We're often asked what the difference between the UL and HD option when selecting the discs on some of our Wilwood big brake kits.
According to Wilwood:
Ultra-Light UL straight vane rotors provide high value performance and rugged durability for a wide range of competition, high performance, and sport driving applications. Don't confuse these rotors with bargain priced off-brand named parts. ULHP rotor are manufactured using a 1:1 turning process which yields tighter tolerances for flatness, parallelism and run-out. Wilwood's modern manufacturing capability, combined with large scale economies, make it possible to offer this level of quality and performance at such an affordable price. The straight vane design also makes it possible to use the same rotor on left or right hand mounting locations.
HD-48 and HD-36 Series directional vane rotors provide superior thermal stability and long service for asphalt late models, modified, open wheel, and most other types of high heat competition applications. All HD rotors are cast from premium grade, long grain carbon iron for long wear, high thermal stability and resistance to distortion. Thick wall pad sweep faces with directional cooling vanes provide superior heat absorption and dissipation qualities to prevent heat fade and realize long service life from the pads and rotors. All HD rotor faces are precision turned to less than .001" for flatness, parallelism, and run-out for smoother engagement and reduced vibration.
So to simplify this, it depends what you're using the brake kit for.
Lightweight car (<1000kg) on fast road, occasional track days and sprints using medium compound pads, we'd recommend the UL rotors.
Heavier car (>1000kg) with lots of track and race use, with race pads, we'd recommend the HD rotors.