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Ok, I'll chime in here. As we [Classic Retrofit] develop high output alternators, we always pay close attention to the belt temperature and slip. Although we recommend a serpentine for running our units at high loads, I did want to find out what causes the alternator belt on the double belt setup to fail. The short answer is heat and belt speed.
These tests were done with a high output unit producing 80 - 90A. When the car is driven, the engine fan serves to cool the belts and even with a high load you will rarely see the belt temperature above 70 deg C (measured with an IR gun). When you stop, however, the temperature in the engine bay rises and the cooling effect is less so the belt temp climbs. The belts are good for about 80 deg C but in traffic, on a summers day, after a fast run I have seen over 100 deg C which is beyond the temperature rating of the belt. Now, in this condition the worst thing you can do is switch the car off. When you do that the hot belt will take a 'set' to the pulleys - if you take it off the next day you will find it is no longer round but egg shaped like the pulleys. So, you start the car, the battery demands some charge and the belt shreds itself in the first few minutes of the drive.
We converted over to the Turbo pulleys and they are 'better' but still we shredded the alternator belt eventually so I started to look into this a bit closer as we never have belt issues with older 911's running the single belt. The problem is that the 993/964 pulley drive ratio is something like 2.6 times the crank speed whereas the 911 is only 1.8. This seems ridiculously high and puts a lot strain on the belt as 1) it is turning fast and 2) it has minimal contact area around the small circumference . What to do?
I noticed that for some reason the recommended size alternator belt rides very low in the pulley compared to the fan belt. Just look at Jlaa's picture above. Why is the belt so low in the pulley? They all seem to do this (even with the correct length Porsche belt) - it doesn't seem right.
I ended up buying a Gates belt that is 3mm longer than standard. With this belt, when shimmed, it sits in the top of the pulley (like the fan belt does). This gives the belt more contact area around the circumference and also slows the alternator down. No problems so far.
belt
belts
shredding
dual
9mm
pulley
update