Pulley sizes

[manbridge 74]

Every June I take five minutes to switch to 131mm from the 116mm. Then when colder temps hit I switch back. When I want to get oil to the front cooler in winter or keep temps above 180F I’ve been known to partially block the deck lid grille.

No issues with overspeeding alternator.

[burgermeister]

I have been trying to make sense of the various pulley sizes and speed ratios quoted in this thread and in the 5 vs 11 blade fan thread.

The pulley sizes listed seem to be the outer diameter of the pulley. Using those numbers "raw", one ends up considerably below the advertised ratios.

Now, the belt does ride a few mm down in the pulley groove. also, the belt has thickness, so the effective radius where the belt acts is going to be someplace between the top and bottom surface of the belt. I measure 3-4mm for the 1st number (belt top surface below pulley outer radius), and the belt thickness for a 10x belt is 6mm, so another 2-3mm below the top surface of the belt.

Subtracting 10-14mm from the advertised pulley diameters gets one close to the advertised ratios, but not actually there. Here is what I come up with subtracting 12mm:

Fan Dia Crank Alt Pulley belt (10x) Ratio (spec) Alt Ratio Fan tip speed at 6800
65-74 245 116 95 710 1.3 1.25 109.3
76-77 245 131 83 725 1.8 1.68 146.2
78-79 226 126 80 725 1.8 1.68 134.9
80-89 245 127 83 710 1.67 1.62 141.3
90-98 279 145 100 775 1.6 1.51 150.1


Anyone have any idea where the extra ratio (1.6 vs 1.5, 1.8 vs 1.7) comes from?

[burgermeister]

Apologies for the garbled text ... apparently spaces and / or tabs don't post. Here is what it looks like in the editor:

[Bill Verburg]

Originally Posted by burgermeister

Apologies for the garbled text ... apparently spaces and / or tabs don't post. Here is what it looks like in the editor:

I believe that you have some errors

Here's what I got using only factory specs for the fans

[burgermeister]

I think I found the error in the calculated ratios ...

On the alternator pulley, the belt rides 3-4mm below the pulley circumference. On the crank pulley, it is only 0 - 1mm below the pulley circumference. Making the correction, the ratios (admittedly incomplete and rounded off) end up almost on the money.




I did leave the turbos out of the picture - thanks for including them Bill!

So ... how fast do the racing folks spin 993 motors? In other words, is there overhead in the 993NA fan to spin it faster without causing fatigue issues in the blades?

The 993TT fan turns faster - I think it has straight blades. I wonder if that was done for durability reasons - more even stress from centripetal acceleration? In which case that answers the previous question with a "no".

It is pretty easy physically to run the 993NA fan faster - the 10x760 alternator belt fits in the fan belt's place with an added 4.0mm spacer (I have the single belt conversion hub). That represents a 2.0-ish ratio - almost certainly too much for the fan to handle in the long run. I drove it around for 40 some miles, it worked fine with a bit more fan noise, but I did not take it above 5000RPM.

I have a kit car with a 993 motor that runs warmer than I'd like during track days. It has a huge oil cooler and no room for anything bigger. It seems some folks have had good success cooling the engines down a bit with more airflow. Also, the 993TT used increased airflow - also suggesting that more airflow is probably a good thing.