Why Is The Ignition Coil Grounded?

[69 Soft Targa]

Most engines photos show a ground wire from the coil to the mounting stud for the coil bracket.

Some CDI engines don't have this ground but still run.

What is the purpose of grounding the coil?

[304065]

What CDI engine doesn't have a coil ground? All the Porsche cars have one, there are only four terminals on the CDI box, ground, +12V, points trigger and the coil positive. The coil negative is a short brown pigtail wire that goes to the fan shroud.

If you mean MSD, there are two wires from the box to the coil, it's a "Floating" ground.

[Jerome74911S]

Floating ground? Good grief, I have enough trouble with mysterious grounds as it is.

[RWebb]

your concerns are groundless

[69 Soft Targa]

Here's a photo courtesy of Shaun's '69T engine for sale.

Short wire, red terminals, coil to ground.

My '69 came to me without this wire and it runs fine.

[dshepp806]

grounds are a matter of "where" you get them.....



Doyle

[steely]

Originally Posted by RWebb

your concerns are groundless

PUNishingly true

[Hugh R]

American cars used to have a capacitor on the coil to ground to reduce static on the radio.

[69 Soft Targa]

This ground is only required for Bosch CDI. True?

This ground can't be used with points and no CDI. No spark.

My engine does not have this ground but has Permatune CDI and runs fine.
This is because it has the "floating" ground like the MSD mentioned in the first reply.

Do I have this right?

[2.70Racer]

Bob,
A coil is an autotransformer. That is it has 3 connections. 1 for the primary winding, (the lower voltage small terminal), 1 for the secondary winding, (the center high voltage wire to the center of the distributer cap, and the 3rd, a small terminal where the primary and secondary are connected together, typically the negative side.
For the coil to produce a spark, a small voltage (12 volts for non CDI systems up to 350/450 volts for CDI systems) is applied across the primary terminal and the 3rd terminal. When the voltage is removed from the primary coil, the secondary side of the coil discharges a much higher voltage (25,000 to 50,000 volts).
The older non CDI ignitons (Kettering), use the points to break the ground side of the coil. Therefore one side of the points are grounded, while the other side goes to the coil primary.
With a Kettering system 12 volts is constantly on one of the coil primary terminals while the other side connects to the points which make and break the ground circuit, causing the secondary to discharge through the spark plugs to ground.
With CDI system the voltage (typically 350/450 volts) makes and breaks from the CDI box as controlled from the distributer. The coil is continously grounded on the other small primary terminal.
Either way the voltage at the primary side is made and broken, whether at the positive or the negative side of the primary winding of the coil.
Further since a CDI coil primary operates at 350/450 volts as compared to a Kettering system operating at 12 volts, the CDI coil is wound at a much lower ratio to the secondary, compared to the Kettering style 12 volt ignition.

[2.70Racer]

Bob,
To answer your question with a permatune or bosch, the small coil negative terminal is continously grounded whether at the alternator housing or at the CDI box.
It is a true ground, not this "floating ground". Ground is ground.
I suppose even ground for your Porsche could be considered a "floating ground" since there is no copper rod driven into the ground like your home wiring.
This is the reason for the flat spring like metal strap imbedded into the ground in F1 pits which contacts the lower frame of F! cars during refueling to prevent sparks.
Or aircraft which are grounded by a cable anchored to the ground prior to refueling.
In the case of a Porsche (and most other cars other than some old English cars) the negative terminal of the battery is grounded to the body and this connection is continued to the engine with a ground strap.
No ground strap and the engine may or may not be grounded.

[James Brown]

You guys are well grounded

[Shaun @ Tru6]

If you are concerned about grounding your coil, just buy my motor, it's ready to go as you can see.

Originally Posted by 69 Soft Targa

Here's a photo courtesy of Shaun's '69T engine for sale.

Short wire, red terminals, coil to ground.

My '69 came to me without this wire and it runs fine.

[Zeke]

Originally Posted by 2.70Racer

Bob,
A coil is an autotransformer. ..................... (etc.)

That has got to be the best explanation ever posted here including those from Early_S_Man.

Recommending for the Wiki.

[James Brown]

How is that project, Wiki, coming along?

[69 Soft Targa]

Thanks Doug,
I agree with Milt that your post is very informative and well explained.
I understand how a coil/transformer and the battery/body/engine ground works. Now I know more about the CDI.

This quote answers my original question.

Originally Posted by 2.70Racer

With CDI system the voltage (typically 350/450 volts) makes and breaks from the CDI box as controlled from the distributer. The coil is continously grounded on the other small primary terminal.

Obviously my Permatune doesn't require the ground at the alternator housing.
That ground (the one I questioned) is neccessary with the Bosch CDI.