I bought new Brake Rotors and Pads

I found a vendor on eBay that offers slotted and drilled rotors for just $25 or so over the price of standard rotors, this is for the complete set of
rotors and semi-metallic pads for both the front and rear axles. The cost-up also includes nickle plating that I hope will keep the rust at bay. The rotors LOOK really good, how they perform is yet to be seen. I'm not really a speed demon, but I like stuff that looks good -- assuming it works good too, which is really the more important quality of stuff.
I decided that if slotted and drilled rotors are good enough for an M3, they should be good enough for my '94 convertible. My Bentley manual states that the the slotted rotors are directional, but does not describe which rotor goes on which side.
My instinct is that the slots and holes should slant toward the rear as I am looking at the top half of the rotor. Does anybody dispute that, and why?
Thanks,
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Interesting question. My first thought would be to start looking at motorcycles but I did find this in an ad for Brembo Rotors on the internet.
" Which direction should the discs rotate? It is a popular misconception that the slots or drillings in a disc determine the direction of rotation. In truth, for an internally vented disc, the geometry of the vanes dictates the direction of rotation. There are three vane types in use:
a.. Straight b.. Pillar vane (comprised of many small posts) c.. Curved vane The first two vane types are non-directional, and can be used on either side of the vehicle. The curved vane disc, however, is directional. A curved vane disc must be installed with the vanes running back from the inside to outside diameters in the direction of rotation. Please see figure. Orienting the disc in the manner creates a centrifugal pump. The rotation of the disc causes air to be pumped from the center of the disc, through the vanes, and out through the outside diameter of the disc. This greatly enhances the disc's ability to dissipate heat.
Additionally, all of Brembo's slotted discs are directional as well, regardless of the vane geometry. The discs should be installed such that the end of the slot nearest the outer edge of the disc contacts the pad first. Please see figure.."
http://www.buybrakes.com/brembo/faq.html#q19

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Interesting. This says that I should install the rotors opposite of what my intuition told me.
POINT OF ORDER BMW uses vented front rotors that have straight vanes.
I wish the figures referred to in the link actually existed ...

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Jeff Strickland wrote:

So your intuition doesn't know physics well. At any given point in the travel of your wheel, the top is always passing through the air faster than the bottom (which is, technically, momentarily stopped). If you want to 'scoop' cool air, the vanes at the top should facilitate that. This makes more sense than trying to expel hot air from the center of the disk since the brake backing plate will tend to block the intake of cool air.

Then it doesn't really matter ...
I'm hoping your considerations don't include noiselessness in your cabrio because drilled or slotted rotors tend to make some noise compared to solid face types. -- C.R. Krieger (Been there; done that)
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I follow. You're right though, I aint a physicist; which is why I'm asking BEFORE I get my wrenches out.

Well, the vents between the disc surfaces are differnt than the slots and holes that are cut and drilled, respectively.

I'm okay with a bit of noise, I have lots of volume remaining on the radio controls ...
If slots and drill holes are okay for an M3, they're okay with me. ;-)
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(disks)UNLESS your BMW is a motorbike. On a bike the disks get wet in the rain and the slots allow water to be dispersed more quickly (less than a single rotation of the wheel). On a car they simply reduce the contact area and probably muck up the wear profile of the disk and the pad.
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That may be true, but lots of high performance cars get slotted and drilled rotors as factory fitment, so I suspect it is not true. At least it is not true to be significantly adverse to the quality of the braking experience. There may be no particular benefit to me since I am not a Ricky Racer, but I have 'em now and I want to put them on in the correct orientation.
PS The M3 gets directional discs on the front and rear as factory fitment. I am not sure if the directional nature is due to slots and drill holes, or if it is due to the cooling vanes between the disc surfaces. Perhaps somebody with an M3 will chime in.
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