I bought a set of Zimmermann cross drilled rotors but the pattern on them appears to be directional. The confusing part is that both front and both rear rotors have the same pattern thus one will be spinning one way and the other will spin in reverse. There are no markings on the front rotors to say if they are left or right and absolutely no markings on the rear. Here is a picture of them. The top two are the front and the bottom two are the rear.
http://www.phillipssoftwarelabs.com/BMW/ROTORS.JPG
It doesn't matter to performance one iota which way the "holes" are spinning. If the vanes on the inside of the rotors are directional (which they're NOT), then it matters which way the rotors are installed. You will want the vanes to be oriented in a way where air will blow from the inside of the hub out for best cooling (again, I assume these rotors do not have directional vanes. Very few does now a days).
If you have "slotted" rotor then you need the slots to slope away from the direction of forward momentum. I don't know how better to describe it to you.
That's interesting! I bought mine two years ago and they were not directional. Are they one-piece rotors? Otherwise, the hats must be removable.
Front rotors on OEM M3(1999 at least) are directional. Vanes face opposite the direction to scoop air.
MikeG
