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
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...
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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