larger dia fan with more pitch, and blades. Thus the SD unit will have a tendency to be engaged for a longer time then it needs too. As our blade can't put enough load on the clutch to disengage the fan. Bob Dunahugh
