Volume 8 - Capacitive Discharge Ignition Installation

The stock ignition on your Volkswagen puts out about 20,000 volts to the spark plugs, yes 20,000. Seems like allot but consider that an internal combustion engine "requires" at a minimum 18,000 volts to run reliably. That's a 10% buffer between good running and poor performance. VW wasn't alone, the Kettering ignition system found in all air cooled VW engines was used by virtually every manufacturer between 1920 and 1980. While points type ignition is reliable, it suffers from serious drawbacks inerrant to it's design. First off, ignition points wear and deteriorate rather quickly. Once this starts, which is almost immediately after installation, you are cutting into the 20% buffer I mentioned. This drawback is easily fixed with the installation of a Pertronics points replacement system. The second drawback is weak spark with further deterioration at higher RPMS.

A stock system uses the car's 12 volts to charge the ignition coil, it's the coil's job to step up the voltage prior to it reaching the spark plug. A CDI module has "capacitor" storage of its own and sends a short, high voltage (about 250+ volts) pulse through the coil. The coil now acts more like a transformer (instead of a storage inductor) and multiplies this voltage even higher. Modern CDI coils step up the voltage about 100:1. So, a typical 250v CDI module output is stepped up to over 25,000v output from the coil. The CDI output voltage of course can be higher, you'll see CDI systems claiming coil output capability over 40,000-60,000 volts!! The main advantage of CDI is the higher coil output and "hotter" spark. The spark duration is much shorter (about 10-12 microseconds) and accurate. When using ignition points to trigger the spark on a CDI, they tend to last 20 to 30 thousand miles because the current is so low, there is no arcing and as a result, they don't burn like a stock ignition system.

The automotive CDI, pioneered mainly by Bosch in the late 70s was called the "Bosch Motronic". Today there are a variety of names to include: Ford's TFI (Thick Film Integrated), GM's HEI (High Energy Ignition), DIS (Distributor less Ignition System), ECU (electronic control unit), and many others.

So why do you need one? Consider this, almost every new automobile made today uses some form of CDI ignition. Manufactures were quick to see the benefits of complete combustion (cleaner engines), faster starts and smooth operation. A stock system will do the job it was designed for, start the engine and get you down the road but that's about it. Start changing things like raising the compression, installing bigger carburetors and you'll quickly reach it's limits.

I chose a unit called the Tiger Model 581 CDI sold by Universal Corp in Colorado. This CDI is a basic, no frills unit selling for $82.50.

Installation:

Required Items:

  • Hand Tools
  • Drill & Bits
  • Electrical Wiring Tools (Crimper, Strippers etc..)
  • Trouble Light

The CDI unit is approximately 5" x 4" by 2 1/2" tall. Since the type 1 engine compartment doesn't have much room and my VW has dual carbs, I was limited in placement options. The unit needs to be as cool as possible while remaining in close proximity to the ignition coil. Keeping that in mind, I decided to place it on the left hand side of the firewall. This would require me to remove the left side carb and air cleaner to gain access. I started off by disconnecting the positive lead on the battery. Since I would be removing carburetors, I didn't need a spark igniting some spilled fuel.

ENGINE COMPARTMENT PRIOR TO INSTALLATION

Since I had it all out, I decided this was a good opportunity to rewire the engine compartment. But first I had to design a mount and figure out the wiring.

BASIC DESIGN WITH WIRING DETAIL

As you can see, the wiring for this CDI is simple. Basically the 12 volt originally routed to the ignition coil now gets run to the unit (Red) and the points wire from the distributor (Green) is now routed to the CDI instead of the coil. The CDI supplies new leads to the ignition coil (White, Black). Since I'm rewiring the entire engine, I've shown it's wiring in this diagram. In the picture below, you can see the plate I made up for mounting the unit. The CDI could be mounted directly to the engine compartment firewall but I wanted some airflow beneath for cooling purposes so I opted to mount a plate spaced away from the firewall with standoffs.

CDI UNIT AND MOUNTING PLATE

A unique feature of this unit is it's ability to return to standard ignition by flipping a connector. All you do is pull out the large black connector and re insert it with the arrow facing the other way. Once the design was completed and the unit mounted to the plate, it's ready for installation into the car. First I removed the LH Carb assembly and separated the wiring. Make sure you place a rag into the manifold to keep anything from falling into the engine.

FIREWALL PRIOR TO INSTALLATION

I held the unit up to the firewall and marked the holes. I elected to use 1 1/2" self tapping screws with 1/2" standoffs to secure the CDI to the firewall. Obviously I couldn't get to the lower RH mounting bolt since the shroud is in the way, 3 screws will be sufficient until the next time the engine is out.

INITIAL PLACMENT

There isn't much room behind the shroud and the Kadron air cleaners are rather large so I had to ensure there was space to allow the engine to move while not interfering with the CDI. My first attempt didn't provide enough room to clear the air cleaner, it pays to properly plan these things.

RELOCATED CDI

As you can see the wiring is completed and taped up and secured a various locations to prevent chafing. Once the unit was mounted and connected up, I needed to re gap the spark plugs. This is where you see the benefit of running a CDI, the more spark that's exposed to the fuel air charge, the better the combustion. In a perfect world, 100% of the fuel air charge would be ignited every time, in reality, it's somewhat less. Typically, the plug gap is set at .024" ~ .028", anything wider and the stock ignition system won't fire reliably. With a CDI, the gap is reset to .045", almost double the stock setting. The extra energy provided by the CDI allows the plug to fire across the wider gap as well as through carbon and oil deposits.

NOTE: Since a CDI produces considerably more voltage than the stocksystem, the ignition (plug) wires must be upgraded to a high energy type. Stock wires will quickly break down which results in the spark jumping out (arcing) of the wires to nearby objects like your hand (ouch!). The stock ignition rotor has an embedded resistor for RF interference which also doesn't like the high voltage produced by the CDI. There are inexpensive replacements available that do not contain this resistor.

COMPLETED INSTALLATION

After re checking the wiring one last time, I was ready to start it up. Upon turning on the key I could hear a faint high frequency tone. This is characteristic on a CDI and is why they're sometimes referred to as a "Singing Brick". When I cranked over the engine, it fired up instantly, the car always started pretty well but with the CDI, it lights off immediately. The Kadron Carburetors I'm running don't have chokes so cold starting usually required 30 seconds of keeping the engine running with the throttle. As I let it warm up, there was much less tendency to stall, in fact, it idled almost like it had already been warmed up. There wasn't a big difference in the engine's performance but then I didn't expect one. One characteristic I noticed was when going around a corner in too high of gear, the engine pulled out of a lugging situation much easier so derivability was increased. At higher RPMs the was definitely a little more pull. I suspect the original ignition wasn't producing much spark over 4000 RPMS which is characteristic of a stock system. The overall first impression is very favorable, time will tell how it effects the fuel mileage.

Chris R.