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Ignition Systems

By Ryan Ballou

Fuel mixture, compression and spark/ignition are the three things that must come together for your engine to operate properly. In this installment of High Performance 101 we are going to take a look at the ignition system and ways to maximize it's potential.

The two most common types of ignition systems are the Inductive and the Capacitive Discharge (CDI) styles. The inductive ignition system, also know as a Kettering Ignition for (Charles Kettering - inventor) is the system your VW is equipped with in stock form. It consists of a power source (battery), a distributor, points, capacitor (condenser) and coil. Current flows through the points creating a magnetic field in the coil while the points are closed. Once the points are opened, the field collapses and the coil discharges through the high-tension lead, the distributor and ultimately the spark plug in the combustion chamber. The purpose of the condenser is to prevent arcing across the face of the points and prolong their lifespan. Premature pitting or erosion of the points is usually an indicator of a faulty condenser.

The inductive system has stood the test of time having been in use now for almost 100 years. They are simple to manufacture and maintain, and are considered by many to be nearly fail-safe. Inductive ignitions also provide a very powerful spark as compared to a CDI, but tend to loose their efficiency as rpms rise due to their slower charge times. This is where the CDI steps in to take over.

A CDI uses a switching power supply to charge and capacitor to around 400-500 VDC. This capacitor then discharges through the coils primary windings. The coil then operates like it normally does, discharging through the secondary (high tension) windings and fires the spark plug. The rpm range at which a CDI is effective is limited only by capabilities of the switching power supply within the ignition unit itself. Since the CDI sends a higher primary voltage to the coil (nearly 40x greater), the coil doesn't need as much time to build the same charge. The rpm range of an inductive ignition is limited by the speed at which the inductor (coil) can complete a charge/discharge cycle with enough energy to fire the spark plug. When comparing conventional retail ignition systems, this difference is about 2.5 times in favor of a CDI.

CDIs also posses benefits over traditional inductive ignitions, most will fire the spark plug multiple times in the lower rpm range. This helps to assure a more complete burn at every combustion event and will generally lead to smoother low speed operation in adverse conditions such as a cold wet climate. Additionally, when points are retained as the triggering mechanism within the distributor the actual current passing through the points is virtually nil. This means the only limiting factor for the lifespan of the points is how long the rubbing block will last.

Along with the benefits offered by the ignitions boxes themselves comes a host of accessories available. First and foremost is the rev limiter. A rev limiter is a device that through one method or another will prevent your engine for surpassing a preset rpm limit. This serves as a failsafe device to protect your engine from over revving, usually from a missed shift. For those into the racing scene there are two/three step options. Basically this is just a more advanced version of a rev limiter offering multiple preset points that you can switch between. Say one rpm limit for the burnout box, one for staging, and one to prevent over revving.

For those running forced induction, a boost retard option is available. These retard the timing in a linear fashion as boost increases. Retard options are also available for those running nitrous. These can either retard timing when nitrous is activated manually, or can be used with available window switches to automatically control nitrous activation. Window switches act a switch for a device only within a preset rpm window. For example you can set up the system to activate your nitrous system and retard timing only between 3k and 6k rpm. Above or below that window, the switch is 'off'.

Installation of an ignition box is quite simple; the hardest part is finding a suitable location for it. My favorite is on the parcel tray behind the rear seat. Here you have easy access to the battery and chassis bolts for direct power and ground. A switched power lead will need to be brought in to turn the unit on. You also have fairly direct access to the firewall to route wires into the engine bay.

Hooking the box up to your existing ignition is very easy. Whatever wires were on the coil (+) lead originally should be removed from the coil but kept wired together. This is a great place to pull your switched power lead from for the box. There should only be one wire on the coil (-) lead unless you have added a tachometer. This is the wire coming from your points, also known as the trigger wire. Most boxes can be triggered one of two ways, by a points style ignition, or by magnetic pickup ignitions. Unless you are running an MSD distributor, you likely have a points style ignition. Points style triggers include Pertronix and Compufire ignitions even though they use magnets.

With nothing left connected to the coil, you can now hook the box up to the coil. The box will have two leads that will be the only two coil connections. Be cautious of these leads when the car is on or running, they no longer carry 12VDC. They now carry close to 500VDC, albeit at very low current, but it's still enough to give you a little jolt. Try to keep these wires and the wire from the points routed separately and away from any spark plug wires. The pickup on the box is sensitive to induced currents and can fire erratically otherwise. This will mean drilling two holes through the firewall, one for the trigger wire and one for the leads to the coil. Make sure to use properly sized grommets in these holes to prevent chaffing the wire's insulation and to keep moisture out.

The last two leads to connect are the ground and main power. I've found the best location for a ground connection is one of the chassis bolts on the rear cross members under the rear seat. Then the main power lead should be direct to the battery, best if wired with an inline 20 amp fuse.

Something worth mentioning is your choice of spark plug wires. I have used stock Bosch wires with my CDI with no apparent problems. However use of solid core wires like these will introduce the possibility of induced cross firing between cylinders and in increased likelihood of interference with radio and computer equipment (FI anyone?). If any problems arise after the introduction of a CDI box, consider and new upgraded set of wires for high energy ignitions systems. Some of my favorites are Jacobs Electronics, Taylor, and Magnecore wires. These will generally use a spiral core wire to self-cancel EMI and RFI.

Another common question is spark plug gap, i.e. do you need to increase it? With a high-energy system, increasing the gap will force the plug to fire a little hotter. In most situations .030" to .040" is all that's needed. Anymore and the increased demand on the rotor, cap, and wires will lead to premature failures of these parts. When using distributor that maintains the same physical dimensions as the stock unit, cross firing within the distributor cap may occur with plug gaps exceeding .035". It's not guaranteed, but a distinct possibility. You'll know this is happening because from your midrange rpm and up, the engine will feel like it's missing or bogging. The solution to this is to drill a small vent hole (or two) in the distributor cap. The hole/s should be about 3/32" in size and located at the same height and centered between the electrodes in the cap. Choose the side/s to drill so that the holes are not facing your fuel pump or fuel lines, as it's possible (but unlikely) a stray spark can exit these holes.

Almost seems too easy doesn't it? Now that you've got your new high-energy ignition system in place and working, all that's left is to drive it like you stole it.


1-This is your typical Capacitive Discharge Ignition; it's an MSD 6AL. The 6AL features a built in rev-limiter unlike the standard model 6A. Mallory has their version called the Hyfire VI with various versions also available for rev-limiting and so on. The advantage of the Mallory over the MSD is that the MSD requires the purchase of 'pills' to set the rev-limiter. The Mallory has a built in dial feature to accomplish the same task.
2-Here you can see the relative size of these boxes and with all the wiring why location can be an issue. The two wires on the end of the box, one exposed one sheathed are for Tach (right) and kill switch (left). The tach output works with most aftermarket tachometers, but an adapter is sold separately if you experience problems. The kill switch is just the unused magnetic trigger with a toggle switch to ground. It has the same effect as grounding out the points. 3-Here you see the rev-limiting installed in the unit. The 6AL comes with 3k, 6k, 7k and 8k rpm chips. If you need to fine tune beyond this, the pills are available in 100 rpm increments.
4-The group of wires next to the fuel filter is for the distributor, one of them being the trigger wire. The orange and black wires next to the center arm on my cross bar linkage are the coil leads. The come through the firewall in separate places and do not share the same split channel tubing. Also notice that they are kept away from spark plug wires as much as possible. 5-Even though the distributor cap on my Mallory distributor is much large than a stock cap, it still comes with a vent hole installed by the factory.  

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