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9 Blade Fan and Clutch




Whether you tow or not, this fan clutch is an excellent addition to any 6.5 for improved cooling/reliability. See the article here: http://www.62-65-dieselpage.com/reviews/fanclutch.htm

Based on my own personal experience with my 1996 GMC, and the reports from many others, the factory fan clutch calibration is much too high for proper engine cooling. The factory clutch is calibrated to engage at 195° as measured at the fan clutch's thermostatic coil. This coil temp, while directly related to engine coolant temp, is a much lower value. In most cases, it takes well over 220° coolant temp through the radiator to achieve this point. In fact, the "kill temp" or the specified max engine coolant temp measured at the thermostat outlet is specified at 240°. At this point the factory clutch is to be fully engaged. The rapid swing from 220°+ down to 180° can be very hard on the cylinder heads and gaskets etc. The revised calibration shifts the engagement pointdownward by 15°f. This downward slide moves the entire fan engagement curve lower, keeping the engine coolant temperature much more stable. By beginning the engagement curve earlier, we also improve air conditioning, transmission fluid, and engine oil cooling In most cases, with this revised curve, complete clutch engagement is not even necessary.


The graph shows the difference between the factory fan drive (red line)
and the 9 Blade Fan and clutch drive(blue line)

Features and Applications:

1) Performance:

The blade is NOT the key here. The engagement curve is what is key, and this is only consistently repeatable through factory calibration when the units are built. The bimetal springs are the same, and no lower temp springs are available. The clutches are built with a change in the proximity of the spring and valve to the internal fluid port so that by 180° the port is open to get full engagement. Since the engagement profile (see graph) is somewhat gradual, cooling throughout the temp range is improved to a point that most vehicles will never have the need for full engagement! These clutches are of the viscous type in that they slip based on fluid viscosity, rpm and the amount of fluid released from the fluid port. The clutch for every given condition will only pull a specific amount of torque. We will use 30 lb/ft as an example at full engagement. Given the 30 lb/ft rating, the addition of a larger blade will NOT pull any more air UNLESS the larger blade is more efficient, simple physics.

In laymen's terms:

Engine RPM = 2500
Input (pulley) speed 3125 RPM (1.25x 2500 engine RPM)
20" blade RPM 2900 @ 30 lb/ft
21" blade RPM 2750 @ 30 lb/ft

Both blades will be pulling similar, if not identical CFM ratings, BUT, the 21 blade will be generating increased slip speed causing greater stress on the clutch.

2) Application:

These will be available for serpentine belt, reverse rotation only. This should cover most 6.2l and 6.5L models in the new body style from 1988 on. If you have a screw on clutch hub with integral pulley, you will need to switch to the 98 water pump/pulley, or wait to see if demand is sufficient to make a production run of this part as well. You will lose nothing by making this change, as the water pump construction/performance is the same. I recommend the High Capacity Cooling modifications first to ensure adequate coolant flow and to minimize the stagnant flow areas reducing the instances of head cracks, and localized hot spots in the engine.

3) Fans:

Pre 1997 models used a 6 blade 20" steel fan with a very large pilot and 6 mounting bolts. 1997 and later used a 9 blade 20" steel fan with a much smaller pilot and 4 bolts. The new clutches will be of the 1997 and later style so if you do not have a small pilot, 4 bolt, 9 blade fan, you will need one. I also have these available. Either 20" steel vs. 21" plastic blade will work fine. The clutch has been designed/tested with the 20 steel blade which will fit with absolute certainty and no clearance issues. The 21 Dmax blade will bolt on, but may require shroud modification. This blade has NOT been tested with this clutch calibration. I personally prefer the 20” steel blade for the direct fit and the rigid construction. It just plain moves air where the plastic unit can flex especially when warm.

4) Thermostat Temps:

Personally, I prefer 195° thermostats. This is based on my observations that this temperature range produces maximum efficiency and power. With the revised calibration of the clutch, we can keep our coolant temperature within in a relatively narrow window of 190° to 215°f which in my opinion is the optimal range, and prevent any rapid temperature swings. It has been said that the 195° thermostat is not fully open until approximately 220°f BUT with dual thermostats, the flow even partly open is quite substantial, and in all but the most extreme cases plenty adequate. The choice is yours, I offer both temp ratings for the dual thermostat setup. I should also note that my engine runs particularly cool, and in winter it is tough to get adequate heat with lower temp. thermostats.




Item Name: Special Calibration Fan Clutch for 6.5-stud mount
Category: 6.2/6.5 GM
Sub-Category: Cooling Upgrades
Price: $250.00
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Direct bolt on unit fits all 6.5 with serpentine (single) belt drive and 4 stud mounting configuration. Requires blade change on all pre 1997 vehicles. The 1997+ 9 wing steel fan is the best match to this clutch. Engage temp is 180°f air temp which is 15° lower than the OE spec.