I've been wondering about this too, and I found this link:

http://www.stoptech.com/faq/data/faq16.html

"The only correct way to determine what side a directional rotor goes on is the internal vane direction."

I haven't looked at mine too much, but it sounds like they should go on opposite of the way everyone has theirs on.

 

yes i have mine mounted opposite of tibby01 , and my braking rocks. i have very little brake dust, with KVR carbon fiber pads, and the SS brake lines are awesome too.
pic of drivers side, rear (i know they are dirty-they are clean now)

 

one way is for braking hard, like the racecar up top, you dont want to mount them that way on an every day driver. This is because the slots get way more grab on the pads, therefore wearing the hell out of them. While the other way make a much more comfy stop on street roads because it gradually digs into the pads instead of full force. Now you know.

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