What?

[EX400rider]

Yea whats a cdi? thanks!!!

[Nick110]

its the same thing as a rev box. the stock CDI has a rev limiter to u dont rev to high and break anything. if u get an after market one it will raise it 1,000 rpms more over stock. therefor better performance. hope this helps

[JOEX]

Not all aftermarket CDI/Rev boxes are the same! And just because you can get to a higher RPM doesn't necessarily mean you're making more power.

[Crazy400EX]

whats the best one to get then? in your oppinions.

[nowukno]

Vortex

[Crazy400EX]

how much is that?

[miken]

Better be careful, raising the rev limit sometimes means an upgrade in valve springs.
Mike -

[wilkin250r]

If you don't know what it is, and what it does, then you probably shouldn't change it.

CDI stands for Capacitive Discharge Ignition. Your stator and fly wheel send two things to the CDI box. The first is a large pulse of power, and it charges up a big capacitor, almost like a battery. Then, your stator sends a small trigger pulse that is perfectly timed off crankshaft position. This trigger is recieved by the CDI, and it calculates RPM and crankshaft position. At the precise moment, the CDI dumps all the power stored in the capacitor into the ignition coil. The ignition coil boosts it to several thousand volts, and that is then fed to the spark plug.