MSD 4,600-6,800 RPM Module Selector Switch. The RPM module selector plugs directly into the RPM module socket on all MSD Soft Touch Rev Controls and accessories that use plug-in modules. The user can then select between twelve different RPM limits by simply turning the knob. Six models available to cover a range from 3,000 RPM to 12,800 RPM in 200 RPM increments.