The ignition coil is the device which takes the battery's 12 volts and converts it to several thousand volts to produce a spark at the plug. The coil actually contains two coils intertwined, but not connected directly. The two coils are named the "primary side" and the "secondary side". The primary side gets the 12 volts. The secondary puts out the thousands of volts.
There are two basic types of ignitions used on old bikes. The type used determines how the ignition coil will be used to produce spark.
The KZ uses a type called a Kettering ignition. It's named after the inventor. In this type, 12 volts is applied to the primary side. The 12 volts is interrupted by the points. When this happens, the primary side induces a short spike of about 400 volts. This spike is stepped-up through magnetism into the secondary which puts out a spike of several thousand volts (but very little current).
The other type is a magneto which also acts similarly capacitive-discharge-ignition. These systems hit the coil primary directly with a few hundred volts. The secondary acts similarly to the Kettering type.
The main difference is that the Kettering produces a spark when the primary circuit is broken. The magneto/CDI type produces spark when the primary side is hit with a spike.
Because the CDI/magneto type coil is hit with a short-duration spike, it needs to be "faster". This means it will have a much lower resistance on the primary side than a typical Kettering coil. A CDI/magneto coil will have between .5 and 2 ohms. A Kettering coil (for bikes) will have between 2.5 and 4 ohms. Kettering coils for v-8 cars will be lower in resistance because they need to be faster in order to fire 4 times per revolution. A KZ only fires 1 time per revolution. But the car still needs to produce as much energy in shorter time which is why car coils are bigger.
The KZ electronic ignition is just a transistorized version of the Kettering design. It is NOT CDI as is listed in some catalogs etc. The transistorized version uses shorter dwell and thus uses a "faster" coil. Points typically uses 4-ohm coils. Transistorized uses 2.5 ohm coils. This is because the transistor can handle more current than the points. The ignitor design also extends the dwell angle slightly at higher RPMs. Points have no way of extending the dwell. Dwell is the amount of time (or angle of crank rotation) during which the primary current is "on". More dwell means more spark energy-available (up to a point).
Upgrading coils is really just a maintenance issue. As long as the motor is stock and in good shape, stock coils do the job and higher-energy coils won't make much difference. Old coils fail suddenly, especially when they get wet. It sucks being stuck.
The final voltage at the plugs is actually determined by the gap and the amount of air in the gap. Higher voltage coils won't make a difference since the actual voltage at the plug will be lower than what the coil maxes out at. Now, if the engine is modified to breath more, that means there is more air insulating the gap. That's when you start to need more voltage.
Higher-energy coils also don't make much difference if the mixture is really good. Higher-energy means longer duration spark for our purposes, since voltage is limited by the gap. If the flame front travels faster than new mixture can enter the gap, (IE decent mixture), the spark is only firing through combusted gases. In this case the engine will run great on high-energy spark or low-energy spark. If the mixture is weak, and the flame front moves slow (relative to the new mixture flowing into the gap), a longer duration spark will produce a bigger initial flame front (sometimes called a kernel). This will produce a better burn.
Two-stroke motors have a lot more crap (oil) in the gap and benefit from higher voltage spark. Two-strokes also have poor mixture at lower RPMs so often benefit from higher-energy ignitions.
CDI produces high voltage, but low energy. To offset that, better ones fire multiple times at lower RPMs (MSD multiple spark discharge).
Luckily, the KZ's have such a good stock engine design and good carburetion, ignitions coils for the KZ are not a critical area for performance improvement. Typically, if an ignition-coil upgrade makes a big difference, there is usually some other factor involved. That is, either the previous ignition coil wasn't working properly, or the mixture is weak, or the engine has condition issues.
These are just general guidelines and are not hard-fast rules.
Post edited by: loudhvx, at: 2006/08/15 12:50
.......awesome.....
