Single Pulley & Unintended Consequences. - With the 911’s long history of breaking alternator/fan V belts many 993 owners like the idea of replacing the OE dual belt fan/alternator system with a single belt arrangement using a more robust belt design. The idea sounds great for track cars where reliability at speed trumps all other consequences.
For street use the tradeoff seems to be that:
- The several hundred dollar pulley conversion is expensive and;
- At idle the alternator spins at a little lower speed which in traffic can be inadequate to maintaining battery charge while supporting the electrical system. There is an aftermarket fix from the pulley suppliers involving adding a small bulb as a load to the alternator that seems to have proven to work well over time.
- The aftermarket pulley turning the alternator at lower speed also makes the alternator produce a whirring or whistling sound that some hear and find annoying.
In reality, the factory belt arrangement seems to work just fine and the belts have a reasonably long service life if the correct belts are used and they are tensioned properly with a tensioning gauge. The outer belt specified is a 9.5mm wide belt that is not as wide as most and if the replacement belt is of a wider belt size some believe it will induce a premature belt failure. I believe the bad wrap the factory belt arrangement gets may be in large part do with folks who don't bother to check the tension when new belts are put on or measure the tension using their thumbs as a gauge.
Also see the section on this page titled "The Factory Pulley Update, Porsche Technical Service Bulletin 1378"
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