Since Hayden changed the specs of their heavy duty fan clutch there's not a whole lot of difference going up to the severe duty. They rate their fans on air temperature going over the fan clutch and they usually say it firms up at 175 or 180 degrees. My experience seems that that translates more to around 210° coolant temperature for advertised full engagement. As stated above the 2747 is the HD unit, and the severe duty unit is 2797. I still prefer the severe duty but that point can be argued. Both clutches have the same advertised specs and Hayden's explanation is the severe duty is for larger fans - but no explanation of their definition of "larger". The 19.5" seven blade fan qualifies in my book as large but maybe not theirs. Either clutch will cool much better with adjusting the thermal coil to have the fan come in earlier. My personal preference is to run a 160° thermostat and have the fan coming in around 185° coolant temperature.
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Mick Batson
1967 original owner Tyro Blue/black top 4-speed HO GTO with all the original parts stored safely away -- 1965 2+2 survivor AC auto -- 1965 Catalina Safari Wagon in progress.
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