CDI stands for "Capacitive Discharge Ignition".  CDI boxes gives you a more consistent and hotter spark than (stock) inductive ignition systems.   The only disadvantage to CDI is that combustion is more efficient with a medium-duration spark - a CDI system can only emit a short, high-intensity spark.  To overcome this disadvantage, almost all CDI boxes run multiple sparks at low RPMs; the MSD brand actually stands for "Multi Spark Discharge".  This multi-spark feature is switched off at 3,000 RPM; a single, intense spark takes over at high revs above 3,000 RPM.

So what advantages does it give you?  For one, a CDI ignition fires a more consistent ignition spark.  This rewards you with a smoother running engine, at both idle and high RPMs; high RPMs is where the ignition system is most challenged.  Dyno plots have consistently showed ignition break-up on the top end of most vehicles; this is the ignition system not being able to fire off the intake and fuel charge consistently and efficiently.  With the more complete combustion, efficiency goes up which will sometimes show as a 1 to 2 MPG increase!  A benefit of the hotter spark will also allow you to run your spark plugs much longer than normal.  Rotary engines are notorious for "eating" spark plugs, and engine performance decreases rapidly with spark plug gap increase; the CDI can consistently fire a larger spark plug gap, so you can keep your old spark plugs in your engine longer!  Replacing spark plugs is not cheap on a rotary, as NGK BUR spark plugs are easily $5 each.

Both the MSD 6A and Crane Cams HI-6 basically do the same thing.  The MSD 6A uses analog circuitry, while the Crane Cams unit uses digital RISC circuitry.  Both do multi-spark up to 3,000 RPM, and switch over to a single spark above 3,000 RPM.  The Crane Cams HI-6 does include a rev-limiter (more comparable to the MSD 6AL, which is the MSD 6A with a rev-limiter), and user-switchable options to turn both the rev-limiter and multi-spark feature on or off.  The more comparable MSD unit to the Crane Cams HI-6 is the MSD Digital6 which is priced over $200.

Pic of internals of an MSD 6AL...

Pic of internals of a Crane Cams HI-6...

Another popular unit is the Jacobs Rotary Master unit.  Priced over $300, it's an expensive option compared to the MSD 6A or Crane Cams HI-6.  HKS offers it's Twin-Power (Rotary application) which control leadings and both trailing ignition channels.  The HKS unit is pricey MSRP $540!
Click here for the CDI box installation page!

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