[T3] Capacitive Discharge Ignitions

[Jim Adney]

Since about 1966, I've been a supporter and user of the Capacitive 
Discharge Ignitions (CDIs) that were designed and sold by Delta in 
Colorado, either as kits or preassembled. Those were sold under the names 
Delta Mark 10, Mark 10 B, and Mark 10 C. Delta also sold them thru Radio 
Shack (ArcherKit), Allied Radio (Knightkit), and Heath (Heathkit.) These 
were all Delta designs with the only difference being the sticker on the case. 
They are all the same quality.

Early Delta Mark 10s were available in both 6 V and 12 V versions, for either 
positive or negative ground. All the ones I've ever had were for 12 V, 
negative ground, but I've seen a postive ground version on ebay. The kit 
versions came with instructions for assembling them for either positive or 
negative ground.

The B & C versions were for negative ground only. I've never seen a C in 
person.

Various proprietary CDI designs became standard on lots of cars, including 
Porsche and Mercedes-Benz by the early '70s.

So what is a CDI and why should I care?

A CDI is a slightly different ignition system that has sevral advantages and 
one disadvantage. It's an electronic system that can be added to any 
standard Kettering (points and coil) ignition system. It boosts the car's 12 
VDC to 400 V, charging an internal capacitor to 400 V and then discharging 
that capacitor thru the coil when triggered by the points.

Advantages:

A CDI spark is only about 10% as long as a standard spark, so the spark 
plug electrodes don't have as much time to heat up and erode, making spark 
plug life much longer. The fast rise time will also sometimes fire carbon 
fouled plugs that would otherwise short to ground and cause a miss with the 
standard system.

The total energy delivered in the spark is about 10x larger than a standard 
spark, so the chances of a misfire are reduced. The spark energy does not 
decline with RPM like a standard ignition does. The B & C models produce a 
3x longer and slightly more powerful spark while starting, which may be 
helpful on some cars.

The current carried thru the points is minimal and is not inductive, so it does 
not tend to arc and wear out the points. The only wear to the points will be 
that of the rubbing block on the cam.

The Delta will work well above 10,000 RPM, so you never need to worry 
about exceeding the limit of your ignition system. For most of us that's not a 
problem, but the standard system gets weaker as the RPMs go up and pretty 
much runs out of steam around 6-7k.

Disadvantages:

If there are weak points in your system, like broken down insulation, carbon 
tracks, etc. this higher energy system will find them. Since these are things 
that you should fix anyway, I don't see this as a real problem.

The B version has a switch that can fail to make good contact after a few 
hours of use. That's fixed by "exercising" the switch back and forth a few 
times. I get around the problem by exercising the switch each time I check 
the oil on a long trip.

Notes:

The Delta CDIs work best with a standard ratio coil. Using them with special  
"High Ratio" or "High Performance" coils is counterproductive.

Adding a Delta CDI to your car is just a matter of adding and rearranging 
wires on your coil, nothing more. The B & C versions also include a switch to 
set the system back to standard/normal, which can be useful.

A Delta CDI works just fine with a Pertonix in place of the points.

So why would you want to add a CDI to your VW?

Very long lifetime for plugs and points. I'm talking over 50k. If your distributor 
cam is in good condition and you keep it lubricated, so it doesn't wear out 
the rubbing block, that means you can go for years without a tuneup. (Yes, 
you still need to change the oil!)

If you have trouble starting, a CDI might help. If your engine misses, you 
should start by checking your cap, rotor, wires, insulators and plugs. If you 
keep having to replace plugs because your engine is burning a lot of oil and 
fouling the plugs, a CDI could provide a workaround until you can fix the 
underlying problem.  

I've been using Delta CDIs since 1966, starting with my Dad's '60 Corvair. 
Currently all my Type 3s have them, using the standard OE Bosch coil and 
points. In all that time, over many, many cars, I've had only 2 failures; both 
were easily fixed. My last failure was in 1974.

For the past few years, I've been buying and repairing Delta CDIs that I find 
on ebay. I take them apart, clean them up, check and redo solder joints as 
necessary, then reassemble with stainless steel hardware. Then each one 
gets bench tested at 3000, 6000, and 12,000 RPM for over an hour, just to 
make sure they are ready to go.  

I currently have several available for sale, some Mark 10s, some Mark 10 
Bs, from well used to NOS. Prices range from $100 to $175, depending on 
the model and the external appearance. I have several more sitting here 
waiting to be prepped and tested.

Let me know if you're interested. I'd like to get a few of these out to good 
homes. I'll warrantee them for a year, with repairs for parts & labor after that, 
but you're extremely unlikely to ever have a failure.

thanks for reading,

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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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