I'm trying to determine if my S6 swap alternator has an under driven pulley or stock diameter. The problem is I don't have my S4 pulley anywhere so I don't know what diameter it is.

If anyone knows or could take a quick measurement it would be much appreciated. Thanks.

 

I just went through this. The FC alternator pulley is ~45mm in diameter & the FC main pulley is ~90mm. The FD maintains the same ratio (2.08:1 according to S4/S5 FSM) with a ~50mm alternator pulley and ~100mm main pulley. So with an FD alternator on an FC, you'll want to get as close to 45mm as possible to avoid underdriving.

Also, the Atkins double pulley is 54mm in diameter, so it does underdrive a lot on an FC. And the pulleys made by Higgi are apparently about 40mm, which would be slightly overdriving.

 

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