Our R-100 (10mm) cable has a thicker insulating jacket, designed for extreme heat situations or where extra insulation is needed for extreme-output ignition systems and where it's not easy to space the wires from one another and/or the engine. Our KV85 (8.5mm) and R-100 (10mm) cables both use the same conductors and provide the same spark energy output, and, as most street and modified production engines don't really need a cable size larger than 8.5mm, using our 10mm size cable is usually overkill (or for appearance). Please understand our 10mm cables, due to their large diameter and bulk, will take more room, and a little longer to fit and loom correctly.