[T3] CDI Marathon!

Jim Adney jadney at vwtype3.org
Fri Feb 5 07:22:04 PST 2021

CDI stands for Capacitive Discharge Ignition, and I've been using Delta Mark 
10 CDI units since 1966. That started out on Dad's Corvair and then moved 
to my '68 Square. They install easily and work with standard points and 
coils. I've been so pleased with their performance that I've bought and built 
(they were available in kit or factory assembled form) several, but in recent 
years, I've managed to buy several on ebay, when they come up.

This past week, I finally got around to testing a BUNCH of them. I clean, 
inspect, and bench test each one, then repair and resolder as necessary. 
I've replaced all the rusty screws with nice stainless ones. When I'm satisfied 
that one is good to go, I run it on the bench for at least an hour and a half, at 
rates equivalent to 3000-12000 RPM. Now I've got a BUNCH ready to sell. 
I've written up more on thesamba, where you can go to see pictures of some 
of the ones I have available, as well as the instruments I use in the testing.


Delta eventually made 3 main versions of this CDI: Mark 10, 10 B, and 10 C. 
I've never had my hands on a 10 C. The Mark 10 is a basic unit that will do 
everything you need. The 10 B has slightly different internal circuitry, but the 
only important difference is that it has a switch to change from CDI to 
conventional. The switch is convenient, but they sometimes cause trouble 
and need to be "exercised" to regain good contact. I make a point of 
exercising mine any time I have the rear hatch open. This has eliminated any 
annoying interruptions.

I have Mark 10s in 6 V and 12 V versions, negative ground.

Mark 10 Bs only came in 12 V negative ground.

CDIs give a hotter, higher energy spark that happens in a shorter period of 
time. This reduces erosion (wear) of the spark plug electrodes. Plugs last 2 
to 4 times longer.  

The CDI unit uses the points to trigger the discharge, but that trigger current 
is small and non-inductive, so the point contacts last forever. The only 
reason to replace the points is if the rubbing block wears too much or the 
wire fatigues and breaks due to the flexing. My points tended to last at least 
5 years, even when I was driving a Squareback every day. Keep a tiny bit of 
sticky grease on the distributor cam to lube the rubbing block.

If you've replaced your mechanical points with one of the electronic points 
replacements, the Delta Mark 10s will also work perfectly with those.

But, let me be clear: If your current ignition system is running well, upgrading 
to a CDI won't give you any more horsepower. There is nothing magic here. 
If there are weaknesses in your high voltage components, the higher voltage 
and faster rise time of the CDI is likely to find them and force you to fix them. 
OTOH, that's what you should do anyway.

What you can expect is smooth running, reduced maintenance, and, 
possibly, easier starting in very cold weather.

thanks for reading,

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

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