There is a new version of the TDC (Turbo Demo Cart). The new model has a turbo switch. The selectable speeds are 1 MHz, 2 MHz or 4 MHz. The TDC has now 2 SID chip sockets and can use 1 or 2 SID's. With the TDC the C64 runs 2x or 4x faster, but it slows down to normal speed when the VIC and SID chips are accessed. Visit the TDC web page for more technical details.