Can some ignition guru explain to me the difference between inductive ignitions and capacitative discharge (CD) ign's. If I understand things correctly, the stock ignition is inductive? Am I right? What are the pros and cons of each type and how are they different? Just trying to clear up my own confusion. Thanks.

 

OK, an Inductive ignition (which you correctly identified as the stock ignition) works in 3 steps. Step 1, 12V is used to build a magnetic field in the primary. Step 2, the ground is removed, resulting in the collapse of the magnetic field. This generates at least 200V in the primary. Through transformer action (75 to 1 primary-secondary turns ratio), up to 15,000 volts is generated in the secondary, causing a spark at the plug. This type of ignition (Kettering ignition) has been in use for over 100 years.

In a Capacitive Discharge (CD) system, a capacitor is charged to over 400 volts DC (Most popular systems run 470V currently, some as high as 525V). When the ground is removed, it is sensed by the electronics in the ignition, and the capacitor is connected across the coil. This results in a signal over 800V being generated in the coil primary, which becomes up to 60,000 volts in the secondary due to transformer action.

Advantages of the CD: Hotter spark. More reliable spark due to higher secondary voltage. Able to fire a larger spark plug gap consistently. Able to fire leaner and richer mixtures than an inductive system. Due to the fast charge time for the Capacitor, voltage does not drop at high rpm (In an inductive system, it takes a long time to saturate the coil, so the higher the rpm, the less saturated the coil is, and the lower the firing voltage becomes).

Disadvantages of the CD: Short duration spark (about 300 microseconds, versus 1200 microseconds for the inductive system). This is no problem at rich fuel ratios, but economy ratios will suffer from lean surge. Modern systems compensate for this by generating up to 12 sparks at 1000 microsecond intervals for each firing signal. MSD (stands for Mutliple Spark Devices) pioneered this. Also, the much higher secondary voltages will aggravate marginal parts (If your wires are leaking voltage, for example, you will find that the car will miss a lot worse with the CD than with the stock ignition!). You may have problems with ECU's/tachs that work off of primary inductive kick of the coil, since the trigger wire is now connected to an electronic circuit, not a real coil. Adapters are available to simulate this if needed (Not a problem on REX's).

Hope this helps. Have fun!

Irv, Keith's dad