The Sonnet Technologies QuadDoubler Processor Card is a drop in replacement for the 68040 processor in the 20 and 25MHz Apple Centris and Quadra Machines that runs at twice the initial speed. There are two versions; a 40MHz version for the C610, and a 50MHz version for the 25MHz machines. If you clock chip the base machine, say a C610 from 20 to 28MHz (standard), and then install a 40MHz QuadDoubler, it might be too fast and overheat, so you should add a fan to the 40MHz QuadDoubler, and if it is still to fast, then drop to a slightly slower speed; 50MHz is not unreasonable.