CDI override, how to?

[Fishman7]

How can you override the CDI on a 72 911?
Milt I know you know how!

[Zeke]

I don't know how, but I have it on paper somewhere. By the time I find it, the question will be answered. Meanwhile, use those terms and Early_S_Man as the poster. It's pretty simple, but you gotta do it right. Just can't pull that one outta my head. Sorry.

[BK911]

I used to carry around a 914 coil (w/internal ballast resister) and a capaciter. I had it all wired up to make quick connects. I am sure I still have it somewhere. I'll see if I can find it.

[HarryD]

You want to build a "kettering" ignition system. This web page shows the basic wiring scheme. http://users.mrbean.net.au/~rover/ketterin.htm

[Zeke]

I think we are making more of this than it is, so I'm bumping it. IIRC, all you have to do is move a hot wire from the CDI to the dizzy. You don't get the benefits of having a CDI, but the dizzy has a condenser in it which will do the job.

[rick-l]

I wouldn't do it.

The coil in a CDI is a transformer where as in the 'Kettering" ignition system" mentioned above the coil is an inductor. Inductors are designed to store energy where transformers are not.

Doesn't the CDI have a ground connection that would also have to be rewired.

I wouldn't think there would be a capacitor in the CDI system capable of storing the amount of energy from the coil to allow the points to open without establishing an arc.

[JFairman]

I don't think the coil makes much difference here. I've put the old durable aftermarket Mallory CDI boxes on different cars in the past and used the origonal coil with no problems.

The Bosch coil and CDI can be picky abount inductance or resistance of the primary windings in the coil and they sometimes eventually burn out when not matched up right... figures.

The coil is a step up transfomer and none of them "store" electricity.
In a step up transformer there is a much greater amount of secondary wire windings around the iron core than primary windings.
FWIW, it's the opposite for a step down transfomer..

When the points close and 10-11.5 volts goes through the primary windings of the coil a magnetic flux field is set up around the internal primary and secondary windings and iron core they are wrapped around.

When the points open the flow of electricity is cut off to the primary windings and the magnetic field drops and voltage is induced in the secondary windings.
This secondary voltage is much higher than 12 volts because of the greater amount of wire windings in the secondary side.

With CDI installed the primary voltage is amplified to around 350-450 volts DC and stored in special electrolytic capacitors alot like an electronic flash unit on a camera up to around 350-450 volts DC. A thyristor controls the charge on the capacitors and older designs make a little whine while doing it.

The capacitor's stored high voltage is then repeatedly discharged by the thyristor very quickly into the primary windings of the coil triggered by the distributors mechanical or electronic trigger circut.. that being points, magnetic, or optical.

The primary voltage magnetic field rise time is wayyyy faster and greater than a 12 volt primary voltage magnetic field so the secondary voltage going to the plugs becomes alot higher and faster than when only 10-12 volts is going through the primary side of the coil.
The ballast resistor used on old points and condensor ignition was there to lower the primary voltage from 12-13.5 volts down to around 10-11 volts so the points wouldn't burn up as fast and the condensor buffered the small voltage arc, also helping to make the points last longer.

Thats the basics as I understand it.
I may be off a bit here and there in my descriptions...

[ischmitz]

As others said: A Kettering ignition simply uses the coil as the storage for energy instead of a capacitor. When the points close ground is applied to the coil. The other end of the coil is connected to switched +12V (Term. 15). When the points close current flows and the coil creates a magnetic field. Once the points open after the dwell time the magnetic field breaks down. As a result a voltage spike is generated that causes the spark.

In an emergency the "bypassing" of a CDI igniton would look like this. Disconnect the 3-pin from the CDI box. The connector carries three signals:

A: Ignition coil
B: Term. 15 (Switched +12V)
C: Trigger signal

To create a Kettering system you need to disconnect the ground wire from the coil and run a jumper wire from Pin C of the 3-pin connector to that terminal of the coil. Then bridge pin A and pin B to get switched +12 Volt to the other side of the coil. You could add a 3 Ohm 10 Watt resistor to protect the coil. Use it instead of a simple bridge between pin A and pin B.

For this to work properly the dwell of the points in the distributor needs to be set properly. Too much dwell time will stress the coil. Further, you might have to adjust timing. With the CDI box the spark is fired when the points close. With the Kettering the spark is fired when the points open again. You need to correct for that.

I personally have not done this before but I don't see any reason why it wouldn't work.

Cheers,
Ingo

[Zeke]

Originally Posted by ischmitz

I personally have not done this before but I don't see any reason why it wouldn't work.

Cheers,
Ingo

Thanks, Ingo. I emailed him to have this answer posted. I couldn't find Early_S_Man's post on the same subject.

Anyway, this will get you home. I wouldn't drive the car excessively or aggressively until you get you CDI repaired. Which Ingo does, BTW (repairs, not drive aggressively. Oh, wait...... )

[Fishman7]

Great responses, thank you guys. What a wealth of knowledge!