Fan Clutch

[John M.]

What is the functional difference between big block and small block fan clutches, (other than the length).
Thanks

[Jack H.]

Re: Fan Clutch

In addition to the shaft length issue (obvious since clearance distance from water pump mounting hub to radiator differs between BB and SB), there are internal construction issues controlled by GM PN drawing and agreement with supplying vendors (Schwitzer and Eaton).

Basically, each fan clutch was designed for unique Temp vs. RPM %-Slip characteristics to accomplish the CFM requirements of air being pulled through the radiator in each car's dynamic operating environment. BB engines produce more underhood BTU's and have different cooling need profiles. The performance characteristic of a given fan clutch is pre-defined by the GM part drawing and a %-Slip response curve is embodied in the drawing based on both temperature as well as RPM. The response curve PRESUMES a specific fan blade is attached to the clutch, since the number of blades, attack angle, and diameter of each fan dictates a unique dynamic rotational inertia profile.

This (rotational inertia profile) is a pretty complex science, not dissimilar to the design/maintenance of aircraft propellers and nautical propellers. We're talking case book studies in fluid dynamics....

Internally, several things change from fan clutch to fan clutch and these include:

(1) The bi-metal actuator.
(2) Degree of fluid fill to the internal clutch cavity.
(3) Specific size/length of internal clutch cavity raceways.

[Richard S.]

Re: Fan Clutch

Would the characteristics of a fan clutch differ for a 66 with air conditioning and a seven blade fan versus a five blade fan?

[Jack H.]

Check your AIM

MOST of the time when C60 was installed both the fan and fan clutch changed. Find the PN's for the base engine fan clutch in your copy of the AIM and compare to the PN's listed for the equivalent engine (SB vs. BB) in the C60 option section of the AIM. If there's a PN difference, then go bet there's a fan clutch internal difference. GM didn't proliferate part numbers when there wasn't a technical need to do so!