This box is the standard of the industry. The new 233-6425 has 2 rotary dials to set the limit in 100 RPM increments and incorporates a sealed connector for all the wiring. Additional rev modules for the discontinued 233-6420 are still available separately. Comes with a built-in rev limiter and plug-in module to change the RPM limits in seconds. Rev-limiting modules in 100 RPM increments with 3000, 6000, 7000, and 8000 rpm modules supplied in the box. Some tachometers need MSD tach adapter for ignition box compatibility.