They are suppose to ‘slip’, that’s the point.  The fan is superfluous at highway speed.  If you had ever dropped a tool into the fan while working on a running engine (or had one come apart at 5000rpm), you would likely appreciate ‘slip’.

6 VOLTS/POS. GRD. NW INDIANA

Slip, when they shouldn't slip, is not good. Dropping tools into the fan of a running engine? I'm 67 years old and have yet to do that. Makes one appreciate fan shrouds all the more.

My experience tells me that the more parts a component has, the more likely it will fail.

The test described earlier - running the engine at "Low Idle" (auto trans in drive and a helper behind the wheel) - and looking for flow through an open radiator cap usually tells the tale. Water needs to be warm enough to fully open the T-stat of course .........and the stat needs to be of the higher flow variety. I built a 289 Cougar before the T-bird arrived and water absolutely gushed in that radiator during this test - at low idle in drive. Ran cool, even at parade speeds. It is possible to find a reduced diameter small block ford pulley that fits the Y-block - and gets the same sort of result. It is cheap, almost undetectable (if that's what you want) - and is a mechanically sound solution. You'll also need a smaller belt.

My T-bird had enough nose room for a viscous clutch (sedans don't) - and just for the lovely summer weather we have down here (105º today), I fitted it with a 7 blade GTO aluminum fan. The viscous drive is supposedly about 10% slippage so something had to make up for that. 

 
 Steve Metzger       Tucson, Arizona

6 VOLTS/POS. GRD. NW INDIANA

Went digging in old computer files to find my parts list. I used a Hayden 2797  "Severe Duty Thermal" fan clutch - their spec says 80-90% shaft speed at full engagement. I believe it was initially created for a Ford 351 truck application. Virtually every manufacturer has a variety of fans suitable for a clutch type mounting - can't find it this morning but I had a part number for a Ford steel six blade that would go on there.
I thought the Poncho 7 blade looked pretty nifty in the bird..



 
 Steve Metzger       Tucson, Arizona

Greenbirds explanation of how to solve my cooling problems with a smaller 302 Mustang waterpump pulley years ago made our car usable anywhere at any time. I have shared his information on many websites. Living where he does and wanting to use his car in any month really helps us with similar problems. Once again thanks for your help. 

56 Vic, B'Ville 200 MPH Club Member, So Cal.

Good point.  It never occurred to me that a viscous clutch might fail, but then it DOES have a bearing.  Also didn’t realize there might not be room for one on a passenger car motor (?).  While mine (CASCO) is likely a basic model, I see there are even more complicated versions yet available (temp controlled/electronic/etc.). SOME added complications are a justifiable trade-off though (IMO).  Not me, but urban Y-Block owners with automatic transmissions might agree (?)

6 VOLTS/POS. GRD. NW INDIANA

I doubt changing the number of fan blades is going to get you where you want to go. Here in Central Florida, the weather is hot and traffic is heavy. At a car show, you may have to wait in line with your engine running for some time.   My TBird had evidence of earlier overheating "fixes".  A previous owner had used a torch to smoke holes in the lower radiator valance in an attempt to increase air flow. This actually made things worse.  To remedy the situation, I started with an aluminum radiator and a wide mouth thermostat. My old original radiator was in bad shape and I could not find anyone in my area to work on it.  I tried many different fans and they didn't have much effect.  All they did was make noise.  For $200, I bought an EBay Champion aluminum radiator and an electric fan.  My current setup is to remove the belt driven fan entirely and use the electric fan continuously. This was a big improvement, but I still had slight issues in the heat of summer traffic.  I learned that Tbirds have a 1" spacer behind the water pump that causes water flow issues at idle speeds.  I installed a Casco improved water pump with longer blades and this worked great. One of the first things you should do is install a quality water temperature gauge so you know what you are dealing with.  Those dash gauges are worthless.  I am currently happy with my Tbird's operating temperature.  Some members have installed a smaller diameter water pump pulley to increase flow.  Casco sells the correct one.  I haven't tried it.

Any benefit from reducing parasitic loss by using an aftermarket fan would be a far second from having a fan that's going to keep air flowing through the radiator to reduce engine temperature. Nevertheless, if you have found the right combination for success, that's great. If Chris Ames's Paragon Industries Tbird wp spacer, Casco's HD wp and lower fan shroud, 6-blade mechanical fan, wide-mouth thermostat, smaller pulley, and 4-row copper & brass U.S. Radiator don't get the job done, I'll add an electric pusher fan between the grill and radiator. I'll also make sure that any rust/scale is removed from the block as part of the rebuild process.

The thermometer reached 105 in the Central Valley yesterday, and 101 expected today. However, my Tbird doesn't have a/c. What that means is when it's too hot for me, it's too hot for the Tbird; it will stay in the garage. 

I forgot timing.LOL