After much debate on the forums, we at MiataSpeed decided to further investigate the differences between the two packages that come standard with the ND Miata. Mazda lists two different part numbers.
All measurements were referenced with the use of a surface plate.
However, based on our measurements, we have not found any difference between the two rotors dimensionaly. The only difference we have found were the internal cooling vanes of each rotor.
Brembo on the left, Non-Brembo on the right.
Each rotor is 280mm x 22 mm
Offset is the same.
The Non-Brembo has a straight vane design and the Brembo has a pedestal-style/pilar style design. Pedestal-style/ vanes are commonly used by manufactures to reduce the overall weight of the rotor assembly, and is not necessarily ideal in performance applications.
This is the main advantage of the MiataSpeed rotor (in addition to the lighter aluminum hat). The curved vane design drastically improves the rotors cooling capabilities, transforming it into an air pump, moving cool air through the rotor to dissipate heat.
Brembo (pedestal-style vane), MiataSpeed (curved-vane), and Non-Brembo (straight vane).
The more mass a rotor has the more resistant it will be to brake fade, as the rotor will be able to act as a larger "heat sink". Therefore, rotor mass is not the place you want to save weight.
MiataSpeed Directional Curved vane two piece rotor.
While the two piece rotor weighs in around 2 lbs less, most of the savings come from the aluminum hat, yet rotor mass is nearly identical.
For more information about the MiataSpeed curved vane: Click here.