TCI vs. CDI....what's the difference?

[reelrazor]

Hey all.

I want to write this to clear up some misconceptions about our igntion systems.

FZR's (all models that I know of) have TCI igntion. here's a link from a Yamaha Vision group...but our TCI works by the same principles :

http://www.jetav8r.com/Vision/IgnitionFAQ.html#a7p0

Our TCI box exists to eliminate moving parts (breaker points and advance mechanism).

Basically, our ignitions are functionally the same as old vehicles with breaker points. It is just a modernized, solid state(transistors taking the place of the points and a logic chip replacing advance weights and cam) 'Kettering type' ignition.

Our coils have full time (keyed) battery voltage to them. The pickup coil by the flywheel 'feels' the trigger bump go by and sends a signal to the TCI box. The TCI looks at RPM (using the frequency of the pickup coil's signal) and determines whether or not to advance the spark timing based on a pre-programmed curve in the logic chip. At idle, it is using the static advance that is determined by the flywheel key. When spark is needed, The TCI interrupts the batt+ to the coils and this causes the field in the coil to collapse. This, in turn, induces the secondary windings to dump and fire the plugs.


The logic chip in our TCI has only ONE variable to look at, and that is RPM as reported by the (single) pickup coil. There is NO throttle position sensor, NO gear position sensor, NO road speed sensor, NO ambient air sensor. It is what is known as 'single map, open loop' logic. The advance curve is the same whether you are revving to limiter in neutral, or you are in second gear dragracing uphill-which means a compromise has been made somewhere. The compromise was made toward the cleaner emissions end of the scale. This is WHY advancing the ignition with an offset woodruff key, or slotting the pickup coil gives immediate results.

CDI works almost just the opposite as TCI. CDI ignition coils sit there, with NO voltage in the primary circuit, until the CDI sends a pulse of hundreds of volts through the primary to ground. That 200+ volts induces the secondary windings to dump and fire the plug.

A CDI box of the same vintage as ours has the same kind of logic as ours. BUT, a CDI also has a set of condensers, which take the Batt+ and store charge, rapidly building to hundreds of volts. When the pickup coil sends a signal, the CDI's logic says 'when' and it dumps that charge of hundreds of volts through the coil's primary windings. CDI's have very high spark voltage compared to TCI, but CDI also has a very short spark duration.

CDI powered coils also can be used in higher rev situations because the coil isn't dependent on time to build up inductance in the secondary coils, it is amplitude dependent. CDI therefore offers consistent spark voltages as high as you want to rev.

TCI powered ignitions are dependent on time. The coil needs a minimum time to saturate to build full power in the secondary windings. TCI ignition spark voltages fall with revs after a certain point.